Inflammatory Atherosclerosis

Atherosclerosis

The Beginnings

1. The Beginnings. A Multicentric Disease “We cannot identify the unknown aspects of modern life that

Proteoglycans and the Extracellular Matrix

lead to atherosclerosis until we know the true nature of the characteristic atherosclerotic lesion.” Earl Benditt, [12]

Previous studies demonstrated that the initial lesion in atherosclerosis is asymmetrical intimal thicken-

The Injurious Agent

ing, the result of increased production of sulfatecontaining proteoglycans (PGs) - primarily Chondroitin Sulfate Proteoglycans (CSPG) and other

Throughout this comprehensive study the term “Injurious Agent” (IA) will be used to refer to any agent or any process, singly or in combination, that might cause cellular dysfunction or injury to the artery wall, resulting in atherosclerosis.

forms of extracellular matrix (ECM) - by resident intimal smooth muscle cells (SMCs) in a focal area of the arterial wall [6–9]. The IA, directly or indirectly, enters the arterial wall from the circulating blood, and then either stimulates or enters the resident SMC, the principal source of vascular PGs

Response-to-Injury

[10], to produce increased amounts of PGs and ECM. Walton [8] showed this mucoid thickening of the intima occurs before lipid infiltration and is com-

The response-to-injury hypothesis states that the

posed primarily of collagen, PGs, and ECM. Thus,

initial event in the pathogenesis of atherosclerosis is

although lipid accumulation in the artery wall is con-

injury to the endothelium [1]. A variety of IAs pro-

sidered an early event in atherosclerosis, lipid

duce an inflammatory response in which leucocytes,

retention is not the initiating event, and the fatty

primarily monocytes, migrate to the area of injury

streak is not the first sign of atherosclerotic injury

[2]. The result is retention and oxidation of lipopro-

[11,12]. This initial intimal thickening is not charac-

teins and transformation of monocytes into

terized by hypercellularity or proliferation of SMCs

macrophages that ingest lipid, particularly oxidized

[13], as is seen in other types of vascular injury [14],

low density lipoproteins (LDL). These form the fatty

but rather by relative acellularity, apparently due to

streak that is an early objective sign of atheroscle-

the increased amounts of PGs and ECM without

rosis [2]. Important considerations in this theory are

associated SMC proliferation [15]. The relative acel-

the precise nature of the IA, and the sequence of

lularity noted in these early lesions is not believed to

events that lead to the retention of lipid. Several

be due to massive cell death of resident intimal cells

studies in experimental animals have demonstrated

[16]. Increased production of PG and ECM, without

that lipid retention occurs before the monocytes

an increase in the number of SMCs, is an unusual

migrate into the intima [3], showing that the mono-

response to injury, suggesting a specific type of IA

cyte is not the cause of the lipid retention [4,5].

and/or a specific type of injury or effect on the SMC

What, then, is the cause or the mechanism of the

[17,18].

lipid retention? Whether the increase in PGs and ECM is a pathologic response and, therefore, to be prevented, or is a physiologic defensive, protective, or reparative 1-1

Atherosclerosis response to the IA is not known [6,10]. The fact that

The Beginnings

Adaptive Intimal Thickening

these intimal thickenings develop very early after wall injury and before lipid accumulation suggest this is a protective, healing, or defensive response [1,6]. This view is supported by the knowledge that CSPG is required and is the predominant PG in normal wound repair [9]. However, if this is a physiologic defense, it fails badly because the IA agent is not halted, proliferation of PGs and ECM continues, and resolution, healing, and stabilization do not occur. The disease continues to progress. In addition, if the production of PGs and ECM is a physiologic defense, why is lipid retained?

Stary, et al. [21], believe many asymmetric intimal thickenings, termed Adaptive Intimal Thickening, reflect a physiologic adaptive response to hemodynamic stress. They found this thickening at points of arterial bifurcation in infant human beings and animals. The authors point out that such physiologic thickenings may also be the site of atherosclerotic plaques. It may be difficult to distinguish thickenings that are physiologic adaptations from those that are pathologic, particularly in the early stages of atherosclerosis. These adaptive intimal thickenings are rich is PGs [21]. Evidence of lipid retention,

The ECM is a visco-elastic material containing pri-

then, is a primary feature that distinguishes physio-

marily CSPG, a biochemically active scaffold that

logic thickening from pathologic atherosclerosis

regulates arterial permeability, filtration, transport of

[21]. The presence of intimal thickening at points of

plasma constituents, and regulation of wall

bifurcation supports the view that these lesions are

metabolism and function [10]. The increased

an adaptation to hemodynamic stresses, but the

amount of PGs produced by the SMC in response

occurrence of the same lesions in areas without

to various growth factors associated with

bifurcations, Figure 1, or areas of low or relatively

atherosclerotic injury have much longer side chains

low hemodynamic stress, suggests other factors are

and form larger aggregates than do the PGs nor-

also involved. These other additional factors may be

mally found in the artery wall [10,13,19]. Thus, there

acting independently or in conjunction with hemo-

is not only an increase in the PGs and ECM pro-

dynamic stresses to transform adaptive intimal

duced, but a change in the structure of the PGs in

thickening into atherosclerotic lesions.

the areas of atherosclerotic injury. This change in PG structure is believed to alter the metabolic properties and biochemical function of the PGs and ECM, resulting in a disturbance in the transfer of substrates through the zone of injury, particularly alteration of interactions with lipoproteins [10,13,19,20]. These structural and functional changes in the PGs as well as their turnover rate, are directly related to the rate of retention of lipid in the interstices of the ECM [8,9]. The alteration in

Whether intimal thickenings are initially physiologic or pathologic, they reflect tissue proliferation to some sort of IA at any age. The fact that some adaptive intimal thickenings progress on to atherosclerotic lesions indicates that all such thickenings may possess the potential to do so and to become a vulnerable site for the IA to enter the wall [2,11].

structure and the increased production of PGs suggest a pathologic component of the disease process, produced and altered, not as a physiologic defense [2,8], but for the specific purpose of retaining lipid, particularly LDL. 1-2

Atherosclerosis

The Beginnings

Figure 1: A, Dissected left coronary artery from a 31-year-old male who died of head injuries. The artery shows no radiographic evidence of atherosclerosis. B-H are contiguous segments of the main left coronary artery, as labeled in A. B, Normal appearing intima (thin arrows) and media. C, Same coronary segment as B, but directly opposite on the other side of the lumen. The intima here is thicker (bracket), with a small focus of relative acellularity (open arrows). D, Slightly increased intimal thickening (bracket) and an increased area of acellularity (open arrows) compared to C. E, Marked increase in intimal thickness (bracket) with loss of tissue and cells (open arrows) consistent with focal degeneration. Lipid-laden SMCs (long arrows) surround this area of degeneration. F, Further increase in intimal thickness (bracket) with areas of acellularity and lipid-laden SMCs (long arrows). No areas of degeneration can be identified. G & H, Intimal thickening (bracket) is decreasing distally from F, but with more prominent and more numerous lipid-laden SMCs (long arrows). White asterisk = lumen, fat arrows = media. Hematoxylin & Eosin (H & E) stain in all photos.

1-3

Atherosclerosis

Early Atherosclerotic Lesions

The Beginnings because the IA has been present and active for a longer period of time than in adjacent sections. Fig-

Figure 1 shows early changes of atherosclerosis in

ure 1E shows the most advanced changes in terms

a 31-year-old white male who died of non-cardiac

of tissue degeneration, cell loss, and lipid accumula-

causes. Asymmetric intimal thickening begins near

tion, located approximately midway between the

the left coronary ostia (Figures 1B, 1C), and contin-

first proximal intimal thickening (Figure 1C), and the

ues through all contiguous coronary segments,

distal thickening in Figure 1H. Therefore, Figure 1E

ending at Figure 1H. Focal areas of relative acellu-

could be the site of initial injury and the develop-

larity can be identified in Figures 1C–1H, consistent

ment of intimal thickening, proximally and distally,

with increased production of PGs and ECM by resi-

may reflect direct spread of the IA agent in both

dent intimal cells, presumably in response to wall

directions. A review of Figures 1C–1H confirms that

injury [21]. Lipid retention in the form of lipid-laden

the amount of intimal hickening and the severity of

macrophages or extracellular lipid, or evidence of

the degenerative changes tend to decrease in both

tissue injury are not evident in unaffected intima

directions from Figure 1E.

(Figure 1B) nor in mild intimal thickening (Figure 1C). This finding supports the view that intracellular or extra-cellular lipid deposition does not occur in the normal artery wall, but only follows the development of intimal thickening [2,8]. The asymmetric intimal thickening involving only a portion of the luminal circumference indicates that the injury is focal and that the IA is present and presumably active at this particular site. Why the IA enters or affects the wall at a particular site has not been fully worked out, but it is probably related to local susceptibility, increased vulnerability, and/or focal injury to the endothelium by a various agents [1,2].

The histologic changes observed in Figures 1C and 1H may represent the leading edge of the injury that is spreading longitudinally from the central site in Figure 1E. Furthermore, if the IA can spread longitudinally then it may also spread circumferentially, with the leading edge of the injury expanding in the direction of the plaque shoulder (Figure 2). If this is the correct interpretation of these histologic changes, it means an IA can establish a foothold in the artery wall in spite of defensive responses. It then has the potential to spread by direct contiguity in all directions from this central focus.

The presence of asymmetric thickening to a similar degree through contiguous coronary segments, as shown in Figures 1C–1H, suggests these thickenings are part of one continuous area of injury extending in a longitudinal direction [21,22]. If this assessment is correct, do these contiguous thickenings reflect injury from a single IA that has spread from a single focal site in a proximal and distal direction, or do they represent multiple, separate foci of injury to the same or different IAs? If we postulate that one agent caused all these lesions from one focus, is it possible to identify the initial injury site histologically? In theory, the initial site of injury should show the most advanced histologic changes 1-4

Atherosclerosis

The Beginnings presence of cholesterol crystals (long arrow) and calcification (short arrow). H & E stain. E, Area near the other plaque shoulder with similar amount of tissue degeneration (open arrows) as shown in B. H & E stain.

Advanced Degeneration Figure 2, illustrates, in the same artery of the same patient, another, more advanced focus of intimal thickening and plaque formation, but distal to that shown in Figure 1A. The plaque in Figure 2A is the largest, most advanced atherosclerotic plaque in this artery, but it is still insignificant in terms of luminal stenosis. The central area of this plaque (Figure 2D), shows advanced degeneration with focal necrosis of tissue, loss of both ECM and cells, lipidladen SMCs, lipid infiltration, formation of cholesterol crystals, and a tiny focus of calcification. The necrosis and calcification indicate this is an “advanced” lesion, according to the classification of the American Heart Association’s Committee on Vascular Lesions [15]. According to this classification, any lesion, regardless of luminal stenosis, that contains an atheroma and/or a fibrolipid plaque and calcification is considered to be an advanced lesion.

The advanced degenerative changes in Figure 2D extend toward each plaque shoulder, where the plaque meets the normal arterial wall, the area of degeneration and necrosis becoming progressively smaller and less severe toward the plaque shoulder, Figure 2: Coronary section taken from the same left coronary artery illustrated in Figure 1, approximately 1 centimeter distal to

Figures 2B, 2E. Again, as in Figure 1, the IA

Figure 1H. A, Low-power view of a small asymmetric plaque with

appears to be spreading by direct contiguity, only

a central core (black asterisk). Martius Scarlet Blue (MSB) stain.

this time in a circumferential direction toward the

White asterisk = lumen. B - E, High-power views of different

plaque shoulder. The tissue immediately surround-

parts of the plaque corresponding to the letters shown in A. B, Focal area of tissue degeneration, cell loss, and lipid retention

ing the central core (Figure 2E) appears devoid of

(open arrows) near one shoulder of the plaque. H & E stain. C,

cells, suggesting that these cells were either

Midway between the shoulder and the center of the plaque,

destroyed by substances contained within the

showing the size of the degenerative area (open arrows) is larger

necrotic core, have undergone apoptosis or died of

than in B. MSB stain. D, Central area of the plaque showing focal degeneration (open arrows) and frank necrosis, including the

some other consequence of the disease process [23,24]. Progressive cell loss is characteristic of 1-5

Atherosclerosis

The Beginnings

advanced lesions and, with cell death, degenera-

responses, whatever they may be, appear to be

tion, necrosis, and formation of a necrotic core

unable to halt, sequester, or neutralize the IA or to

follow [23–26].

effect healing and resolution of the injured area.

The plaque in Figure 2A extends proximally into the

Unanswered Questions

adjacent coronary section and distally into two addi-

What is the IA and what is the mechanism of injury?

tional segments, again suggesting active longitudi-

What kind of an IA can enter the wall and injure or

nal spread of the IA. The remainder of the coronary

stimulate resident SMC to produce excess amounts

artery distal to the segment illustrated in Figure 2

of abnormal PGs without stimulating the prolifera-

showed two additional focal, widely separated

tion of SMC or attracting other SMC from the

asymmetric intimal thickenings, similar to, but less

media? Does the IA enter the SMC without injuring

severe than those shown in Figure 1. Altogether

the extracellular matrix? Why does this particular

there were four separate lesions separated by nor-

form of PGs retain lipid, when the normally present

mal arterial wall in this one artery. Wilens [11] noted

PGs do not? What is the relationship between the

that IAs can attack the wall at multiple sites, setting

IA and the retained lipid? What is the mechanism by

in motion atherosclerotic disease at multiple points,

which the IA spreads throughout the intima? Why

emphasizing atherosclerosis is a multicentric dis-

do some mucoid swellings degenerate and become

ease.

necrotic very early in plaque development while others do not? These questions will be explored in

In Review

subsequent chapters.

The IA causing atherosclerosis appears to enter at a focal point in the artery wall. In some way it stimulates the resident intimal SMC to produce increased amounts of an abnormal form of PGs, resulting in asymmetric intimal thickening and lipid retention. Asymmetric intimal thickenings are ubiquitous throughout the coronary tree because atherosclerosis is multicentric in origin and the IA, present in circulating blood, may enter the wall at any vulnerable point. The production of an abnormal form of PGs appears to be a pathologic component of the disease process, produced specifically to retain lipid. The IA appears to establish a locus or focus of injury and then spreads in all directions from this central focus, to contiguous areas within the intimal layer. Lipid-laden SMC are an early, but not the earliest sign of atherosclerosis. Degeneration, necrosis and calcification of plaque tissue can occur very early in plaque development. The defensive 1-6

Atherosclerosis

The Smooth Muscle Cell

2. The Smooth Muscle Cell. The Pivot in Atherosclerosis nism gone awry [2], but a pathologic component of “Whereas the precise nature of the initiating event for

the disease process from the very beginning. There

atherosclerosis is not known, it is clear that the failure of the

is no reason to think the FP response is physiologic

smooth muscle cell to maintain its normal differentiated phenotypic state becomes a key contributing factor in the

in any way because all atherosclerotic plaque tissue

progression of atherosclerotic disease.”

represents diseased tissue, regardless of the cir-

GK Owens, [17]

cumstances under which it formed.

The Fibroproliferative Response

This chapter presents evidence supporting the view that the intimal SMC is specifically targeted, its

Atherosclerosis is generally considered a fibroprolif-

intracellular functions altered by an IA, to produce a

erative (FP) disease because atherosclerotic

specific type of pathologic fibrous tissue. We

plaques contain large amounts of fibrous tissue [2].

hypothesize that inherent SMC functions are altered

This FP response is believed to be a defensive,

to such an extent that the SMC becomes a partici-

protective, physiologic response to injury, designed

pant, mediator, and perpetrator of the disease

to wall off, contain, enclose, or sequester the IA,

process, as illustrated in the following examples.

and then to assist in resolution of the injury [2]. However there is no evidence to show that the FP response ever functions in a defensive or protective

Atherosclerotic Fibrous Tissue

fashion. Typical plaque fibrous tissue, illustrated in Figures Continued plaque growth suggests the resident

3A and 3B, is found throughout atherosclerotic

intimal SMCs and any SMCs migrating from the

plaques. The tissue architecture consists of a

media are being stimulated to produce more and

meshwork of collagen bundles embedded in ECM,

more fibrous tissue, but this continued growth does

with many lacunar-like spaces containing SMCs

not prove that the FP response is a physiologic

staining positive with SMC actin antibody (Figure

defensive response to injury. If the growth and pro-

3C). The collagen and other components of the

liferation of fibrous tissue were a defensive

ECM are believed to be produced by these SMCs

response, then why isn’t the disease process halted

(2,4,12,27), forming an unusual and distinctive type

in its very earliest stages, and why does the FP

of fibrous tissue. The SMC nucleus is flattened

response progress above and beyond that required

against one side, suggesting compression or

to repair a small area of injury [2,12]?

increased pressure within the lacunar space (Figures 3A, 3B). These SMCs appear to be identical to

Plaque tissue is produced primarily by intimal SMCs, not by fibroblasts, the usual cell type normally involved in wound repair: it therefore differs from the normal fibrotic response to injury [12,27].

those shown in Figure 1, except that these are larger and enclosed in a uniform fibrous meshwork. Other investigators have observed these flattened, attenuated SMCs [28,29].

The FP response may not be a defensive mecha2-1

Atherosclerosis

The Smooth Muscle Cell

The fibrous tissue shown in Figures 3A, 3B is relatively acellular, similar to the asymmetric intimal thickening shown in Figure 1, representing a further developmental stage due to the large amount of collagen and ECM. The SMCs within these lacunar spaces appear to be filled with clear material, presumably lipid, that is compressing the nucleus. The type of lipid has not been identified, but it appears to be in a different form than that found in macrophage foam cells (Figure 3B). Lipid droplets are not obvious, so the SMC may metabolize ingested lipid differently from monocyte-derived macrophages [30,31]. The absence of a typical foam cell configuration may be due to a lack of esterification of lipid by the SMC. Other investigators, using electron microscopy, have noted that SMC do contain lipid droplets, showing that at least some of the lipid ingested by SMC is esterified, but seldom to the same extent as monocyte-derived macrophages [17,31].

Figure 3: A, Typical fibrous plaque in a 65-year-old white male. Note the spindle-shaped, lipid-laden, SMCs (small arrows). These cells vary greatly in size, and some appear to be coalescing with neighboring cells (long arrows). The SMC nucleus is flattened along one side of the lacunar spaces (open

2-2

Atherosclerosis

The Smooth Muscle Cell type of fibrous tissue [12]. Recent evidence shows

arrows). There are relatively few cells present, but a large amount of fibrous tissue. H & E stain. B, High-power view of the

different types of SMCs in plaques, but the histo-

subendothelial area in the same section as A. There are foam

logic picture is the same in all plaques [12] and no

cells (white arrows) and other lipid- containing cells just beneath

pathognomonic changes distinguish one IA from

the endothelium. Two very large, lipid-filled spaces, apparently

another.

formed by the joining of adjacent, but dead, SMCs are present (black arrows). Asterisk = Lumen. H & E stain. C, A small fibrous plaque in a 50-year-old female, stained with SMC actin, showing the lipid-laden cells (arrows) within the lacunar spaces are, in

The Smooth Muscle Cell

fact, SMCs. Asterisk = Lumen. D, Intimal hyperplasia in a 75year-old white female who received PTCA at this site 9 months earlier. Note the hypercellularity, the disorganization of these

The SMCs found in plaques have an altered pheno-

cells, the presence of stellate cells (arrow), and the absence of

type compared with normal SMCs, but what controls

any lipid-laden SMCs. H & E stain. E, Intimal hypercellularity in a

these phenotypic changes and whether they are the

6-month-old vein bypass graft in a 53-year-old white male. The

cause or the consequence of active atherosclerosis

SMCs are oriented parallel to the endothelial surface, and none appear to be lipid-laden. Note the hypercellularity near the

is unknown [17]. These SMC show altered lipid

endothelial surface (arrows). Asterisk = Lumen. H & E stain. F,

metabolism, altered growth factor production,

Oil Red O stain of a fibrous plaque showing the fibrous tissue

altered ECM production, smaller size, fewer intercel-

diffusely infiltrated with lipid droplets, giving the fibers a granular

lular junctions, and the presence of fatty vacuoles

appearance. Note the scattered cholesterol crystals (arrows).

Other Types of Vascular Fibrous Tissue

[12,32]. In addition there is a decrease in myofilaments and various proteins, with an increase in golgi and rough endoplasmic reticulum [17]. Although this phenotypic change is often considered a physiologic response to an injurious stimulus, the

Contrast the relatively acellular tissue in Figures 3A

possibility that this change is actually a pathologic

and 3B, with the hypercellular response seen follow-

alteration in SMC function and a key factor in the

ing percutaneous transluminal coronary angioplasty

development of plaque lesions cannot be excluded

(PTCA), (Figure 3D), or a recently placed saphe-

[17].

nous vein bypass graft, Figure 3E. These structural differences plus the absence of lipid-laden SMCs in Figures 3D, 3E, indicate that these tissues are not,

The structural meshwork of the tissue shown in Fig-

or at least not yet, atherosclerotic. Presumably the

ures 3A and 3B suggests these lacunar spaces may

FP response following PTCA is related to physical

be created by the ingestion of lipid by the enclosed

injury produced by the balloon, and the thickening of

SMC [33]. The flattening of the SMC nucleus and

the vein graft wall is a physiologic, hyperplastic

the variation in size of lacunar spaces suggest over-

response to increased hemodynamic stress within

ingestion and/ or unregulated uptake of lipid by the

the vein graft lumen.

SMC is greater or more advanced in some SMCs than in others. The inability to regulate the uptake of

These different cellular and structural responses to

lipid is one of the features of monocyte-derived

different types of vascular injury suggest different

macrophages found in plaque tissue [31]. The lipid-

IAs stimulate the SMC to produce different FP

laden SMC in Figures 3A, 3B may be SMC

responses, illustrating the pleuri-potential nature of

macrophages that have also lost that regulatory

the SMC [17]. The monoclonality of the SMC, pro-

ability [1,21,30,31]. The loss of lipid regulatory

posed by Benditt [32], suggests the SMC is altered

capacity is believed due primarily to the presence of

in a very specific way to produce a very specific

scavenger receptors on the SMC [34]. 2-3

Atherosclerosis SMCs are reported to transform into macrophages and to take on the appearance of foam cells, but they are not as efficient as monocyte-derived macrophages in the uptake of lipid, possibly because SMCs do not express scavenger receptors to the same degree as do monocytes [2,31]. Further, some of the larger lipid-laden SMCs appear to be fusing with adjacent lipid-laden SMCs resulting in the formation of small lipid lakes, suggesting active and continuing accumulation of lipid (Figure 3B) [10,35].

The Smooth Muscle Cell halted, or the injury resolved by the activity of the SMC macrophages. The inability to remove lipid from the artery wall, for instance, by reverse transport, may be related to a decrease in mobility of SMCs related to the thick fibrous network. Or the SMC, like the monocyte-derived macrophages, may be partially disabled and have decreased mobility through the excess intake of lipid [30]. Is it possible that some or all of these SMC functions are pathologic components not physiologic defenses, of the disease process? Whatever the mechanism, the SMC appears to be a key player, their normal func-

If, in fact, these are SMC macrophages, why have they formed in this location, what is their purpose, and why are they ingesting lipid? The general purpose of the macrophage is to neutralize and remove any IA and/or harmful or toxic compounds and to

tions targeted and subverted by the IA, serving to mediate the subsequent development of atherosclerotic disease.

Degeneration and Necrosis

participate in the removal of dead and injured tissue [36,37]. Since this is atherosclerotic plaque tissue, we can assume that three of these factors, the IA, cytotoxic compounds, and injured tissue are present within a plaque.

The fibrous tissue shown in Figures 3A and 3B appears to be fatally flawed. All such tissue, given sufficient time, may degenerate and undergo necrosis, forming a lipid-laden necrotic core or atheroma. The SMCs die as result of toxic chemical agents

Figure 3F shows plaque fibrous tissue is heavily

such as oxidized LDL, from the over-ingestion of

infiltrated with lipid, is extra-cellular [33] and is pre-

lipids, from hypoxia or from apoptosis [25,37,39–

sumably the source of lipid ingested by the SMC.

41]. The uptake of oxidized LDL by SMC

The metabolism or oxidation of this retained lipid, an

macrophages may promote apoptosis [39]. The

expected consequence of lipoprotein trapping [19],

death of the SMC leads to discharge of the ingested

results in the formation of cytotoxic compounds

lipids and other cellular elements into the extracellu-

such as oxidized LDL [35,38]. These compounds

lar space, and to the degeneration of non-viable

may produce the harmful stimulus necessary to

fibrous tissue. Degeneration and necrosis of the

transform the SMCs into macrophages [31]. We can

lipid-laden fibrous tissue lead to a further increase in

assume that one of the reasons these SMC are

the extracellular lipid content, and eventually to a

taking up lipid is because it is a toxic substance or

lipid-rich necrotic core.

compound [30,31]. However, these SMCs do not seem to neutralize or

Figure 4 illustrates degeneration and destruction of

remove the lipid or toxic compounds, but simply

tissue surrounding the necrotic core. Figure 4A

ingest more and more of them, resulting in fibrous

shows a relatively small plaque with a necrotic core

tissue laced with lipid-laden SMCs of different size

and an overlying fibrous cap. A fibrous cap is gen-

(Figures 3A, 3B). Nor is there any evidence that the

erally considered to be a protective layer of fibrous

IA and/or the injurious process have been slowed, 2-4

Atherosclerosis

The Smooth Muscle Cell

tissue separating the core from the lumen, formed

quent destruction of this same tissue [39] means the

primarily to contain and prevent communication

IA and subsequent atherosclerotic disease are

between the lumen and the necrotic core [4].

basically and ultimately destructive in nature [43].

The necrotic core in Figure 4A appears to be expanding toward the shoulders of the plaque, illustrating the tendency for the plaque and the necrotic core to grow circumferentially, as in Figure 2A. Erosion through the fibrous cap will probably occur first at the lower shoulder of the plaque where the fibrous cap is thinnest. The histologic structure of the fibrous tissue in this fibrous cap is the same as that illustrated in Figures 3A, 3B. This fibrous cap, already diseased and weakened by dysfunctional SMCs, is destined for eventual destruction because the SMC in the cap will eventually die, the cap will degenerate, and the plaque will rupture. In other words, the IA appears to affect and destroy the SMCs in the fibrous cap and elsewhere, by creating a toxic necrotic core that destroys surrounding tissue, including the fibrous cap. Thus, the fibrous cap has become a pathologic component of the disease process, not a protective structure designed to enclose the necrotic core.

In Figure 4B, the fibrous cap adjacent to the lumen in the upper part of the photo separates the lumen from the deeper necrotic core. The tissue between the necrotic core and the fibrous cap is in the process of degeneration and destruction and will soon be part of the core, presumably as a result of the toxic substances within the necrotic core, the growth and expansion of the necrotic core, and the resulting death of SMCs [25,40–42].

Figure 4: A, Crescent-shaped atherosclerotic plaque in the proximal LAD coronary artery of a 53-year-old white male. Red and blue injection mass has mixed in the lumen (asterisk). Note

The expanding destruction of tissue surrounding the

the major part of the plaque is composed of whitish material (fat

necrotic core is vividly apparent in Figure 4C. These

arrows), and microscopic examination shows this white material

photos illustrate gradations and zones of tissue

to be the necrotic core. The fibrous cap is made up of what appears to be clear or transparent tissue (thin arrows) and varies

damage and destruction around the necrotic core

in thickness from one side of the plaque to the other. The fibrous

from the center of the core outward. They also show

cap is quite thin at the plaque shoulder near the bottom of the

that although there is initial proliferation of fibrous

photograph (open arrow). B, High-power view of the plaque

tissue in atherosclerotic disease [31], the subse2-5

Atherosclerosis shown in A. The necrotic core is identified by a black asterisk,

The Smooth Muscle Cell

Unanswered Questions

and the arterial lumen by a white asterisk. The fibrous cap has structure similar to that shown in Figures 3A and 3B, is adjacent

How does the IA enter the wall? Why is the SMC

to the lumen. The fibrous tissue (arrows) between the lumen and

targeted and intracellular functions altered? Why

the necrotic core has undergone partial degeneration and

does the IA first stimulate the proliferation of fibrous

necrosis. MSB stain. C, SMC actin stain of a fibrous plaque surrounding a necrotic core (asterisk) from another patient. The

tissue, then proceed to destroy this same tissue? Is

tissue adjacent to the necrotic core has undergone degeneration

this the nature of the disease process, and if so,

with loss of SMCs, but the basic fibrous structure remains

what purpose is served by destroying this tissue?

(Bracket 1). The next layer outward from the necrotic core

What is the mechanism of growth and expansion of

(Bracket 2), shows the SMCs are still apparently viable and stain appropriately with the SMC actin stain, but many are lipid-laden

the necrotic core? Are the SMCs responsible for the

(long arrows), of different size, with several small lipid lakes and

FP response the same cells that transform into

with early degeneration of tissue at the right side of the

macrophages and ingest lipid, as Owens suggests

photograph (open arrows). Beyond this second layer is a layer of

[17]? Can the SMCs that migrate from the media

cellular fibrous tissue (Bracket 3) composed of viable-appearing SMCs, and only occasional small lipid-laden SMCs (short arrows).

undergo a phenotypic change from producing contractile fibers to synthesizing fibrous tissue, then subsequently transform or be transformed into a

In Review

SMC macrophage? If so, does this confer a survival advantage [17], or are these migrating SMCs subverted in the same manner as resident SMCs,

Atherosclerotic fibrous tissue is diseased, patho-

becoming part of the disease process?

logic tissue produced by SMC’s that have been targeted and affected and whose intracellular func-

Can one type of SMC perform all these various

tions have been altered by the IA to produce a

functions, or are there various subsets of SMCs that

specific, pathognomonic, type of fibrous tissue.

transform along certain cell lines, each performing a

Plaque fibrous tissue is laced with lipid laden SMCs,

different function [44]? Is failure to regulate SMC

probably SMC macrophages, whose normal func-

differentiation [17] due to the effects of the IA? How

tion has been subverted by the IA, leading to the

can a cell that is partially disabled, say with scav-

eventual death of the SMC and degeneration of the

enger receptors, continue to manufacture and

fibrous tissue. The SMC is the key cell affected by

secrete sophisticated and complicated metabolic

the IA, and it mediates the onset and progression of

compounds, such as growth factors and pro teolytic

atherosclerotic disease. The fibrous cap is not a

enzymes? Whatever the answers to these ques-

specialized protective structure but is a typical

tions, clearly the SMC plays a major role in the

atherosclerotic fibrous tissue formed by phenotypi-

progression and development of atherosclerotic

cally altered SMCs. It only appears to be a cap

lesions.

because the necrotic core tends to form first in the central part of the plaque. Atherosclerosis, in the final analysis, is basically and primarily a destructive disease process.

2-6

Atherosclerosis

Inflammation

3. Inflammation. A Sign of Active Disease “The lesions of atherosclerosis represent a series of highly specific cellular and molecular responses that can best be described, in aggregate, as an inflammatory disease.” Russell Ross, [18]

Chronic Inflammation Atherosclerosis is a chronic inflammatory disease characterized by migration of monocytes and T lymphocytes to the area of arterial wall injury [1,18,45]. Early investigators also noted that the lipid-rich atherosclerotic plaque may develop secondary to a primary inflammatory process [46]. Inflammation per se, acute or chronic, is believed to be primarily defensive or protective in nature, its principal aims being to neutralize and remove the IA, and to initiate the process of tissue repair and healing [18,47]. However, the inflammatory mediators associated with inflammatory cells are also potentially harmful because they can damage tissue and aggravate injury. The presence of T lymphocytes with an atherosclerotic plaque indicates that the immune system has been activated, that the IA, or a product thereof, is a foreign agent or antigen, and that antibodies are being produced against it [48–50]. We can surmise, based on the migration of monocytes and T lymphocytes to the area of injury, that the IA initiating the development of atherosclerosis represents a significant threat to the organism, and that all appropriate defenses are being mobilized against it. It should be emphasized that inflammatory cell infiltrates are found only within or overlying atherosclerotic plaques. They are not found in relation to a normal intima that has no evidence of atherosclerosis [51,52], (Figure 5, Figures 5A,5B). The migration and infiltration of chronic inflammatory cells, i.e., monocytes and T cells, reflect “active” inflammatory atherosclerotic disease [18,48,53]. 3-1

Atherosclerosis

Inflammation the entire circumference (arrows), and this is classified as a Grade IV inflammatory response. H & E stain. Magnification x11.7.

“Active” Inflammatory Atherosclerotic Disease Active, inflammatory, progressive, expanding atherosclerotic disease is characterized by plaque growth and the development of luminal stenosis, presumably due to continuing and expanding injury produced by the IA. If atherosclerosis is related to arterial wall injury and this injury results in a chronic inflammatory response, then the inflammatory infiltrate associated with an atherosclerotic plaque is a marker of active, injurious atherosclerotic disease [53]. The extent and severity of atherosclerotic lesions, in terms of plaque size, should reflect the extent and severity of arterial wall injury. The severity of this injury, in turn, should be reflected in the extent, magnitude, and/or number of inflammatory cells present, as seen histologically in the artery wall.

Adventitial Inflammation Figure 5: A, Proximal RCA section from a 32-year-old Asian male showing a small asymmetric plaque, with a necrotic core (black asterisk) and a fibrous cap (black arrows). White asterisk

Table 1 illustrates, in 83 patients who died of acute coronary disease (ACD), the relationship between

= lumen. H & E stain. Magnification x19.5. B, High-power view of

adventitial inflammatory infiltrates, primarily T lym-

rectangle in A, of the adventitia showing scattered Tcells (thin

phocytes, and the extent and severity of atheroscle-

arrows). This amount of inflammatory response was classified as

rotic involvement of the epicardial coronary tree, in

Grade I. Media = fat arrows. H & E stain. C, Large asymmetric atheroma (asterisk) in a 51-year-old white female with Grade II

terms of luminal stenosis. In order to assess the

inflammation of the adventitia (arrows). Lumen at top of photo. H

severity and extent of underlying arterial wall injury,

& E stain. D, Mid-RCA section from a 72-year-old white male.

adventitial inflammation was estimated and graded

The luminal stenosis is estimated to be 80% and the lumen

on the basis of circumferential involvement by T

contains a thrombus. Two foci of T cells can be seen in the adventitia (arrows) on opposite sides of the lumen. This is

lymphocytes in the adventitia in each coronary

classified as a Grade III inflammatory response. H & E stain.

segment. Inflammation or inflammatory cell infil-

Magnification x11.2. E, Distal RCA section from a 59-year-old

trates of the intima were not considered in this

white male. This section was taken immediately distal to an

grading system, but adventitial inflammation was

occluding thrombus with fragments of thrombus still present in the lumen. A thick, heavy band of T cells extends virtually around

believed to reflect intimal injury [54].

3-2

Atherosclerosis

Inflammation

Table 1:. Comparison of luminal stenosis and circumferential extent of adventitial inflammatory involvement of the coronary wall in 83 patients who died of acute coronary disease.

IC %

#

None %

#

%

Severity of Inflammation I–II # %

Degree of Stenosis (%)

# of Sections

<50

3221

46

957

30*

2264

70

899

28

58

50–80 >80

2458 1377

35 19

1623 1042

66 76*

835 335

34 24

1453 864

59 63

170 178

Totals

7056

3622

51

3434

49

3216

45

406

III–IV #

%

2a 7 13a 6

IC = Adventitial inflammatory cell involvement; * = p =<0.001; a = p =<0.001

Adventitial inflammation in patients with atheroscle-

Seventy-six percent of all coronary segments with

rosis is commonly present as discrete foci of T

>80% luminal stenosis had adventitial inflammation,

lymphocytes that have congregated at a particular

compared with 30% of coronary segments with

site in the adventitia. They are easily recognized

<50% luminal stenosis, p=<0.001, proving that the

histologically (Figures 5B–5E), and were graded on

frequency of adventitial inflammation is directly

the basis of I to IV according to the following sys-

related to plaque size. Comparing the frequency of

tem: The circumference of each coronary segment

Grades III/IV adventitial inflammation in the 178

was divided into three, 120° quadrants, with the

coronary segments with >80% luminal stenosis to

presence or absence of adventitial inflammation in

the 58 segments with <50% stenosis shows Grades

each quadrant recorded. Grade I, the T cells were

III/IV inflammation is significantly more common in

rather diffusely scattered in the adventitia overlying

those with >80% stenosis, p=<0.001. Both the fre-

a plaque without a definite discrete focus (Figures

quency and severity of adventitial inflammation, in

5A,5B). Grades II, III and IV showed discrete foci in

terms of circumferential extent, are directly related

one, two, or all three of the 120° quadrants (Figures

to plaque size [52,55]. We conclude that the size of

5C–5E). Segments with Grades I, and II inflamma-

the plaque and the circumferential extent of adventi-

tion were considered a “mild” injury, whereas

tial inflammation are directly related to the severity

Grades III and IV were considered “severe” injury of

or magnitude of the arterial wall injury.

the artery wall.

The Magnitude of Injury In these 83 patients, 3,835 (54%) of over 7000 coronary segments examined histologically showed

What factors related to the IA determine the magni-

more than 50% luminal stenosis. These patients

tude of wall injury? Since plaques vary in size and

had widespread injury and significant plaque devel-

composition, the amount of IA may not only vary

opment throughout the coronary tree. Similarly,

from plaque to plaque but vary in potency, toxicity,

3,622 (51%) of coronary segments showed adventi-

virulence, or antigenicity [56]. The susceptibility of

tial inflammatory infiltrates, confirming the presence

the individual patient to the IA must also be taken

of widespread, active, atherosclerotic injury and

into account. This variability in the inherent charac-

resulting disease [54].

teristics of the IA could explain why plaques vary in size, in speed of development, and in the degree to which the histologic changes become “advanced” 3-3

Atherosclerosis

Inflammation

without narrowing the lumen [15]. For example, Fig-

cant luminal stenosis, it is still a vulnerable plaque

ure 2A is a very small plaque with “advanced”

with a relatively thin fibrous cap. Theoretically it

changes, including calcification and necrosis, but

could ulcerate early in plaque development [57].

without adventitial inflammation or stenosis. The IA

Figures 5C–5E are examples of Grades III and IV

may be sufficiently potent, or present in sufficient

adventitial inflammatory involvement associated

amounts, to cause focal but severe injury. Although

with increasing luminal stenosis that illustrate the

the IA may be potent, toxic, or virulent, because this

association between inflammation and luminal

is still a small plaque, the IA is not yet present in

stenosis.

sufficie/nt amounts to activate the immune system or present long enough to cause a large plaque to form. In considering the pathogenesis of atherosclerotic injury, we may be dealing not only with the amount, but also the potency, toxicity, virulence, or antigenic potential of the IA.

Luminal Stenosis and Inflammation

Failure of Inflammatory Defenses The inflammatory defensive responses detailed above appear to be no match for the IA. Specifically, these inflammatory defenses, in spite of what appears to be a vigorous response, fail to neutralize, contain, or remove the IA and/or to halt the spread of the disease at an early stage of plaque development. This observation is similar to those

Figure 5 illustrates the direct relationship between luminal stenosis and adventitial inflammation. Figures 5A and 5B show a small asymmetric plaque causing approximately 20–30% luminal stenosis along with a Grade I adventitial inflammatory infiltrate, consistent with a small area of focal injury and beginning activation of the immune system. The presence of a necrotic core suggests the presence of an IA sufficiently potent to cause early destruction and necrosis of tissue, similar to, but more advanced than in Figure 2A. Of 23 coronary segments examined from this artery in Figure 5A, 14 showed various degrees of plaque development causing <50% luminal stenosis. Seven of the segments showed Grade I adventitial inflammation, and one segment showed Grade II inflammation. There were no ulcerated plaques (UP) or thrombotic

made in regard to the FP response (Chapter 2) in that the FP response also failed to halt the spread of the IA. The failure of these defensive measures suggests the monocytic and/or T lymphocyte responses are either inadequate to deal with the strength or toxicity of the offending agent, or that these inflammatory responses have been altered changed, or subverted in some way from performing their usual protective, reparative functions following wall injury [47]. Leibovich and Ross [36] noted the importance of normally functioning macrophages in wound healing. Therefore, continued plaque growth may reflect either an overwhelming or resistant IA and/or a disabled inflammatory response to injury on the part of macrophages and/or T lymphocytes.

Natural History of Wall Injury

lesions in this artery, nor any coronary calcification. This artery may have been injured in multiple focal

What is the natural history of atherosclerotic wall

areas by a potent IA, actively growing and expand-

injury and the associated inflammatory response

ing and beginning to cause activation of the immune

caused by the IA? Is the IA ever completely neutral-

system, but not yet causing any plaque ulceration

ized and removed, or can it persist indefinitely in a

(PU), erosion, or calcification. However, even

dormant, inactive state? Table 1 showed 24% of all

though the plaque is small and produces no signifi-

coronary segments with stenosis greater than 80% 3-4

Atherosclerosis

Inflammation

were without adventitial inflammation, similar to the

plaque size, and reflect the magnitude of the injury.

results of other investigators [54]. Some arteries

Plaque growth and the development of luminal

seem able to heal, and the inflammation may sub-

stenosis may be due to the continued presence of

side. This implies that the IA can, in some circum-

an active IA that spreads to adjacent tissue, expand-

stances, be neutralized, removed, or become

ing the area of injury and producing increased

dormant, with the injured area undergoing resolution

inflammatory responses. Atherosclerosis and

and healing. Plaque growth may be phasic with

adventitial inflammation are diffuse and widespread

exacerbations and remissions of the active inflam-

in patients who die of ACD. The magnitude of the

matory disease process, but the factors causing

injury and the speed of plaque development appear

remission are not known.

to be directly related to the amount, potency, toxicity, or virulence of the IA and/or individual suscepti-

Clinical Manifestations of Atherosclerotic Inflammation One of the great mysteries surrounding active atherosclerosis is the absence of clinical manifestations of inflammation in spite of diffuse inflammatory involvement of the coronary tree (Table 1). Virtually all clinical symptoms of active, progressive

bility to the IA. The inflammatory response to injury fails to halt the growth and spread of the IA. The IA may be too powerful or resistant to these inherent inflammatory defenses, or the macrophage and/or T lymphocyte functions may have been subverted, thereby preventing their neutralizing the IA.

Unanswered Questions

atherosclerosis are related to obstruction of coronary blood flow, and to resultant ischemia or infarc-

What kind of an IA is this that can apparently alter

tion of the myocardium. They are not related directly

intracellular mechanisms without killing the cell and

to inflammation. Patients with active atherosclerosis

can subvert normal defensive responses? What is

do not exhibit fever, pain, increased sedimentation

the mechanism of spread and expansion of the IA?

rate, or a significant increase in white blood count,

Does the IA replicate and, if so, by what mecha-

or other signs of an active inflammatory disease.

nism? Why is the adventitia the site of such heavy

This suggests that the IA is not exposed to the nor-

infiltration of T lymphocytes compared with the

mal bodily defenses that recognize and remove

intima, the area of injury? Does the IA, or a byprod-

most IAs. The IA may be residing within a cell and

uct thereof, pass to the adventitia via the lymphat-

thus escape detection.

ics, with the adventitia functioning much like a regional lymph node? What is the life cycle of the IA

In Review

and what is the energy source that drives growth and expansion of the plaque?

Inflammation, in the form of adventitial T lymphocyte infiltration, follows and responds to atherosclerotic injury and subsequent plaque formation. It is a marker of active, injurious atherosclerotic disease [54,55]. Adventitial inflammation is found only overlying atherosclerotic plaques. It serves to identify the site of injury and, presumably, the location of the IA. The frequency and circumferential extent of adventitial inflammation are directly related to 3-5

Atherosclerosis

Atheromas Are Caseous Abscesses

4. Atheromas Are Caseous Abscesses As stated in Chapter 3, plaque size, as reflected by “This lesion (the atheroma), undoubtedly the most dangerous of all prethrombotic processes, has often been called an

the severity of luminal stenosis, is directly related to

atheromatous abscess.”

the magnitude, severity, and extent of injury caused

Meyer Friedman, [60]

by the IA, as shown in the magnitude of the T lymphocyte response in the adventitia. The same may

The Necrotic Core Atherosclerosis is characterized by the proliferation, then destruction of intimal fibrous tissue, resulting in the formation of an atheroma, as originally proposed by Virchow [39]. The presence of a necrotic core is objective evidence that atherosclerosis is ultimately a destructive, not an FP disease, and the necrotic core is a component, not a complication of atherosclerotic disease. The precise mechanism of atheroma formation has not been fully worked out [12,37], but apparently the IA, directly or indirectly, is responsible for cell death, leading to non-viable degenerative tissue and eventual necrosis [58]. The overall cell count within a plaque decreases as the plaque enlarges, indicating that the lost cells are not being replaced, and that plaque enlargement is due primarily to the growth of the necrotic core rather than to the growth of fibrous tissue [58]. The mechanism of cell death is believed to be either coagulation necrosis or apoptosis, possibly related to the toxic effects of oxidized LDL or pro-apoptotic mechanisms [25,37,39,59]. Both of these responses, active tissue proliferation and progressive enlargement of the necrotic core, result in increasing lumi-

be true of the formation and growth of atheromas. If plaque size is directly related to the magnitude of the injury caused by the IA, then it follows that the size of the atheroma, also produced as a result of activity of the IA, is also related to the magnitude of the injury. The formation and growth of an atheroma may also be directly related to the toxicity, virulence, concentration, or dose of the IA causing atherosclerosis, and/or to the susceptibility of the patient to the ongoing, progressive, destructive activity of the IA.

Size of Atheroma and Luminal Stenosis Table 2 illustrates the relationship between luminal stenosis and the presence of an atheroma in 83 patients who died of ACD. Atheromas were present in 2,223 (32%) of 7,056 coronary segments taken from these 83 patients, with 88% in plaques with more than 50% luminal stenosis. There was a significant difference in frequency, p=<0.001, of atheromas in those segments with >80% stenosis compared with segments having <50% stenosis. These results show that the frequency of atheromas increases as the plaque enlarges.

nal stenosis and ultimately, if unchecked, to obstruction of coronary flow.

4-1

Atherosclerosis

Atheromas Are Caseous Abscesses

Table 2:. Comparison of luminal stenosis and size of atheroma in 83 patients who died of acute coronary disease.

Antheroma # %

Degree of Stenosis (%)

# of Sections

%

<50

3221

46

387

50–80 >80

2458 1377

35 19

1037 799

Totals

7056

2223

None

Size of Atheroma I–II # %

III–IV

#

%

#

%

12*

2834

88

383

12

42 58*

1421 578

58 42

983 711

40 52

4a 54

0.1

88a

2.0 6.0

32

4833

69

2077

29

146

2.0

* = p =<0.001; a = p =<0.001

The circumferential extent of intimal involvement with an atheroma, also determined by a review of each microscopic section, was graded in the following manner: Each artery segment was divided into four, 90° quadrants, then graded on how many of the four quadrants contained an atheroma. Grade I indicated one quadrant contained an atheroma; Grades II, III, and IV indicated involvement of two, three, or four additional quadrants contained an atheroma. Table 2 shows the relationship between luminal stenosis and the severity and extent of atheromas in each coronary segment. The relationship between luminal stenosis and the circumferential extent of the necrotic core showed Grades III and IV atheromas were significantly more common, p=<0.001, in those segments with >80% luminal stenosis than in those with <50% stenosis. Therefore, a direct relationship exists between plaque size, as reflected in the severity of luminal stenosis,

Atheromas Are Similar to Bacterial Abscesses Atheromas are abscess-like in the sense that they are associated with inflammatory infiltrates, grow in size, are composed of necrotic, amorphous material, are acellular, and contain toxic, irritating, and antigenic material [37,60,61]. Atheromatous degeneration is similar to caseation observed with tuberculosis in that both have a high lipid content and appear “cheesy” on gross appearance. Figures 6A, 6B, illustrate the gross and microscopic appearance of atheromas, showing a yellow, acellular, lipidladen, cheese-like necrotic core with a semi-solid consistency. These photos show the tendency of atheromas to grow circumferentially from a central focus toward the plaque shoulder, and are frequently associated with adventitial inflammatory infiltrates (Figures 5B-5E).

and the circumferential extent to which each coronary segment is involved with an atheroma. Pro-

The natural course of events for most bacterial

gressive and expanding injury caused by the IA

abscesses is to point and drain spontaneously,

results in a progressive increase in plaque size and

removing the offending organism and the toxic core

a progressive increase in the proportion of that

material. The spontaneous rupture of a bacterial

plaque that is atheroma.

abscess is related to increasing volume and pressure within the necrotic core, plus the action of digestive, proteolytic enzymes on the surrounding tissue, particularly on the overlying cap. Resolution and healing of such abscesses commonly follows this spontaneous drainage, provided the necrotic core is sufficiently emptied of necrotic material to 4-2

Atherosclerosis

Atheromas Are Caseous Abscesses

allow resolution to take place. The over-all inflammatory response associated with localization and encapsulation of the necrotic tissue and of the offending agent that characterizes a bacterial abscess is an important component. This is thought to be a physiologic defense against the growth and spread of a necrotizing, destructive organism. The development of a pathologic condition or disease as a result of this spontaneous drainage does not mean these physiologic defenses, per se, are at fault. For example: Rupture of a bacterial bowel abscess that results in peritonitis does not mean the defensive responses leading to spontaneous rupture and drainage of the abscess are pathologic, or that these defensive responses can or should be prevented. Proper treatment is to focus on and treat the IA, not the defensive responses. The same may be true of atheromas.

Figure 6: A, Large necrotic, yellowish, crescent-shaped, asymmetric atheroma in the proximal CIRC artery of an 81-yearold female. Virtually all fibrous tissue in the core area has been digested, leaving the lipid-laden core (asterisk). Note thin fibrous

4-3

Atherosclerosis

Atheromas Are Caseous Abscesses architecture, and it stains differently from neighbor-

cap (fat arrows) and small embolus in lumen (open arrow). Magnification x9.25. B, Microscopic section of large necrotic

ing, viable cellular tissue. If these proteolytic

atheroma in the CIRC marginal branch of a 78-year-old male.

enzymes are essential to the physiologic resolution

The intima is totally necrotic, and plaque hemorrhage has

of injury, then the action of such enzymes in the

occurred (open arrows). The media and adventitia are intact.

formation of atheromas is, in the final analysis, also

Partially digested fibrous tissue is present along the left side of the photo (long arrows). MSB stain. Magnification x19.5. C,

a physiologic response, not a complication of the

Atheroma in the mid-RCA of a 69-year-old white male. The

disease. In other words, these enzymes may pro-

fragment (asterisk) appears to be undergoing digestion (open

vide a key defense by promoting the removal of

arrows) with moth-eaten irregular border. Red blood cells (RBCs)

degenerating tissue and toxic agents from the artery

and fibrin (small arrows) suggest communication with the lumen. MSB stain. D, Abrupt termination of a strand of fibrous tissue

wall.

within an atheroma (open arrow) of a 74-year-old male. The digestive process appears to be attacking the fibrous tissue, and portions of undigested fibrous tissue have been separated from

Some of the fibrous fragments are quite large, have

the main fiber (arrows). MSB stain. E, Same patient and section

been completely separated from the surrounding

as D, but in a different area. Small fibrous fragment dangling

wall, and are free-floating within the lipid core (Fig-

from the wall of the atheroma (thin arrows). The fragment

ure 6F). Should a major PU occur, these tissue

appears to be undergoing digestion with loss of fibrous architecture and lighter staining amorphous tissue (fat arrows).

fragments would constitute a sizable embolus to the

MSB stain. F, Atheroma with a large free-floating fragment (open

distal circulation. Because of their fibrous structure,

arrow) in an UP, located in a empty necrotic core in the proximal

they may be difficult to remove by circulating

RCA of a 41-year-old white male. H & E stain.

enzymes and may cause ischemia and/or focal

Proteolytic Enzymes

infarction [64]. The digestion of these fragments by MMPs, while the atheroma is still intact and before rupture or ulceration, may reduce the pathogenetic

Atheromas contain a number of proteolytic

potential of such tissue emboli.

enzymes, particularly matrix metalloproteinases (MMPs), derived primarily from monocyte-derived macrophages present in and around the necrotic core of atheromas [62,63]. These digestive enzymes, because of their ability to digest injured or degenerated tissue, play a major role in the enlargement and growth of atheromas, but they are also necessary for the repair of injury [62]. Therefore, the digestion and destruction of dead or injured tissue by MMPs is a component of active atherosclerotic disease and constitutes one method of converting dead and damaged tissue to a semisolid form, presumably for the purpose of removing it.

“Exit” Tracts Another potential action of MMPs is the creation of communicating channels or tracts between deeplying atheromas and the artery lumen. Figures 7A7C illustrate an example of a tiny, narrow, long, serpiginous tract that connects the lumen with an underlying necrotic core. The presence of red blood cells and injection mass within these tracts proves their in-vivo existence and excludes the possibility that they are cutting or post-mortem artifacts. Furthermore, the presence of fibrin, red staining material on Martius Scarlet Blue (MSB) stain, (Figures 7A, 7B), shows pre-mortem communication with the

Figures 6B-6F illustrate digestion of fibrous tissue,

lumen, activation of the clotting system, and forma-

taking place within different atheromas in different

tion of intraintimal thrombus. These tracts pass

patients. The fibrous tissue undergoing digestion is

through acellular, degenerated fibrous tissue (Fig-

acellular with loss of normal tissue substance and

ure 7C). They are presumably formed by the action 4-4

Atherosclerosis

Atheromas Are Caseous Abscesses

of MMPs secreted by macrophages that line the tracts, and possibly with other digestive enzymes as well [63], providing a route for plaque contents to enter circulating blood. Specifically, these tracts are NOT formed by the splitting and disruption of normal, cellular, viable fibrous tissue by external forces, such as hemodynamic stresses [63], but are due to actions and metabolic activities taking place within the core [65]. Such tracts may be called “exit” tracts because they contribute to core decompression and reverse lipid transport. These communicating tracts are similar to the sinus tracts observed in chronic osteomyelitis and may open and close at recurrent intervals, depending on factors contained within the atheroma. Perhaps all plaques, at some time in their existence, will develop such tracts that serve to reduce plaque size.

4-5

Atherosclerosis

Atheromas Are Caseous Abscesses the layered appearance on gross examination to which we are referring, (Figures 7D–7F), is due to alternating layers of fibrous tissue and necrotic core tissue, caused by different light-absorbing properties of these two major tissue types. Tracing this layering through subserial sections often shows these seemingly separate foci of necrosis are actually caused by extensions of a single necrotic core, located either proximal or distal to this site. The layered appearance may be the result of a proximal/ distal expansion or burrowing of an atheroma, probably facilitated by MMPs, growing and expanding in a longitudinal direction within the plaque at different

Figure 7: A - C, Coronary section from the proximal LAD of a 74-

depths [32]. Also, adjacent atheromas may commu-

year-old white male. A & B, Low and intermediate power views of

nicate and fuse with one another to form intercon-

a plaque showing a tiny tract (arrows) traversing the fibrous cap at the shoulder, communicating with a small necrotic core, rich in

necting necrotic cores that can extend long dis-

fibrin (open arrow). The deeper, larger core in A (long arrow) also

tances throughout the intimal layer and be

stains positive for fibrin, suggesting the tract also communicates

associated with multiple cleavage planes or tracts

with this large core. C, High-power view of rectangle shown in B

within a given atheroma or between atheromas [66].

at mouth of tract, showing injection mass (solid arrows) and RBCs (open arrows) within the tract. MSB stain. Fibrous tissue

The cleavage planes are often oriented parallel to

stains blue, fibrin orange-red, and RBCs yellow with MSB stain.

the fibrous cap, with the necrotic core generally

D, Mid-LAD coronary artery of a 39-year-old male showing an

directed or oriented toward to plaque shoulder (Fig-

asymmetric plaque with two lipid cores (asterisks) separated by a

ures 7D–7F). Some of these cleavage planes may

fibrous layer (thin arrow). Note a finger-like extension (open arrows) of one lipid core in the direction of the shoulder of the plaque, in the lower part of the photo, and thinning of the fibrous

represent a previous exit tract, now healed (Figure 7E).

cap at this point (white arrows). The different tissue characteristics create a “layering” effect on visual examination. Magnification x12. E, Cleavage plane (arrows) in a fibrous plaque in the LAD coronary artery in a 33-year-old male. The cleavage plane contains no injection mass and is partially closed by fibrous

If this is the correct assessment of these cleavage planes, is the presence or formation of such tracts

tissue. MSB stain. F, Microscopic view of mid-LAD of a 53-year-

an important feature of atherosclerosis? The cleav-

old white female showing two atheromas (asterisks) separated

age planes may be important if blood enters the

by a partially calcified fibrous strand (thin arrows). Note these two

core at one site, as through an UP, and then

atheromas are oriented toward the shoulder at the upper margin of the plaque (fat arrows). H & E stain. Magnification x19.75.

extends along cleavage planes, taking the path of least resistance through the atheroma, with result-

Cleavage Planes

ing plaque swelling and an increase in luminal stenosis. The planes may also be important from the standpoint of drainage of plaque contents in that

Atherosclerotic plaques often present a layered

an exit tract at one point may serve to drain a

appearance, both on gross and histologic examina-

necrotic core some distance away.

tion (Figures 7D–F). Stary, et al, suggest this layered appearance may be due to repeated disruptions of the lesion surface associated with hematomas or thrombotic deposits [15]. However, 4-6

Atherosclerosis

Plaque Shoulder Ulcerations

Atheromas Are Caseous Abscesses Incision and drainage followed by resolution and healing are the proper treatment for all bacterial

It is well known that macrophage foam cells and the

abscesses. Incision and drainage may also be the

MMPs they produce are heavily concentrated at the

proper treatment for atheromas. PTCA is basically a

plaque shoulder [63], and that there is a propensity

form of incision and drainage because the atheroma

for the plaque to ulcerate at this site. Ridolfi, et al.,

is split, its contents drained by this procedure. Fol-

noted a fibrous cap that was eroded on the under

low-up studies after PTCA show a reduction in

surface over a necrotic core, supporting the view

luminal stenosis consistent with resolution and heal-

that the fibrous cap is eroded from within [67].

ing [68]. As long as the PTCA is accomplished without such complications as thrombosis or restenosis, this may be the preferred method of

Figures 8A–8C, illustrate ulcerations involving the

treatment for a large atheroma or a vulnerable

shoulder of the plaque in different patients. The

plaque [69]. The use of coronary stents may con-

fibrous cap is thin and attenuated at the point of

tribute to the success of the PTCA by assisting in

ulceration. The free end of the fibrous cap appears

complete drainage, preventing or reducing post-

to be valve-like in structure, (Figures 8A–8C), so it

PTCA dissection along cleavage planes, and pro-

could allow the extrusion of plaque contents in dias-

moting healing and subsequent resolution.

tole, then close in systole, preventing the ingress of blood from the lumen. If this is correct, nature may provide a mechanism for plaque contents to be extruded, at the same time preventing or reducing the inflow of large amounts of blood into the core area. The force of arterial pulsation may pump or “milk” plaque contents into the lumen in this manner. Thus, the necrotic core may be intermittently drained or partially drained in this manner.

None of these UPs are associated with significant luminal or occlusive thrombosis. Therefore, spontaneous PU and debulking, early in plaque development prior to the development of significant luminal stenosis, may be beneficial. Ulceration and drainage of the necrotic core early in plaque development are rarely associated with thrombosis and may be beneficial if resolution, reendothelialization, and fibrotic scarring stabilize the plaque and reduce luminal stenosis [57]. These observations lead us to question the prevailing opinion that PU is a pathologic event that must be prevented if we are to prevent acute coronary disease [63].

4-7

Atherosclerosis

Atheromas Are Caseous Abscesses

Figure 8: A, RCA section of a 37-year-old white male showing a large shoulder ulceration (arrows) with the tract penetrating to the necrotic core (black asterisk). Magnification x15.6. B, UP (arrows) containing injection mass, involving the proximal CIRC of a 58-year-old white male who developed cardiogenic shock following acute inferior myocardial infarction. This was not the culprit lesion. C, High-power view of the rectangle in B showing plaque fragments near the mouth of the ulceration (white arrows) within the artery lumen. D, Mid-RCA of a 62-year-old white male who died within 24 hours of coronary artery bypass surgery. The cholesterol crystals are aligned parallel (long arrows) in the exit tract. The tract also contains many foam cells (short arrows). Small amount of fibrin (open arrows) is present within the core. MSB stain. E, UP in the proximal CIRC of an 82-year-old white male who died of cardiogenic shock in the hospital. The cholesterol crystals are again aligned parallel, as if being extruded under pressure from within the core. RBCs and fibrin surround the cholesterol crystals. H&E stain. F, Proximal RCA section of an 82-year-old white male who died SCD out-ofhospital. The cholesterol crystals appear to form a volcano-like eruption. Note the disruption is through the center of the fibrous cap and the marked adventitial inflammatory cell infiltrate (fat black arrows). The eruption is associated with tiny luminal thrombosis (white arrow), hemorrhage, and fibrin within the core (thin black arrows). H & E stain. Magnification x12.6. In all photos, white asterisk = lumen.

Increased Intraplaque Pressure Evidence for increased pressure within an atheroma comes from observation of the actions of cholesterol crystals within the necrotic core. Figures 8D-F, illustrate parallel alignment of cholesterol crystals at the site of PU, suggesting that these crystals are being 4-8

Atherosclerosis

Atheromas Are Caseous Abscesses

actively and forcibly extruded under pressure from

bacterial abscess [65,71]. Although virtually all UP

within the necrotic core. This alignment of choles-

are associated with inflammation and could there-

terol crystals may be a marker of impending PU in

fore be considered “hot,” there are many plaques

the intact atheroma.

with associated severe inflammation that have not ulcerated [57]. Therefore the use of this tool to iden-

Figure 8F illustrates an explosion-like eruption of a

tify the vulnerable plaque ready to ulcerate may be

relatively small atheroma, preceded by parallel ori-

difficult. At present, the determination of thermal

entation of cholesterol crystals within an exit tract. If

heterogeneity is an invasive technique, but its

disruption of the fibrous cap were caused by exter-

potential for identifying the most active plaques

nal hemodynamic stress, we would not expect to

would be a major step forward.

see such uniform orientation, but rather a total disorganization of the cholesterol crystals. These

In Review

findings provide further support for, and are consistent with, increased intraplaque pressure and spon-

Atherosclerosis is initially an FP disease, but it

taneous rupture or ulceration of the necrotic core

evolves into a destructive process that leads to

[66,70].

luminal stenosis, caused primarily by the growth and expansion of lipid-laden, necrotic atheromas.

We propose actively growing atheroma result in

PU and subsequent drainage of plaque contents are

increased pressure within the necrotic core. This

components of and a natural consequence of the

increased pressure contributes to spontaneous

growth of an atheroma, not, per se, a pathologic

ulceration and drainage of plaque contents, similar

event. Attempts to prevent PU may not be appropri-

to that observed with bacterial abscesses.

ate. Atheromas grow primarily by the digestion of degenerated, lipid-rich, fibrous tissue, through the

The parallel orientation of the cholesterol crystals in

action of proteolytic digestive enzymes, particularly

Figures 8D and 8E, raises further questions about

MMPs. The growth and expansion of atheromas

these crystals. Are they actually sharp and needle-

result in increased pressure within the necrotic core,

like as they appear, and could they act like a batter-

leading to the formation of cleavage planes that

ing ram to pierce or damage tissue, thereby

communicate with adjacent atheromas and to the

facilitating or promoting PU? If so, then this is

formation of exit tracts and/or UP, primarily at the

another mechanism to consider in the pathogenesis

shoulder of the plaque. Decompression and/or

of PU.

debulking of atheromas through PU, early in plaque development, prior to significant luminal stenosis,

Thermal Heterogeneity Further evidence that atheromas are actively growing, inflammatory abscesses is provided by studies of thermal heterogeneity. These studies show

may be beneficial in stabilizing the plaque, reducing luminal stenosis, and halting active progression of the disease process at that site.

Unanswered Questions

active, progressive, growing, inflammatory atheromas are “hot” compared with adjacent normal

What is the natural history of an UP that is not com-

tissue. Increasing vascularity is associated with this

pletely drained of necrotic contents at the time of

inflammation and is similar to the increased temper-

plaque rupture or ulceration? Do such partially

ature associated with an inflamed and swollen

drained atheromas act like an incompletely drained 4-9

Atherosclerosis

Atheromas Are Caseous Abscesses

bacterial abscesses? Do they continue to fester as a chronic, indolent lesion that intermittently drains and releases plaque contents, as in chronic osteomyelitis? Do all plaques with a necrotic core ulcerate and drain, not just once, but repeatedly during their development? What are the implications, in terms of potential complications, of a single necrotic core extending long distances along the course of the coronary artery? Is PU beneficial if it can be accomplished without pathologic sequelae? Could the alignment of cholesterol crystals or intraintimal fibrin be detected by Magnetic Resonance Imaging (MRI) or other modalities and be used as a means of identifying the vulnerable plaque with impending rupture [72,73]? These are some of the questions that must be considered in dealing with the pathogenesis of atherosclerosis and the subsequent sequelae.

4-10

Atherosclerosis

Calcification

5. Calcification: A Physiologic Defense the pathogenesis of injury and calcification. This “Atherosclerotic calcification is an organized, regulated

chapter will show that vascular calcification is a

process similar to bone formation that occurs only when other

physiologic defense against active, progressive

aspects of atherosclerosis are present.” L Wexler, et al., American Heart Association Writing Group, [74]

atherosclerotic disease, that it is produced by physiologic mechanisms similar to those required for normal bone formation [74,77,78], and that it is potentially reversible [79,80].

Considerable insight into the nature and character of the IA causing atherosclerosis can be gained from the study of coronary calcification. Figure 9 shows postmortem X-rays illustrating different patterns of coronary calcification in intact human hearts (Figures 9A,B,D) and one case of dissected, uninjected coronary arteries (Figure 9C), in four patients of differing age and sex who died of ACD. Calcific deposits may be small, discrete, widely separate foci, measuring only a few mm, or large, confluent masses measuring several cm in length (Figures 9A-C). Calcification does not occur in a normal vessel wall [74] and, when present, provides objective evidence of arterial wall injury and the presence, location, and extent of atherosclerotic plaque formation within the coronary tree [75]. Widely separate foci of calcification, as shown in Figure 9, confirms that atherosclerosis is multi-centric in origin, with the IA being present and simultaneously active at multiple separate sites within the coronary tree.

Confluent, uninterrupted blocks of calcification (Figures 9A-C), reflect active longitudinal growth with fusion of adjacent plaques [76], as reported in Chapter 2. The most extensive calcification is often found in the proximal half of the main epicardial coronary arteries, particularly the Left Anterior Descending (LAD) artery [75], (Figures 9A,C,D), suggesting hemodynamic forces contribute to wall injury and plaque growth. Extensive calcification, however, is often present in the distal half of a coronary artery as well (Figures 9A-C), implicating additional factors besides hemodynamic stress in 5-1

Atherosclerosis

Calcification

Figure 9: Post-mortem X-rays of 3 hearts before injection of barium gelatin mass (A, B, D) and the dissected coronaries of one heart not injected (C). A, 42-year-old white male who died of cardiogenic shock shows extensive, confluent calcification of the LAD coronary artery (long arrows) and focal discrete calcium deposits of the RCA (short arrows). B, 54-year-old white male who died of SCD outside the hospital. Extensive calcification of all three coronary arteries, particularly the RCA (white arrows), with diffuse confluent calcification. Note the circumferential calcification of the CIRC (black arrow). C, Coronary arteries of a 62-year-old white male who died SCD outside the hospital. The RCA (short thin arrow)shows focal calcification extending into the posterior descending artery. Extensive, confluent calcification is seen in the LAD coronary artery (long arrow), and the intimedius branch (fat arrow). D, 93-year-old white female who died as a result of cardiogenic shock secondary to a large anterior myocardial infarction. Relatively minimal calcification is present in the RCA (short white arrow), the LAD coronary artery (long black arrows), and CIRC (short black arrow) in this elderly patient.

Aging and Vascular Calcification The presence of widely spaced calcified plaques in a young patient, shown in Figure 9A, indicates atherosclerotic injury occurred early in life and at multiple locations, leading to premature, accelerated atherosclerosis and to early death from ACD. We 5-2

Atherosclerosis

Calcification

can assume, based on the multiple foci of calcifica-

spreading, IA for a long period of time. Age, per se,

tion, that the IA was active at multiple locations

plays no direct role in the pathogenesis of coronary

within the coronary tree, spreading proximally, dis-

calcification.

tally, and circumferentially, with fusion of adjacent

Calcification and Luminal Stenosis

plaques. Extensive calcification in young persons, Figure 9A, and less extensive calcification in older persons,

Table 3 compares the relationship between coro-

Figure 9D, demonstrate calcification is not, per se,

nary calcification and luminal stenosis in the same

an aging or age-dependent process [81], even

83 patients who died of ACD, shown in Table 1,

though the extent and amount of vascular calcifica-

Chapter 3, and Table 2, Chapter 4. The amount of

tion tend to increase with age. If calcification were

calcification was determined histologically by view-

an aging process, then all older persons would

ing each microscopic slide and estimating the

show calcification, and the amount and extent of

circumferential extent of calcification. Each coronary

calcification would be directly related to age. Calcifi-

segment was divided into four 90° quadrants and

cation reflects and is related to atherosclerotic

graded according to the presence or absence of

injury, caused by an IA that can and does occur at

calcification in each quadrant. Grade I indicated one

any age. The increase in extent and severity of cal-

quadrant showed calcification whereas Grades II,

cification that occurs with increasing age is due to

III, and IV indicated the number of additional quad-

the patient’s harboring and surviving an active,

rants with calcification.

Table 3:. Comparison between the frequency and circumferential extent of calcification and luminal stenosis, determined histologically, in 83 patients who died of acute coronary disease.

Degree of Stenosis (%)

# of Sections

%

% Calcification # %

<50

3221

46

574

50–80 >80

2458 1377

35 19

Totals

7056

None

Amount of Calcification I–II # %

#

%

18*

2647

82

548

17

1196 877

49 64*

1262 500

51 36

1046 654

43 47

2647

38

4409

62

2248

32

III–IV #

%

26a 150

1

223a 399

6 16 6

* = p =<0.001; a = p =<0.001

The results: 2,647 (38%) of 7,056 coronary segments contained calcification. Of these segments, 2,073 (78%) were associated with >50% luminal stenosis. Calcification was significantly more common, P=<0.001, in segments with >80% stenosis than in those with <50% stenosis. The frequency of calcification is directly related to the severity of luminal stenosis or to plaque size. Plaque size, as discussed previously, is directly related to the mag-

calcification is similar to adventitial inflammation and to the presence of a necrotic core in that all three basically reflect the magnitude, extent, and severity of injury caused by the IA. Calcification is a component and another consequence of active atherosclerotic disease, not a specific pathologic complication of the disease process [82,83] that can or should be prevented, apart from preventing atherosclerotic disease itself.

nitude of the injury caused by the IA. Therefore, 5-3

Atherosclerosis

Calcification

Further examination of Table 3 shows extensive calcification of a given segment, Grades III/IV, is significantly more common in those segments with >80% stenosis than it is in those with <50% stenosis, p<0.001. These results confirm our previous observations, Chapters 1 and 4, that the IA tends to spread in a circumferential direction, and that calcification of the injured tissue produced by the IA follows in the wake of this expanding injury [Figure 9B].

Figure 9B, and Figures 10A and 10B, illustrate, in one patient, the direct relationship between the magnitude of atherosclerotic injury, the extent of calcification, and the severity of luminal stenosis. The entire right coronary artery, Figure 9B, is involved with what appears to be a confluent block of calcification, presumably formed by the growth, enlargement, and fusion of many adjacent plaques. The calcification is so severe that the lumen, even though filled with injection mass, cannot be detected in Figure 10A. In Figure 10B, after decalcification, it is evident the patient has severe and extensive obstructive disease throughout the course of this artery, although the channel itself is not completely obstructed. The amount and extent of calcification reflect the plaque burden as well as the extent and severity of atherosclerotic injury. Further, no acute lesions such as thrombosis and/or UPs were present in this artery, so extensive calcification is not synonymous with nor indicative of acute or unstable lesions [74].

5-4

Atherosclerosis

Calcification The primary purpose of EBCT is to identify the presence and extent of atherosclerotic involvement of the artery wall and the overall “plaque burden” [86,87]. However, calcific deposits must be >2 mm to be seen by EBCT, and the reproducibility of the scans is low [88], making it difficult to follow the progression of the disease at a specific site in the coronary tree. The identification of coronary calcification by EBCT has broad implications for increased risk (89), but EBCT cannot detect the current activity of the IA [74]. It is not possible to distinguish an actively growing, progressive plaque from an inactive, stable plaque on the basis of coronary calcification. Therefore EBCT is of limited value in determining or predicting acute events [90].

Figure 10: Radiographs of dissected coronary arteries in 2 patients, taken before (A, C) and after (B, D) decalcification. A & B, Themid-RCA of the same patient shown in Figure 9B.

Figures 10C, 10D, are examples of diffuse, obstruc-

Removing the calcium deposits in A reveals extensive

tive atherosclerotic disease of the right coronary

obstructive disease of the artery in B. C & D, The RCA from a 63-year-old white male who died in hospital of cardiogenic shock,

artery, with one tiny fleck of calcium at the point of a

secondary to a large anterior myocardial infarction. This artery

90% stenosis in a 63-year-old, insulin-dependent,

showed small flecks of calcification,one area located just

diabetic patient. Multiple, widely separated areas of

proximal to a 90% luminal stenosis (white arrow). The artery

70% stenosis were present throughout the length of

beyond this obstruction shows no significant angiographic stenosis, but histologic examination showed multiple areas with 70% stenosis.

this coronary artery without associated calcification, but no acute lesions, thrombosis, or UPs were present in the artery. These observations empha-

Luminal Stenosis without Calcification

size the difficulty of predicting the extent and severity of stenosis on the basis of coronary calcification, even in high-risk patients.

Table 3 shows that 51% of all coronary segments with 50–80% stenosis, and 36% of segments with >80% stenosis did not show calcification. Atherosclerotic plaques can become quite large, significantly occluding the lumen, and still not show calcification, confirming that not all plaques become calcified and that the absence of calcification does not exclude significant stenosis [74]. These observations limit the usefulness of electron beam computed tomography (EBCT) to predict plaque volume or the severity of stenosis at a specific site in the coronary tree based on the amount and location of

However, a previous study from this laboratory showed virtually all acute coronary lesions to be associated with calcification at the site of the acute lesion [57]. The absence of calcification, even at points of significant stenosis, is strong evidence against the presence of an acute lesion at such sites [57]. The absence of calcification at the site of significant stenosis suggests a stable, nondangerous, non-vulnerable lesion. At the same time, the consistent presence of calcification at the site of UPs, with or without thrombosis [57], means that

calcification [84,85]. 5-5

Atherosclerosis

Calcification

calcification must play some role, discussed below,

as nucleators for vascular calcification and may be

in the pathogenesis of PU and the development of

present within degenerating SMCs or be excytosed

ACD.

into the extracellular space (Figures 11A, B), the way matrix vesicles pinch off from chondrocytes in

Calcification Is Similar to Bone Formation Vascular calcification is now recognized as an active, organized, complex, highly regulated process that is similar, if not identical, to normal bone formation [60,74,77,91]. It is NOT the result of passive precipitation of calcium salts in or on degenerating fibrous tissue [74]. Vascular cells are induced to calcify by the same set of genes as those

bone formation (Figure 13D) [74,93,97]. Matrix vesicles tend to clump and fuse together causing a breakdown of the matrix vesicle membrane, exposing their contents to intra- or extracellular fluid containing bone matrix proteins (Figures 11A, B), followed by the formation of HA. Matrix vesicles are an objective sign of cellular injury, death or apoptosis [25,96,98], are an essential element required for calcification, and are commonly present in most atherosclerotic plaques.

expressed during bone formation [92]. Specifically, the calcification of atherosclerotic plaques involves virtually the same biologic reactions as normal physiologic bone formation and is not the result of a pathologic biochemical calcification process. The calcium deposits in vascular calcification exist primarily in the form of hydroxyapatite (HA) that is identical to the HA in bone [77,93]. Although the sequence of events leading to normal bone formation is well known, the sequence of events leading to vascular calcification is not completely understood. Various mechanisms may be involved [74,91,94].

Matrix Vesicles

The tiny focus of calcification shown in Figure 11B is composed of many matrix vesicles that have fused together in a rather random, disorganized fashion in an area of tissue degeneration containing many other isolated matrix vesicles. In contrast Figure 11C, the calcification process appears to be quite organized with the formation of a relatively uniform calcification front [93,94], surrounded by many matrix vesicles. The calcification front identifies the dividing line between viable and nonviable tissue. Note how the degenerating fibrous tissue in Figure 11C serves as scaffolding for calcium deposition and how the basic fibrous architecture is maintained after calcification. Why an organized calcification front develops in some plaques while in

Figures 11, 12 and 13 illustrate the similarity

others tiny foci of calcification are scattered

between vascular calcification and normal bone

throughout the plaque is not clear.

formation. Vascular calcification is believed to begin with the formation of matrix vesicles within degenerated SMCs [25,82,95], Figures 11A–11C, while in bone formation matrix vesicles are formed within degenerating chondrocytes (Figure 13D). Matrix vesicles are tiny, round structures of variable size and density, containing microcrystalline calcium derived from mitochondria, other cell organelles, or from influx of calcium from the extracellular space [25,74,95,96]. Matrix vesicles are believed to serve 5-6

Atherosclerosis

Calcification

5-7

Atherosclerosis

Calcification

Figure 11: A, This section was taken from the proximal LAD coronary artery and shows “dead” SMCs containing intracellular matrix vesicles, clumped together and forming tiny foci of calcification. The cell spaces are of different size, suggesting adjacent cells have fused together (arrows). H & E stain. B, A small focus of calcification composed of multiple matrix vesicles that appear to have fused together (open arrow) in an area of cellular and tissue degeneration. Cell loss and degeneration of fibrous structure have occurred with many matrix vesicles in the area (solid arrows). H & E stain. C, Calcified plaque lying adjacent to the lumen of a 68-year-old female. Note the relatively uniform calcification front (thin arrows) and the presence of numerous matrix vesicles (fat arrows) surrounding the front. Viable tissue is above the front and calcified tissue below. The tissue architecture is preserved and serves as a scaffolding for the advancing front. Asterisk = lumen. H & E stain. D, Low-power view of a calcified plaque (solid arrows) and an adjacent mass of amorphous globular material (open arrow), staining red with MSB stain. E, Higher-power view of the material in D, showing its “knobby” appearance and attachment to the calcification front (arrow). F, Monoclonal antibody staining for osteopontin of the osteoid-like tissue shown in D & E. A brown, positive staining reaction for osteopontin (arrows), is present throughout this tissue, suggesting active calcium deposition.

5-8

Atherosclerosis

Calcification

Figure 12: A, A calcified plaque stained for osteopontin (brown reaction product), illustrates positive staining of the calcification front (open arrows), the surrounding matrix vesicles (thin arrows), and lipid laden SMCs (fat arrows). B, A focus of calcification showing a large number of matrix vesicles (arrows) congregating and fusing along a calcification front. The matrix vesicles stain red, in the same manner as they do in degenerating cartilage in Figure 13D, with the MSB stain. A calcified plaque shows multiple calcification layers (open arrows), suggesting appositional growth. C (H&E stain) and D (MSB stain). Clumps of matrix vesicles (thin arrows) along the outer layer suggest an active, advancing calcification front. E, High-power view of a calcification front (arrows) with matrix vesicles within SMC spaces. H & E stain. F, Same area as in E with MSB stain, demonstrating positive MSB staining material within the degenerated SMCs (solid arrows), as well as fusion and enlargement of the lacunar spaces (open arrows).

5-9

Atherosclerosis

Calcification contain large amounts of osteopontin (arrows). C, Interface between cartilage and bone Note chondrocytes (open arrows) and surrounding cartilage do not stain red with the MSB stain, but the encroaching osteoid, cancellous bone, stains deeply red (solid arrows) similar to the red staining matrix vesicles in SMCs in Figure 12E & 12F. D, Degenerated chondrocytes (long arrows) containing matrix vesicles (short arrows) that stain red with the MSB stain suggest that osteoid and bone matrix proteins (open arrows) line the margins of the lacunae.

Osteoid Normal bone formation requires osteoid, the organic, unmineralized matrix of bone produced by osteoblasts. Osteoid is composed of matrix vesicles, osteopontin, collagen, and other noncollagenous bone matrix proteins. It often has a knobby appearance. The deposition of calcium salts in both normal bone formation and in vascular calcification is believed to begin and progress under the influence of osteopontin and other bone matrix proteins, which act as mediators of the calcification process [74,93,94,99]. Once osteoid is formed, calcification is believed to progress rapidly, within one or two weeks, with rapid disappearance of the osteoid [99]. Although osteoblasts involved in bone formation have not been identified as such in atherosclerotic plaques, several investigations have shown pericytelike cells, possibly originating in SMCs, within the plaque. These function much like bone osteoblasts [77,78,92,100]. It is reasonable to believe these pericytelike cells have the ability to produce osteoid.

Osteoid and Atherosclerotic Calcification Calcification begins with the exposure of HA in the Figure 13: All sections in this figure were taken from the bone of the sheep heart. A, Low-power view showing marrow elements

matrix vesicles to osteoid [99] and to bone matrix

(short arrows) and bright red staining of cancellous bone (open

proteins that are found in atherosclerotic plaques,

arrows). MSB stain. B, High-power view of the periosteal layer of

but not in normal vessel wall [93]. Osteopontin is

the bone shown in stained for osteopontin, showing osteoblasts

synthesized by vascular SMCs, macrophages, and endothelial cells [100], has calcium-binding sites, is 5-10

Atherosclerosis

Calcification

regulated by cytokines, binds readily to HA, and is

active calcification may have ceased. The MSB

found at the sites of early calcification [74,99,101]. If

stain may be a convenient method of identifying

atherosclerotic vascular calcification and bone are

osteoid in the artery wall. To further explore the

similar in formation and composition, then osteoid or

possibility that this amorphous tissue in the artery

osteoid-like tissue should be present in atheroscle-

wall may be osteoid, we performed similar stains on

rotic plaques. Demer [99] believes some of the

normal bone.

atherosclerotic matrix resembles osteoid, but osteoid, per se, has not been identified in calcified atherosclerotic plaques.

Normal Bone and Cartilage Figure 13 illustrates the staining features of normal

Figures 11D–11F, are examples of tissue in the wall

bone, cartilage, osteoid, and matrix vesicles in the

of a calcified coronary artery that has features con-

bone and cartilage found within the sheep heart

sistent with osteoid. These deposits consist of an

[102]. Cancellous bone, Figure 13A, is immature

amorphous, relatively acellular mass of tissue, stain-

bone, recently formed from osteoid, that stains

ing bright red with MSB stain, knobby in appear-

bright red with MSB like the amorphous masses in

ance, and located adjacent to a calcified plaque.

Figures 11D, 11E. Figure 13B shows osteoblasts at

Immunohistochemistry staining of this tissue with

the periosteal surface, the site of new bone forma-

osteopontin monoclonal antibodies shows intense

tion, stain strongly with osteopontin antibody, con-

staining, (Figure 11F), suggesting this material may

sistent with active bone matrix formation. Normal

be osteoid. Connected to the amorphous mass is a

cartilage, (Figure 13C), does not stain red with the

calcification front, also staining bright red with MSB

MSB stain, confirming that normal cartilage does

(Figures 11D, 11E), and also with osteopontin anti-

not contain osteoid. However, osteoid-like tissue

body, Figure 12A. The amorphous mass and the

appears to be advancing into and replacing the car-

calcification front may be related to and composed

tilage. Finally, in Figure 13D, the matrix vesicles,

of the same tissue. If this amorphous tissue is

like the matrix vesicles in Figure 11A, are contained

osteoid, then it may be a site of active calcification

within degenerating chondrocytes, and stain bright

and of an active expanding, spreading calcification

red with MSB. This is strong evidence that the

front.

amorphous mass, staining bright red with MSB stain in Figures 11D and 11E, is osteoid.

Figures 12C, 12D illustrate layering, apparently produced by an advancing calcification front. This

Evolutionary Purpose

appositional growth of the calcified plaque is similar to the appositional growth of bone. Note that the

What is the significance or evolutionary purpose of

leading edge of the calcification front and associ-

bone or bone-like deposits in the wall of an

ated matrix vesicles stain red with MSB, but those

atherosclerotic artery?

residual calcification fronts behind the leading edge, not surrounded by matrix vesicles, do not stain red

Why is it necessary that these calcific deposits be

with MSB. Therefore, a calcification front staining

bone, formed by a very complex series of biochemi-

red with MSB and surrounded with similarly staining

cal reactions, rather than, say, by the passive

matrix vesicles, (Figures 12 B-F), may indicate

precipitation of salts [99]? Is there a larger purpose

active calcification is occurring, and the calcification

here than just the calcification of injured, degenerat-

front is advancing. If these features are absent,

ing tissue? Is vascular calcification a physiologic 5-11

Atherosclerosis

Calcification

defense against the action of the IA or is it a patho-

ing for the deposition of calcium salts. A second

logic process, a complication, of atherosclerosis,

evolutionary purpose of calcification may be to

that should be prevented, as theorized by some

delay the longitudinal spread of the IA by creating

investigators [82,83]? If vascular calcification is the

barriers between adjacent plaques, preventing or

same as normal bone, formed by the same bio-

delaying the formation of cleavage planes between

chemical reactions as normal bone, it is difficult to

plaques and adjacent necrotic cores. Atheroscle-

understand how this physiologic process suddenly

rotic calcification is an abnormality, but that does

becomes a pathologic process, a complication that

not prove it is pathologic in nature. Seen from the

must be prevented.

standpoint of a threatening, advancing, destructive, IA, calcification may be an essential physiologic

Calcification, in general, is often regarded as an important component of the healing process, generated as a physiologic defense designed to contain, wall off and stabilize injured or damaged tissue and/

defense, in delaying destruction of the artery wall.

Reversibility of Coronary Calcification

or foreign agents [99]. Calcification of a ghon complex is an example. Atherosclerotic calcification, however, appears to have a different purpose because the IA is not sequestered or walled off. In fact, calcification appears to have little if any direct influence on the activity of the IA because the disease continues to progress. The preservation of the fibrous tissue architecture behind the calcification front, as illustrated in Figures 11C and 12A, indicates the injured and degenerated tissue was calcified quickly, before the onset of necrosis. If there is prompt formation of pericytes or osteoblastlike cells and these cells produce osteoid, calcification of degenerated fibrous tissue by an advancing calcification front could occur rapidly, utilizing the same biochemical mechanisms as normal bone formation that are already present. The rapid calcification of degenerating fibrous tissue may be viewed as a physiologic defense that can be mobilized relatively quickly and efficiently.

Several histologic observations suggest coronary calcification is reversible. Patients who died from malignant disease have a very low frequency of coronary calcification, indicating that calcification may be reversed in wasting conditions [75]. Clinical studies of patients receiving HMG-CoA reductase inhibitors to lower blood cholesterol show a reduction in calcium score on serial testing with EBCT [79,80]. Apparently the activity of the IA can be slowed or arrested, and calcification can be reversed. Figures 14A–14D, are typical calcified plaques from two patients of different ages. In Figure 14A and B the central portion of the calcified plaque contains many lacunae, of different size, apparently formed from dead SMC, that appear to be fusing or coalescing as a result of degeneration, destruction, or resorption of the calcified fibrous septa between them. In Figure 14C and 14D, these lacunae have fused to form a small lipid lake presumably formed by the fragmentation, erosion, and

Therefore, one evolutionary purpose of vascular

dissolution of the calcified, lipid-rich, fibrous tissue.

calcification may be to preserve the fibrous tissue

Resorption, destruction, or dissolution of the central

architecture by rapidly calcifying this tissue to main-

portion of calcified plaques may have important

tain wall integrity, delaying the onset of necrosis. It

implications for plaque progression.

is important to remember that necrotic tissue, per se, does not undergo calcification, primarily because necrotic tissue has no structure or scaffold5-12

Atherosclerosis

Calcification

What is the mechanism of this apparent resorption?

Support for this view is provided in Figure 15D,

Several possibilities can be considered. First,

another calcific deposit at the rim of a necrotic core,

Jeziorska, et al [103], found multinucleated giant

but stained with osteopontin monoclonal antibody.

cells in close apposition to carotid calcification that

Again, we have the irregular moth-eaten border

demonstrated all the normal features of bone forma-

consistent with degeneration and resorption, and

tion and resorption. Vascular calcification may

this degeneration is associated with free-floating

seemingly undergo resorption in the same manner

fragments within the necrotic core, also staining

as bone. Second, bone undergoes resorption and

positive for osteopontin. The calcific deposit

avascular necrosis when deprived of its blood sup-

appears to be breaking up rather than forming. The

ply. The same may be true of vascular calcification

rim of calcification commonly found at the edge of a

that has no nutrient blood supply [104]. Third, SMC

necrotic core may be a remnant of a previously cal-

produce and contain MMPs. These enzymes may

cified plaque and the necrotic core may have origi-

remain active after the SMC dies and be a signifi-

nated and been formed by the resorption and

cant factor in the degradation and resorption of

dissolution of the calcified plaque. If this is correct,

calcified vascular tissue [94,105], particularly when

then the chemical agents and biologic compounds

resorption originates within the central part of calci-

that are by-products of calcification resorption will

fied plaque. Vascular calcification may not be a

be added to all the other compounds present within

permanent or inert deposit, but an active, dynamic,

the necrotic core. Since HA is very irritating when

changing structure capable of being reversed,

injected into joints [96] it is possible that it may also

removed, and/or remodeled [99]. If this is correct,

be very irritating and toxic when present in the

then all calcified plaques have the potential to

necrotic core.

undergo reversal and resorption. What role, if any, does plaque calcification play in Figure 15A shows a large, circumferential calcified

the formation of the necrotic core? What is the

plaque containing a similar circumferential, struc-

sequence of events in the growth and development

ture-less, necrotic core. Figure 15B, in contrast,

of the calcified plaque? What comes first, the

shows a very small, calcified plaque with similar

necrotic core or calcification? How can necrosis and

findings, showing that such a structure-less central

the formation of an atheroma occur before calcifica-

core can develop in calcified plaques of any size.

tion, as suggested by Stary, et al. [15], when matrix

Figure 15C illustrates a large necrotic core with a

vesicles form at the time of SMC death, fuse, form

tiny rim of focal calcification that has an irregular,

calcific deposits, and calcify degenerated fibrous

moth-eaten border facing the necrotic core. This

tissue well before the onset of actual necrosis? The

calcified rim appears to be undergoing digestion or

presence of microcrystalline calcium within a

destruction, presumably by substances contained

necrotic core [15] suggests calcified tissue was

within the core. This calcific deposit does not stain

digested in the formation of the core and that calcifi-

bright red with the MSB stain, suggesting osteoid

cation preceded the development of necrosis.

formation and active calcification are not taking place. This rim of calcification may be receding, not forming.

Evidence presented shows that calcified plaques may be reversible. This reversal appears to result in the destruction and dissolution of the lipid-rich scaffolding of fibrous tissue, and leads to the formation 5-13

Atherosclerosis

Calcification

of a structureless central core that, in time, may lead to the formation of a necrotic, atheromatous core.

Figure 14: A, Low-power view of a calcified plaque (open arrows) from the CIRC coronary artery of a 51-year-old male. The calcified fibrous tissue appears to be undergoing degeneration, and the lipid-laden lacunar spaces fusing together (solid arrows). B, High-power view of the same plaque as A.

5-14

Atherosclerosis

Calcification

Lacunar spaces vary in size, presumably formed by the fusion of adjacent lacunae formed from dead SMCs. H & E stain. C, A calcified plaque showing fusion and enlargement of lipid-laden lacunae (solid arrows) forming a small lipid lake (open arrow). D, High-power view of rectangle in C showing dissolution of the calcified fibrous tissue (solid arrows) and fragmentation of tissue at the edge of the lipid lake (open arrows). H & E stain.

Figure 15: A, Section from the mid-LAD coronary artery in a 58year-old male. A large circumferential calcified plaque (arrows) surrounds a large lipid rich necrotic core (asterisks). H & E stain. Magnification x11.5. B, Small calcified plaque (arrows) in the distal RCA of a 58-year-old male, showing a small structureless central core (asterisk). H & E stain. C, Large necrotic central core with small rim of calcification (solid arrows). Note the irregular, moth-eaten border (open arrows) that faces the core. MSB stain. D, Small calcium deposit at the rim of a necrotic core (solid arrows), immunostained for osteopontin (brown reaction product), showing that these calcium deposits stain intensely for osteopontin. Note that the loose fragment (open arrow) also stains positive for osteopontin.

Sequence of Events Insight into the sequence of plaque development can be gained by reviewing Table 1 in Chapter 3, Table 2 in Chapter 4 and Table 3 in this chapter. These tables show 51% of all coronary segments have inflammatory cell infiltrates, Table 1; 38% show calcification, Table 3; and 32 % show the presence of a necrotic core, Table 2. The frequency of calcification in Table 3 is greater than the frequency of a necrotic core at all levels of stenosis, whether <50% stenosis, 50—80% stenosis, or >80% stenosis. Inflammation develops first following the injury, followed by calcification and then by 5-15

Atherosclerosis

Calcification

the formation of a necrotic core. This evidence sup-

formation of an atheroma by adding the byproducts

ports our view that many atheromas have their

of resorption to the necrotic core. The sequence of

origins within a calcified plaque. The reversal or

events in plaque development following injury is

resorption of plaque calcification may explain why

inflammation, followed by calcification of the dam-

EBCT studies are not reproducible, and why plaque

aged tissue, ending ultimately in the formation of a

volume cannot be quantitated from the amount of

necrotic core.

calcification present [88].

In Review Calcification of the coronary arteries identifies the site and extent of atherosclerotic involvement. Calcification is a component not a complication of atherosclerosis. Uninterupted blocks of calcification indicates the IA spreads in a longitudinal direction and adjacent plaques fuse together. Age plays no direct role in the pathogenesis of calcification. The amount of calcification reflects the magnitude, extent and severity of the injury caused by the IA. The amount of calcification tends to reflect plaque size, but is not sufficiently accurate to predict the severity of luminal stenosis. Calcification does not detect the current activity of the IA or the site of currently active disease, limiting its clinical usefulness. The absence of calcification is strong evidence against the presence of active disease even with significant luminal stenosis. Calcification plays some role in PU.

Vascular calcification is believed to utilize the same biochemical processes and substances as in normal bone formation, and the composition of both bone and vascular calcification is similar. Matrix vesicles derived primarily from degenerating SMCs act as nucleators for the deposition of calcium salts in atherosclerotic plaques, with calcification commonly occurring on a scaffolding of degenerated fibrous tissue, in the same manner as bone forms on the scaffolding of degenerating cartilage. Calcification serves to preserve the integrity of the artery wall, delaying and/or retarding the spread of the IA. Calcification is reversible and may contribute to the 5-16

Atherosclerosis

Adventitia – The Ultimate Defense

6. Adventitia – The Ultimate Defense plaque, but is distributed uniformly over the entire “Another feature of importance is the great tensile strength of

circumference of the plaque. This diffuse thickening

the adventitia.”

suggests that the IA causing intimal injury and

MC Winternitz, et al., [104]

atherosclerosis is also responsible, directly or indirectly, for producing adventitial thickening. The IA

Previous chapters have shown how the IA spreads

itself, or a product thereof, may diffuse or pass

in both a longitudinal and circumferential direction

through to the adventitia from the plaque by direct

within the intima. The possibility exists, judging by

perfusion, convection [106] or via the lymphatics,

intimal destruction and the formation of a necrotic

precipitating this thickening. The actual mechanism

core, that the destructive process could also spread

of growth may be the release of growth factors,

outward or laterally through the arterial wall, empty-

such as Fibroblast Growth Factor, that are released

ing plaque contents into the pericardial space.

by intimal macrophages and pass to the adventitia

However, erosion, destruction, necrosis or perfora-

[107]. Some stimulus related to plaque formation

tion of the artery wall do not occur in the course of

and growth causes the adventitia to respond in a

active atherosclerosis. The elastic lamina, the

consistent, uniform, and characteristic way to the

media, and the adventitia appear to be resistant to

injurious process occurring in the intima.

the effects of the IA, preventing lateral spread. Winternitz, et al. [104] showed the adventitial layer of the artery wall to be extremely strong with a very high tensile strength. It was virtually impossible, using a balloon, to rupture an artery with an intact adventitia. These same arteries were easily ruptured by removing the adventitia. The media and the elastic lamina do not compare to the adventitia in tensile strength. The adventitia may be the structure that contributes most to preventing outward growth and expansion of the IA, and may be extremely important in maintaining the integrity of the artery wall.

Adventitial Thickening over Atherosclerotic Plaques The adventitia often thickens over atherosclerotic plaques [60], but not over unaffected artery wall, (Figures A-D). A relationship exists between the injury occurring in the intima and the resulting adventitial FP, but the precise relationship is not clear. The adventitial thickening illustrated in Figure 16 is not localized in just one area overlying the 6-1

Atherosclerosis

Adventitia – The Ultimate Defense

Figure 16: A, Proximal CIRC coronary artery from a 43-year-old white male who died in the hospital of cardiogenic shock following an AMI. The asymmetric plaque is calcified and contains two necrotic foci (asterisks), and the adventitia overlying this plaque is thickened in a rather uniform manner. The adventitia is not thickened over normal, uninvolved wall. B, Highpower view of rectangle B in A showing heavy Tcell infiltrates in a thickened adventitia (short fat arrows) and associated with a similar infiltrate in the intima (long arrows). The intimal fibrous tissue adjacent to inflammatory infiltrates appears to be undergoing digestion. The media appear to be intact (open arrows). A large adventitial arteriole is close by (white asterisk). MSB stain. C, High-power view of rectangle C in A. The intima (white arrow) is of normal thickness and is not involved with atherosclerosis. The media (open arrow) and the adventitia appear normal without injury, thickening, inflammation, or increased vascularity. MSB stain. D, High-power view of rectangle D in A, containing a small portion of the intima (asterisk), the internal elastic lamina (solid arrows), the media (open arrows), and the adventitia (bracket). The adventitia is composed of layers of thick collagen fibers, often wavy in appearance and oriented in a circumferential direction. Relatively few cells are present and typical intimal SMCs cannot be identified in the adventitia. H & E stain.

Evolutionary Purpose of Adventitial Thickening Why should the adventitia thicken when the primary injury involves the intima? Shi, et al. [108] demonstrated adventitial thickening following balloon injury 6-2

Atherosclerosis

Adventitia – The Ultimate Defense

to the coronary arteries in pigs. They noted striking

adventitia is an extremely strong barrier that, in

similarities between ordinary wound healing and the

essence, serves to restrict the IA to the intima,

adventitial response to injury. In circumstances of

where the only outlet for the expanding necrotic

traumatic injury, clearly the adventitia plays a key

core is into the arterial lumen.

role in resolution and repair, with fibromyoblasts migrating to the intima [108]. Perhaps the adventitial thickening associated with active atherosclerosis is produced in response to intimal injury, particularly to progressive and expanding injury by the IA.

Histologic Features of Adventitial Thickening What characteristics of the adventitia combine to make it resistant to atherosclerotic injury? Typical

What role or what evolutionary purpose does adven-

features of adventitial thickening, shown in Figures

titial thickening play in atherosclerosis? Is adventitial

16A and 16D, are tight, thick layers of sparsely cel-

thickening a pathologic response that should be

lular collagen, wavy in appearance and oriented,

prevented, or is it a physiologic response to injury?

band-like fashion, circumferentially around the

The most logical explanation is that the thickening is

artery. These histologic features are markedly dif-

a physiologic, defensive response to outward

ferent from the intimal FP response shown in Fig-

spread and expansion of the IA, similar to the FP

ures 3A and 3B. There are significant structural

response observed in many chronic inflammatory

differences between the fibrous tissues of the intima

conditions [109].

and the adventitia to support the observed differences in their physiologic responses.

Adventitial Resistence to the IA Adventitial fibrous tissue has been termed “fibrillar Fibroblasts, not SMC, are the predominant cells of

collagen” [10] because of its thick fibers, and

the adventitia. They do not succumb to the IA, do

because it has been shown to be relatively stiff,

not transform into macrophages, and do not pro-

hard, and rigid compared with the fibrous tissue of

duce an abnormal form of extracellular matrix that

the intima, producing a stiff collar around the artery

reacts with and retains lipid. Lipid-laden

[10,108). Some of this stiffness may be related to

macrophages, foam cells, or excessive amounts of

differences in the amount and composition of the

extracellular lipid are not a prominent feature of the

ECM. Intimal fibrous tissue has a higher content of

adventitial thickening over plaques, and lipid-laden

PGs and is more visco-elastic and compressible

SMC are not present in these areas (Figure 16D).

than the adventitia where the PG content is quite

Adventitial cells and tissue do not contribute to the

low [43]. These different physical characteristics of

destructive process and do not undergo degenera-

tissue may impart tensile strength to the adventitia

tion or necrosis as the disease advances. There-

and may also contribute to its inherent resistance to

fore, unlike monocyte macrophages and intimal

the IA.

SMC, the adventitial fibroblast does not become a component of active atherosclerotic disease. The

Significance

failure of atheromas to erode through the adventitia and ulcerate into the pericardial space, and the fail-

The adventitial FP response and inherent resistance

ure of a necrotic core to form in the adventitia

to the IA may have teleological significance

indicate an inherent resistance of the adventi tia to

because preservation of arterial wall integrity is

the aggressive, destructive effects of the IA. The

more important to the organism than preserving the 6-3

Atherosclerosis

Adventitia – The Ultimate Defense

luminal diameter of the artery. In other words, loss of arterial wall integrity by the IA’s eroding through to the pericardial space means certain death, but loss of luminal diameter due to the activity of the IA that is confined to the intima may be tolerated and be compatible with life. Adventitial thickening, the infiltration of T lymphs into the adventitia, and the marked increase in vascularity due to the profuse development of the vasa vasorum may be physiologic responses that limit the growth and expansion of the IA.

In Review The adventitia is resistant to the IA causing atherosclerosis, and it prevents the destructive process from eroding outward through the arterial wall into the pericardial space. The adventitia thickens and acts to maintain wall integrity as well as to confine the IA to the intima, where the only outlet for the necrotic core is into the artery lumen. Adventitial fibroblasts and adventitial collagen are not affected or subverted by the IA and they do not contribute to the growth and expansion of the disease process.

6-4

Atherosclerosis

Surface Erosions

7. Surface Erosions “Erosion of proteoglycan-rich and smooth muscle cell-rich

Endothelial Vulnerability or Dysfunction

plaques lacking a superficial lipid core or plaque rupture is a frequent finding in sudden death due to coronary thrombosis.” A Farb, etal., [110]

The observation that surface erosions are limited to the endothelial surface overlying a plaque suggests the endothelium in this region has become altered

The IA causing atherosclerosis is believed to enter

in some way [18,111] and may be more vulnerable,

the artery wall from the circulating blood through a

dysfunctional, or susceptible to the IA than is the

breach in endothelial integrity, localizing in the

endothelium over normal wall. Something or some

intima and initiating the injurious, destructive pro-

agent contained or acting within an atherosclerotic

cess that leads to plaque development [18]. The IA,

plaque appears to adversely affect the overlying

however, is not necessarily confined to the depths

endothelium, particularly in its ability to withstand

of the intima, but may also localize at the endothe-

injury by the IA [111]. Endothelial dysfunction or

lial surface overlying the plaque and cause injury,

injury is apparently multifactorial in origin [18], but

erosion, and destruction of the endothelium and

this does not mean the IA is also multifactorial.

subendothelial tissue (Figure 7)[110]. The histologic

However, surface erosions are not present over

features surrounding these surface erosions are

every plaque so there may be variable responses

similar if not identical to those found in the deeper

by the endothelium overlying atherosclerotic

necrotic core, including the presence of

plaques.

macrophage foam cells, tissue debris, and evidence of tissue digestion and destruction. Both deep and superficial lesions may be caused by the same IA.

Judging by the histologic changes taking place at the endothelial surface, an IA, such as a virus, may

Surface erosions occur only over atherosclerotic plaques, not over normal wall. They therefore follow and are superimposed on an already established plaque. This means there is a time interval between initial plaque formation and the subsequent development of surface erosions, suggesting that the IA initially traversed the endothelial barrier without affecting or injuring the endothelial cell. If both deep and superficial lesions are caused by the same IA, but at different time intervals, then the IA must be

actually enter the endothelial cell and alter intracellular mechanisms and functions [112]. For example, Figure 17A shows a single layer of endothelial cells distended with lipid, suggesting they have transformed into macrophages, ingested lipid, and subsequently developed into macrophage foam cells. The plaque tissue underlying this layer of endothelial cells, although diseased and abnormal, does not contain macrophage foam cells. The IA may have targeted and entered these endothelial cells directly.

present and persist in the circulating blood for long periods of time. The subsequent development of a

The macrophage foam cell is an afflicted cell that is

surface erosion results in two active, destructive

unable to migrate within tissue, to regulate the

processes proceeding simultaneously, but at sepa-

uptake of lipid or to maintain endothelial integrity

rate locations within a plaque, with surface erosions

[30]. The protective, defensive responses that char-

arising separately, independent of the deeper injuri-

acterize a normally functioning macrophage have

ous process.

apparently been altered in some way by the IA, pre7-1

Atherosclerosis

Surface Erosions

sumably to its advantage. If the IA enters the endothelial cell and alters intracellular function to suit its own purposes, then the endothelial macrophage becomes a subverted pawn of the IA and a pathologic component, fostering the progression of the disease process.

Figure 17B and C, show a surface erosion or a superficial ulceration involving the endothelial and subendothelial tissue that is much more extensive than that in Figure 17A and is associated with many loosely attached macrophage foam cells. This erosion involves a rather sizable portion of the luminal circumference. It is also associated with a rather deep penetrating ulcer near the center of the plaque. Close examination of the tissue interface at the base of the erosion shows evidence consistent with active digestion and destruction of plaque tissue (Figure 17C). This histologic picture is of an active, expanding, spreading, destructive process involving the endothelial and subendothelial surface in an erisipelas-like fashion. In addition, since the destructive changes are most severe in the central portion of the erosion, this area may have been the initial focus of injury. Note also that the depth of the superficial erosion tends to taper in both directions from the central area to the lateral extent of the erosion in Figure 17B. The IA seems to have estab lished a focus of injury in the central part of the erosion, then spread contiguously in a circumferential direction. The central, penetrating ulceration shows the IA is also spreading downward toward the center of the plaque toward the necrotic core. This picture of a spreading destructive IA, localized to the endothelial surface, is similar if not identical to the spreading destructive process, proposed in Chapter 1 and 4, involving the center of the plaque and the formation of a necrotic core.

7-2

Atherosclerosis

Surface Erosions

Proteolytic Enzymes The spread of the IA and the destruction of endothelium and subendothelial tissue may be promoted by production of proteolytic enzymes, such as MMPs, by the macrophage foam cell, possibly through stimulation by oxidized LDL [113]. These proteolytic enzymes can digest and break down surrounding tissue, and have been shown to play a key role in PU and rupture [37,114]. Perhaps they play a similar role in facilitating the spread of the IA at the endothelial surface. Perhaps the production of these proteolytic enzymes is also the result of altered intracellular mechanisms, produced by the IA to foster its spread along the endothelial surface.

Longitudinal Spread of the IA Figures D and E, illustrate an extreme case of surFigure 17: A, Endothelial cells have transformed into foam cells (short arrows) overlying an atherosclerotic plaque in the mid-

face erosion involving the proximal 5cm of the right

RCA of a 76-year-old white female. Endothelial integrity has

coronary artery of a young male who died sudden

been breached, with injection mass visible beneath these foam

cardiac death (SCD) outside the hospital.

cells (long arrows). Lipid-laden SMCs are present in the underlying plaque tissue (open arrows). White asterisk = lumen. H & E stain. B and C, Same patient as in A, but approximately 1

Eighteen contiguous coronary segments, beginning

cm distal to A, shown in low-power (B) and high-power (C). A

at the RC ostia, were involved with this erosive pro-

surface erosion has destroyed a portion of the endothelium

cess which included virtually the entire luminal

(arrows), subendothelial tissue, and, in addition, a deep ulceration (rectangle) appears to be penetrating into the plaque

surface of each segment. It was almost as if a cor-

in the direction of the necrotic core. Many foam cells and

rosive acid had been poured down the artery. The

considerable tissue debris are present. In C, note the irregular,

endothelium was eroded and the intima was virtu-

moth-eaten border of the ulceration (arrows), consistent with

ally destroyed down to the internal elastic lamina in

digestion and destruction. D, X-ray of dissected RCA of a 38year-old white male who died SCD outside the hospital. There is

a number of places (Figure 17E), but no significant

no evidence of significant luminal obstruction, but the luminal

mural thrombi were present at any site. The post-

margins are fuzzy and indistinct in the proximal 1/3 of the artery

mortem angiogram, (Figure 17D), shows no signifi-

(arrows). Asterisk = RC ostia with cannula in place. E, A typical

cant luminal stenosis, but there are areas of

section taken from the proximal portion of the artery shown in D. Large portions of the intima have been destroyed down to the

calcification and considerable evidence of T cell

internal elastic lamina (thin arrows). A small focus of calcification

infiltration in the adventitia, consistent with active,

is present (fat arrows) plus marked adventitial inflammatory

ongoing injury (Figure 17E). This particular case

response (open arrows). Considerable tissue debris and

shows that the IA is able to spread, not only in a

microemboli fragments are present in the lumen. H & E stain. F, Superficial erosion and partial destruction of the fibrous cap

circumferential direction, but also longitudinally

(arrows) overlying a large necrotic core (asterisk) of a 86-year-

along the endothelial surface. The magnitude and

old white male who died SCD in the hospital. H & E stain.

extent of the surface erosion may reflect the inherent pathogenicity of the IA. 7-3

Atherosclerosis

Destruction of the Fibrous Cap

Surface Erosions fibrous cap. Such chronic lesions could result in elevation of acute phase reactants, like fibrinogen

Figure 17, figure F illustrates a surface erosion that

and C Reactive Protein, that persist for long periods

involves the fibrous cap overlying a large necrotic

of time [115,116].

core. The center of the fibrous cap is undergoing erosion and destruction from the luminal side, but

Frequency of Surface Erosions

this destructive process has not yet penetrated into the necrotic core - shown by the absence of injection mass in the core. Destruction of the central portion of the fibrous cap is potentially more serious than a shoulder ulceration because the center of the core will be exposed to flowing blood, increasing the potential for acute thrombosis and acute coronary events. Surface erosion, independently or in conjunction with erosion from beneath the fibrous capm [67], is another mechanism of PU that can lead to acute coronary events.

A previous study from this laboratory showed that approximately 25% of all UPs that are present WITHOUT luminal thrombosis are of the surface erosion type, often multiple in any given patient [57]. They are consistently associated with infiltration of T cells into the adventitia, and to calcification of the artery wall, all indications of active, progressive atherosclerotic disease. The vast majority of these superficial erosions are not associated with significant luminal stenosis. They may be relatively

Absence of Resolution

asymptomatic. However, in the presence of significant luminal stenosis, these surface ulcerations may form the substrate for occlusive thrombosis and

A brief review of the illustrations in Figure 17 shows

acute coronary events [110]. Surface erosions are

that none of these erosions is associated with evi-

not benign lesions, even though they may be cur-

dence of resolution or healing. That is, there is no

rently asymptomatic, not associated with luminal

evidence of mural thrombosis, even though large

stenosis.

amounts of subendothelial collagen are exposed, nor is there evidence of the hypercellular response that would be expected in the normal resolution of

Significance of These Findings

injured tissue. Certainly there is no neointimal, FP response resembling that seen following PTCA. The

Surface erosions offer further information on the

IA, directly or indirectly, apparently prevents such

nature of the IA, showing it to be capable of attack-

responses. Presumably, there are also inheremt

ing, localizing, and destroying the endothelial and

thrombolytic factors acting at the site of erosion that

subendothelial tissue overlying an atherosclerotic

prevent thrombosis and other normal healing

plaque. If these findings are correct, it should be

responses. Therefore, a surface erosion over an

possible to find the IA in the area immediately sur-

atherosclerotic plaque is not an ordinary injury in

rounding a surface erosion. This also means that

which the IA comes and leaves quickly, followed by

any therapeutic agent designed to neutralize, kill, or

resolution, but is consistent with an IA that contin-

remove the IA can readily reach the IA in this super-

ues to be present and active on a long-term basis.

ficial location, as opposed to an IA buried deep

Because the natural history of these surface ero-

within a plaque.

sions has not been determined, they may exist as chronic, festering lesions and persist for indefinite periods invading, expanding, and destroying the 7-4

Atherosclerosis

Surface Erosions

In Review Surface erosions are probably caused by the same IA responsible for initiation of the plaque and formation of the necrotic core. The IA, directly or indirectly, appears to alter and subvert intracellular functions, converting endothelial macrophages into pathologic components of the disease process. The evolutionary purpose of these alterations may be to create intra and extracellular conditions favorable to the growth and replication of the IA. Replication and growth of the IA inevitably lead to and result in the spread of the IA in all directions from a central focus. This is aided by the production of MMPs by macrophage foam cells. Surface erosion and associated destruction of the fibrous cap by the IA may contribute to PU, thrombosis and acute coronary events. Surface erosions may exist as chronic lesions for long periods of time without resolution or healing, producing chronic elevations of C Reactive Protein, Fibrinogen, and other acute phase reactants. Surface erosions are very frequent in patients with ACD and are a component of active, progressive atherosclerosis. They may serve as a substrate for acute thrombosis when underlying luminal stenosis approaches 80% of the cross-sectional area [57].

7-5

Atherosclerosis

Blind Pockets and False Channels

8. Blind Pockets and False Channels results in the emptying or “wash out” of plaque con“An unusual variation of this phenomenon was found in even

tents into the distal circulation. The end result is a

fewer cases where rupture and subsequent extrusion of plaque

“shelled out” appearance to the necrotic core, as

contents occurred, leaving only a shelled out mural plaque.” RL Ridolfi, et al., [67]

shown in Figure 18, reducing plaque bulk and, possibly luminal stenosis also [67].

We have previously emphasized (Chapter 4) that the growth of the necrotic core is associated with increased intracore pressure leading to spontaneous ulceration and drainage of the core itself [66,117,118]. Spontaneous ulceration represents the culmination of factors and forces internal or external to a necrotic core, resulting in the removal or partial removal of necrotic material from the artery wall [4]. This chapter will further explore the mechanism of PU and show that some plaques may drain by forming false channels that course through the body of the necrotic core. It will also consider the role of blind pockets, created by the discharge of plaque contents, in the pathogenesis of acute coronary occlusion.

False Channels Figure 18 illustrates replacement of necrotic core contents by injection mass in five different patients, reflecting pre-mortem ulceration and drainage of the necrotic core. None of these UPs was associated with an occlusive thrombosis, nor did any of these patients receive thrombolytic drugs. The tissue surrounding the margins of these empty atheromas have red blood cells, providing further evidence of pre-mortem formation. The fibrous cap overlying the empty core is intact, suggesting these empty plaques are, in reality, false channels. Histologic examination, both proximal and distal, showed these false channels extended anywhere from 4-to25mm in length. The formation of a false channel requires both a proximal entrance point and a distal exit, and the flow of blood through this channel 8-1

Atherosclerosis

Blind Pockets and False Channels the necrotic core at a proximal site of ulceration may then exit through a distal ulcerated site without disrupting the overlying fibrous cap, creating a false channel [119].

Figure 19 is an example of a false channel involving a long segment of the mid-LAD coronary artery in a 58-year-old man who died of inferior wall cardiac rupture several hours after receiving streptokinase for an acute right coronary thrombotic occlusion. The fibrous cap is beginning to disintegrate in the mid-portion of the plaque, Figures 19C and 19D, but the cap is still intact proximally, Figure 19B, and distally, Figure 19E. The core contents are completely gone from this plaque except for some residual remnants at the distal end of the false channel, Figure 19E. Significant luminal stenosis is absent in the area of ulceration, and there is no evidence of thrombus formation.

Figure 18: A - E. These figures show empty, “shelled-out,” atheromas in 5 different patients. The fibrous cap (thin white arrows) is intact in all figures, but plaque contents are replaced

We hypothesize that before ulceration and drainage, this was a relatively large plaque with a

by colored injection mass. White asterisk = false lumen.

long necrotic core that subsequently drained and

Significant stenosis of the true lumen is present in C and E.

debulked without human intervention. Presumably,

Thrombus is absent in both true and false lumen in all figures. An

with core contents removed, the area will resolve,

embolic fragment of plaque tissue (fat white arrow) is present in the lumen in E. Adventitial inflammation (black arrows) is present in C.

stabilize, and reendothelialize, resulting in an overall reduction in luminal stenosis [68,120]. The absence of thrombus suggests that resolution and reendothe-

Formation of False Channels

lialization take place by in-growth of surrounding endothelium, probably over a bed of platelets. The process of resolution of an UP is probably aided by

Chapters 1 and 5 pointed out that plaques grow

complete or relatively complete drainage of plaque

both circumferentially and longitudinally, and that

contents from the area, similar to a bacterial

adjacent, but separate, plaques often extend and

abscess.

fuse together to form larger and longer plaques. A long plaque with a long necrotic core will have an extensive shoulder area, extending down both sides of the plaque. Since PU commonly occurs at the shoulder of the plaque, the possibility exists that a long necrotic core may develop one or multiple shoulder ulcerations along its length. Blood entering 8-2

Atherosclerosis

Blind Pockets and False Channels

Figure 19: A, Post-mortem angiogram showing extensive ulceration (white arrows) without luminal stenosis of the proximal LAD coronary artery of a 58-year-old white male. A blind pocket is present at the distal end of the UP (fat white arrow). B-E, Contiguous coronary sections taken proximal (B) to distal (E) from the area of ulceration. The ulceration is approximately 25 to 30mm in length. These sections show the presence of a large false channel (white asterisk) in which the fibrous cap (white arrows) overlying the false channel has become thin in B, begins to disintegrate in C and D, and then is again intact in E. Note the absence of thrombus in all figures and presence of residual plaque contents (black arrows) in E. Reproduced with permission [see Ref 57].

8-3

Atherosclerosis

Frequency of False Channels

Blind Pockets and False Channels or dividing the necrotic core, a very thick and/or strong fibrous cap covering the distal extent of the

How often do false channels develop in the course

core, calcification of intimal tissue distal to the core,

of active progressive atherosclerotic disease? Is the

and the presence of an arterial bifurcation. These

development of false channels a rare phenomenon,

anatomic features will be different in every patient

or a common occurrence with ulcerating athero-

and every plaque and may help to explain the great

mas? Are spontaneous coronary dissections or

variation among UPs and their associated complica-

coronary dissections following PTCA in reality false

tions.

channels coursing through a necrotic core along cleavage planes, as illustrated and discussed in Chapter 4? A previous report from this laboratory

Blind Pockets and Acute Occlusions

showed the longitudinal extent of 109 UPs without associated luminal thrombosis [57]. Fifty-five per-

Figure 20 illustrates examples of blind pockets in

cent (55%) of these UPs involved two or more

two patients who died of ACD. In Figures 20A and

contiguous segments and 32% involved three or

20B an UP is associated with both a proximal and

more contiguous segments. Since the coronary

distal blind pocket. A mural thrombus is located at

arteries were cut at 2-3mm intervals, this means

the site of the UP and a remnant of fibrous cap is

that 1/3 of these UPs were, at a minimum, 6-to-9

mixed with the thrombus. The patient received

mm in length. It is relatively common for UPs to

streptokinase to treat an acute anterior myocardial

extend long distances within the artery wall, with the

infarction, thought to be caused by this lesion, but

potential to develop a false channel. We hypothe-

died shortly thereafter of anterior wall rupture. This

size that all UPs have the potential to extend longi-

patient illustrates the association between a blind

tudinally and to develop false channels if the

pocket, dislodgement of the fibrous cap, and forma-

underlying pathologic substrate is suitable or favor-

tion of an occlusive thrombus. Dislodging the moor-

able for their formation.

ings of the fibrous cap, especially that portion of the cap covering the distal end of an UP, may lead to

Blind Pockets

acute occlusion of the lumen, caused by a flap of fibrous cap. The thrombus in this patient was par-

The majority of plaques that ulcerate do not go on to develop false channels. It is our theory that this is due to anatomic and structural features or to “geo-

tially lysed by the streptokinase, but the pathologic substrate, consisting of a blind pocket and a flap of fibrous cap, were not changed by this clot lysis.

metric” changes within and adjacent to the ulcerated plaque [121]. The formation of a false channel

Figures 20C-F, shows severe luminal stenosis pro-

depends upon the development of a distal exit point,

duced by swelling and expansion of a blind pocket

and failure to develop a false channel creates what

at the distal end of the false channel. This example

is essentially a “blind pocket” or cavity in the wall of

illustrates how sudden swelling of a blind pocket

the artery. Blind pockets form in association with

can lead to rapid and severe luminal stenosis, with-

UPs when there are obstructions within the necrotic

out the formation of an acute thrombus. It is easy to

core, particularly at the distal end, and/or the sur-

visualize how the pulsating head of pressure that

rounding arterial wall, that prevent the formation of a

drives coronary blood flow could act much like a

distal exit point (Figure 19E). These obstructions

battering ram, not only in distending the core area,

include the presence of fibrous septa criss crossing

but also in dislodging the fibrous cap. Acutely 8-4

Atherosclerosis

Blind Pockets and False Channels

developing luminal stenosis, caused by either sudden swelling of the plaque or by a flap of fibrous cap, could precipitate acute coronary events in the absence of occlusive thrombosis.

8-5

Atherosclerosis

Blind Pockets and False Channels However, penetration of the obstruction, presumably a fibrous cap flap, with a guide wire allows the thrombolytic drug to perfuse down the artery and to lyse the thrombus [123]. Similarly, acute occlusion caused by distention and swelling of the plaque, as in Figures 20C-F, will not respond to a thrombolytic drug because the occlusion is not caused by a thrombus. This may explain why PTCA is more successful than thrombolytic drugs to treat acute coronary occlusion [124]. For example, balloon dilatation of an obstruction caused by a flap of fibrous cap or a distended blind pocket, followed by stent placement, will serve, first, to open the channel by breaking up any fibrous obstruction, and, second will

Figure 20: Post-mortem angiograms of dissected LAD coronary

produce closure of blind pockets, tacking up loose

arteries of a 71-year-old male (A) and a 62-year-old male (C). B

ends of fibrous cap [125]. This approach tends to

& D are schematic diagrams of the X-rays in A & C. UP = ulcerated plaque. Tc = thrombus. In A, the ulceration involves the

correct the pathologic substrate of a blind pocket

central part of a long necrotic core, with injection mass filling the

and a flap of fibrous cap, and to close any potential

proximal portion of the atheroma and also a distal blind pocket. A

dissection planes that were responsible for the

mural thrombus partially occludes the lumen at the site of the

occlusion in the first place [126]. Stent place ment

ulceration and includes a remnant of the fibrous cap (not shown). E & F show both a true lumen (white asterisk) and a much larger

may also be beneficial in producing more complete

false lumen (black asterisk), with the false lumen ending in a

drainage of the necrotic core through squeezing or

blind pocket in the patient shown in C. A small mural thrombus is

compressing the plaque against the artery wall.

present (black arrows) in E.The true lumen is reduced to a tiny

Complete drainage of the necrotic core through the

slit, apparently the result of distention and swelling of the necrotic core. The true lumen and the false lumen both contain thrombus distal to E (not shown).

use of a stent tends to stabilize the plaque, facilitate resolution and healing, and, in the end, to reduce luminal stenosis. We hypothesize that the patho-

Clinical Implications

genesis of many occlusive thrombi associated with an UP develop when blood cannot exit the necrotic core, or, in essence, are due to the presence of a

Some acute coronary occlusions respond to throm-

blind pocket and the failure to develop a false chan-

bolytic drugs, but others do not [122]. If all acute

nel.

coronary occlusions were caused by pure thrombus, then we would expect 100% of thrombi to be lysed by a thrombolytic drug. But there are different types and kinds of acute coronary occlusions, and one treatment is not suitable for all. A thrombus formed proximal to an occluding fibrous cap may not lyse, with blood flow restored, because the fibrous cap does not respond to a thrombolytic drug.

In Review Spontaneous ulceration and discharge of an atheroma may result in a diverticulum-like structure in the artery wall. It contains a mixture of plaque contents and blood in what is essentially a blind pocket. Over-distention of this blind pocket by inflowing blood can lead to increased luminal stenosis and obstruction to coronary flow, before throm8-6

Atherosclerosis

Blind Pockets and False Channels

bus formation. Pulsatile blood flow, acting like a battering ram within the blind pocket, may promote the formation of a distal point of reentry, creating a false channel within the artery wall. Formation of a false channel with relatively complete drainage of core contents results in a reduction in luminal stenosis without human intervention. The formation of a false channel is another mechanism by which toxic plaque contents are removed from the wall, and it may occur without producing occlusive thrombosis or acute coronary events. All UPs have the potential to develop a false channel. The presence of a blind pocket, associated with the failure to develop a false channel, constitutes a common pathologic substrate that underlies the development of acute coronary events. Obstruction of coronary flow by a fibrous cap flap is one explanation for the failure of thrombolytic drugs in patients with acute S-T segment elevation myocardial infarction.

8-7

Atherosclerosis

Thrombosis and the Injurious Agent

9. Thrombosis and the Injurious Agent becomes pathologic only under certain circum“Unless proven otherwise, arterial thrombosis as now

stances [127,128]. Marcus [129] believes thrombo-

comprehended is primarily a misdirected or amplified form of

sis is a misdirected or amplified form of primary

primary hemostasis and is modulated by blood platelets.” AJ Marcus, [129]

hemostasis and is modulated by blood platelets. If this is correct, the ongoing and continuing hemostatic responses surrounding platelet activation,

Platelets, Injury Repair and Thrombosis

over and above what is required for injury repair, are pathologic and may lead to obstructing thombosis [129]. Specifically, thrombus formation reflects

The adhesion, activation and aggregation of platelets that lead to the formation of intravascular thrombosis are generally considered a pathologic process, rather than a physiologic response to injury. The same hemostatic response, however, that occurs with injury to tissues outside the vascular system, with the platelet response and with the same structure as intravascular thrombus, is considered a normal, necessary physiologic response to injury. Although intravascular thrombosis may

loss, failure, or inability of the thromboregulatory system to halt or control basic hemostatic responses, and can be interpreted as a pathologic breakdown of normal defense systems [129]. What causes this loss of thromboregulatory control, and what is the role of the IA in the pathogenesis of thrombosis?

Pathologic Hemostasis and Thrombosis

progress to produce pathologic disease states, this does not mean that the hemostatic responses taking place inside the vascular tree are pathologic. The basic, initial hemostatic responses of platelet activation to intravascular injury cannot, per se, be considered pathologic in nature (127). Fibrin and mural thrombus formation are beneficial and essential to the resolution of injury. They provide the scaffolding for in-growing fibroblasts and other elements essential to injury repair. For example, the basic hemostatic responses following PTCA injury are essential elements in the healing, repair, and stabilization of the PTCA site [68]. It is important to distinguish pathologic thrombosis from physiologic hemostasis within the vascular tree.

Figures 21A-F, is an example of apparently uncontrolled hemostatic responses that resulted in the formation of a large occlusive coronary thrombus. The thrombus is composed of large amounts of mature fibrin, with a superimposed platelet thrombus on the luminal surface (Figure 21D). The thrombus does not contain plaque contents, so it may have formed on a large endothelial surface erosion rather than on an ulcerated or ruptured plaque. Remnants of the fibrous cap are encased in the thrombus (Figure 21F). This thrombus could also have formed on the exposed surface of what was once a large and extensive false channel. It has continued to grow, presumably because of continued platelet activation, aggregation, and the

Platelet activation and aggregation are, broadly

formation of fibrin. Such a thrombotic response is

speaking, a physiologic defense whose primary

far in excess of that needed to repair this injured

purpose is to stop blood loss and to initiate the

artery.

repair and resolution of injury. This response 9-1

Atherosclerosis

Thrombosis and the Injurious Agent

Figure 21: A–F are sections taken from three contiguous segments of the proximal LAD coronary artery of a 34-year-old male who died SCD outside the hospital. A & B are from the proximal segment, C & D from the middle segment, and E & F from the distal segment. Gross (A) and microscopic (B) are views of a large, circumferential, but incompletely organized mural thrombus (arrows) producing severe luminal stenosis. Magnification x15. Low- (C) and high-power (D) views of the mural thrombus showing the surface is covered with a platelet thrombus, indicating the thrombus (arrows) is actively growing. Low- (E) and high-power (F) views of the distal end of the thrombus showing many platelet fibrin microemboli (arrows) and a great reduction in the size of the thrombus. In F, a remnant of the fibrous cap has been incorporated into the thrombus (solid arrows). A marked infiltration of T cells (open arrows) into the deep area of the intima is also present. B - F, All MSB stain.

Thromboregulatory Control The failure to modulate and control platelet activation and pathologic thrombus formation may be due in part to an imbalance in the concentration of thrombolytic and thrombogenic factors in circulating blood. The ratio of active Tissue Plasminogen Activator (TPA) to active Plasminogen Activator Inhibitor (PAI-1) is 1:8 in healthy male subjects, but 9-2

Atherosclerosis

Thrombosis and the Injurious Agent

in men with atherothrombotic disease, the ratio is

ters. The creation or development of a prothrom-

1:50 [130], indicating that these patients suffer from

botic state in patients with active disease appears to

a systemic abnormality of the thromboregulatory

be another example of subversion of normal regula-

system, resulting in a prothrombotic state. The

tory and defensive responses by the IA. In other

cause of this imbalance is not clear. It appears to be

words, the IA, directly or indirectly, is the cause of

due, at least in part, to increased production of PAI-

and actively promotes thrombus formation as a

1 by one or more of the following mechanisms:

component of active, progressive atherosclerotic disease (136). What other mechanisms related to

First, as recent studies show, increased production

the activity of the IA alter or affect the thromboregu-

of PAI-1 by endothelial cells and vascular SMCs

latory system and promote thrombus formation?

that have been stimulated by various growth factors, including fibroblast growth factor and platelet derived growth factor, are produced by

Luminal Stenosis and Thrombosis

macrophages or lipid-laden foam cells [131–133]. Chapters 3, 4, and 5 showed the direct relationship Second, studies show a marked increase in

between active atherosclerotic disease, caused by

Angiotensin Converting Enzyme (ACE) activity in

the IA, and the development of luminal stenosis.

the endothelium and other intimal vascular cells,

Luminal stenosis is an essential element required

especially lipid-laden macrophages within

for thrombus formation, primarily by causing shear

atherosclerotic plaques [134]. This increased ACE

forces that activate platelets and thus promote

activity results in increased conversion of

thrombosis [121,137,138]. However, luminal steno-

Angiotensin I into Angiotensin II. Angiotensin II has

sis and associated platelet activation alone do not

been shown to be a strong stimulus of the increased

necessarily lead to pathologic thrombosis, and

production of PAI-1 [134,135]. The increase in ACE

many severely stenotic lesions remain relatively

activity within the plaque , producing an increased

unchanged and stable for many years [139]. Lumi-

amount of PAI-I, may disturb the thromboregulatory

nal stenosis is not synonymous with thrombosis, but

balance within the plaque and attenuate inherent

the activation of platelets may have implications for

thrombolytic mechanisms [134].

other acute lesions, such as UPs [57], within the coronary tree. Although luminal stenosis is essen-

Third, Angiogtensin II is also involved in platelet

tial, other factors must be present for thrombi to

activation and aggregation and could contribute to

form.

the continued and excessive platelet responses involved in thrombus formation, increasing the

Wall Injury and Thrombosis

thrombogenic potential [134,135]. Breach of endothelial integrity will activate platelets The increase in PAI-1 and the failure of the

and provide the substrate for thrombus formation.

thrombo-regulatory system can be traced to the

The IA, by producing active, expanding, atheroscle-

lipid-laden macrophages and foam cells within the

rotic disease that affects the overlying endothelium,

atherosclerotic plaque. These are the same SMC

is responsible, directly or indirectly, for producing

and lipid-laden macrophages formed in response to

the pathologic substrate necessary for thrombus

the IA that are responsible for many of the

formation. Data show that virtually all surface ero-

atherosclerotic lesions described in previous chap-

sions or UPs are associated with adventitial inflam9-3

Atherosclerosis

Thrombosis and the Injurious Agent

matory cell infiltrates, an objective sign of active,

thrombus development beyond that required for

injurious atherosclerotic disease [57]. We believe

tissue repair. Patient management should aim to

that excessive, pathologic thrombosis is caused by

reestablish thromboregulatory balance, prevent fur-

and promoted by the IA by altering hemostatic fac-

ther injury or breach of the endothelium, and pre-

tors, producing luminal stenosis and injuring or

vent the development of luminal stenosis [129].

eroding the endothelium, Virchow’s Triad [130].

Evolutionary Purpose

However, if thrombosis is a complication of active atherosclerosis, then it is a pathologic event that must be prevented. As noted above, we do not wish

If thrombosis is a component, not necessarily a

to prevent thrombosis, per se, we want to prevent

complication, of active atherosclerotic disease, what

excessive throm bosis that leads to obstruction of

is the evolutionary purpose of thrombus formation?

coronary flow. Treating thrombosis as a pathologic

Why should the IA subvert or alter the thromboregu-

condition or eliminating it may also prevent or elimi-

latory system to promote thrombosis? Is thrombosis

nate desirable hemostatic responses necessary to

just one of many features of active atherosclerosis,

repair the injured artery. Eliminating all hemostatic

or does thrombus formation benefit the IA in some

responses may lead to further pathologic conditions

way? Does thrombus supply lipid or some other

and problems, such as cerebrovascular accidents

energy source for the replication, growth, and

and acute coronary events [141]. If thrombosis is

expansion of the IA, as we postulated in Chapter 4?

approached as a component of active disease that

Apolipoprotein (a) (Lp(a)), closely resembles plas-

will eventually happen, then management will be

minogen. It is found in thrombus and may have

aimed at preventing excess growth of thrombus

evolved to play a role in wound healing by delivering

rather than preventing all thrombus formation.

cholesterol and other lipids to sites of fibrin deposition where membrane synthesis is required. Lipoprotein (a),(Lp(a)), due to its similarity to plasminogen, may also interfere with plasmin generation and inhibit thrombolysis [140]. We speculate that thrombus provides a more readily available supply of lipids or other elements necessary for the IA to survive than does plaque tissue.

Clinical Implications

The essence here is to identify the patient with active disease. Identifying patients with active disease will aid greatly in risk stratification and will focus on measures to avoid or prevent excessive thrombus formation. The use of C Reactive Protein and Fibrinogen, acute phase reactants, may be useful in identifying the patient with active atherosclerotic disease, but these tests do not identify the location of the active disease whether intra- or extra-cardiac [142]. Not only do we need to know

If thrombosis is a component, not a complication, of

the presence or absence of active disease, we also

active atherosclerotic disease, what difference does

need to develop some way to measure the degree

this make in management of the patient with active

of activity, and to locate the most active sites. Spe-

coronary disease, and what is gained by making

cific identification of the most active sites would

this distinction? If thrombosis is a component, then

allow specific treatments or interventions to be

it is an expected development or event, and all

planned under controlled conditions designed to

patients with active atherosclerotic disease should

prevent excessive thrombus formation. For exam-

receive antithrombotic, antiplatelet therapy, ACE

ple, if it were possible to identify and quantify the

inhibitors, or other drugs to reduce or prevent

activity of “vulnerable” plaques [143] it might be 9-4

Atherosclerosis

Thrombosis and the Injurious Agent

possible to focus treatments, medical or interventional, designed to stabilize or remove such plaques, particularly if they were causing significant, >80%, luminal stenosis. Recent studies of thermal heterogeneity, using infra red technology have been able to identify “hot” plaques -those with active inflammatory disease. These studies may be very useful in the future [65,71].

In Review Pathologic thrombosis is due to uncontrolled hemostatic responses, modulated by platelet activation over and above what is required for injury repair. An imbalance in TPA/PAI-1 ratio develops as a result of breakdown of normal defense systems, leading to a hypercoagulable state in patients with coronary atherosclerosis. Thrombosis is actively promoted by the activity of the IA. It is a component, not a complication, of active, inflammatory, atherosclerotic disease. The recognition that thrombosis is a component of active atherosclerotic disease alters our view of the pathogenesis of thrombosis, and of our approach to the treatment and prevention of ACD.

9-5

Atherosclerosis

Chronic Ulcerated Plaques

10. Chronic Ulcerated Plaques [57,148]. If UPs are of a different age, can their age “It seems certain that many patients do survive plaque

be determined? If so, what is their natural history?

fissuring without developing any symptoms…”

Do they reseal and progress, forming a larger

MJ Davies, et al., [120]

plaque with a larger necrotic core, as Davies suggests [120,149,150], or do they persist as chronic

Chronicity of UPs

ulcerations (57,148) without resealing and without

Nagatomo [144] showed UPs persisting as chronic

that PUs associated with an occlusive thrombus

ulcerations for weeks, months, or years before pro-

were still present one month after thrombolysis, but

gressing to luminal stenosis, thrombosis, and acute

the angiographic degree of occlusion had

events. Other investigators [145–147] showed com-

decreased. Plaques do ulcerate, empty, and

plex lesions, present in patients with unstable

decompress with regression of stenosis, but they

angina, persisting as chronic lesions, and progress-

may not quickly resolve and reendothelialize. The

ing to stenosis and acute coronary events. Our

UPs shown in Figure 22, Figures 22A and 22B,

results, Table 4, show UPs were ubiquitous, multi-

show no evidence of resolution, no platelet aggrega-

ple, occurring in the same artery, but at a different

tion, mural thrombosis, recanalized thrombus or

location, or in a different artery from the one contain-

other evidence of repair. They do not present a his-

ing the culprit thrombus. These ulcerations did not

tologic picture of actively resolving injury.

growth of the necrotic core? Nakagawa [151] noted

develop simultaneously, and are of different ages Table 4:. Comparison of the incidence of coronary thrombosis and of ulcerated plaques without thrombosis with the degree of luminal stenosis in four different acute coronary syndromes. Degree of Luminal Stenosis (%) 50–59 60–69 70–79 80–89 90–99

<50 Cardiogenic Shock No. 28

SCD w/ AMI No. 18

SCD w/o AMI No. 26

Cardiac Rupture No. 11

Totals No. 83

UP Tc Total UP Tc Total

5

11

5 3

19 24* 43

44 56

11

3 8 11

4

6

4 2 6

1 2 3

1 8 9

12

7 4 11

3 4 7

6 6

9 9

36 23* 59

61 39

7

1 3 4

6 6

16 14* 30

53 47

5 25 30

48 48

109 102* 211

52 48

UP Tc Total

15

3

8

UP Tc Total

4

3

4

27

48 52

2 6 8

6

27

38 41* 79

10 4 14

4

UP Tc Total

(%)

7 2 9

3

15

Totals 100

18 18

8 3 3 6

7

1 2 3

24 5 29

28 10 38

7 14 21

21 21

12

10-1

Atherosclerosis

Chronic Ulcerated Plaques

UP = Ulcerated plaque without thrombosis; Tc = Occlusive coronary thrombus; SCD = Sudden cardiac death; AMI = Acute myocardial infarction; * = p =<0.001 (Reprinted from reference 57 with permission)

We hypothesize that UPs, once the fibrous cap has been breached, will not permanently reseal, and the atheroma will not completely resolve until the necrotic core is completely empty. Without complete drainage of the necrotic core the factors that caused the plaque to ulcerate in the first place are still present including the active, destructive IA, the toxic core contents, and the various proteases. We further hypothesize that many UPs without thrombosis are, in reality, indolent, festering, inflammatory, chronically draining lesions that persist indefinitely until either fibrotic stenosis without associated atheroma or occlusive thrombosis develops.

Figure 22: A–C are examples of UPs without luminal thrombosis and without significant luminal stenosis in 3 different patients. The fibrous cap has been breached (fat white arrows) in all patients, portions of the necrotic core have been extruded and replaced by injection mass. Inflammatory cells are scattered in the adventitia (not shown).These UPs were not considered to be the culprit lesions. A, Mid-LAD artery of a 53-year-old white female who died SCD outside the hospital, 6 weeks following an acute subendocardial infarction. B, Main left coronary artery of a 69-year-old white female who died in hospital of cardiogenic shock associated with an AMI. C, Proximal CIRC artery of a 67year- old white male who died of cardiogenic shock following an AMI. All stains H & E. D, Section taken from the mid-LAD artery

10-2

Atherosclerosis

Chronic Ulcerated Plaques inner surface of the intima, and was confined to a

just proximal to an occluding thrombus of a 43-year-old white

single coronary segment. A Grade II ulceration was

male who died SCD out of hospital. A portion of the fibrous cap (short black arrows) is involved with thrombus (long white

also confined to a single segment, but penetrated to

arrows), primarily on the luminal surface. Most of the core

the depths of the intima. A Grade III ulceration

contents have been extruded. E, High-power view of rectangle in

involved two adjacent segments, and a Grade IV

D, showing no thrombi have formed on the exposed surface. D & E = PTAH stain.

involved three or more contiguous segments. Deep and extensive UPs were as frequent in plaques with <50% stenosis as they are in plaques with 70 to

Depth and Extent of UPs

79% stenosis. The frequency, depth, and extent of the UPs was not related to the severity of luminal

The relationship between luminal stenosis and the

stenosis. Severe luminal stenosis is not required for

depth and extent of the 109 UPs without thrombo-

PU, and severe luminal stenosis does not necessar-

sis, shown in Table 4, is presented in Table 5. The

ily develop before PU. These findings support other

UPs were graded in the following manner to deter-

investigations that show PU is not related to plaque

mine the extent, depth, and severity of the ulcera-

size or the degree of luminal stenosis [152,153].

tion [57]: A Grade I ulceration involved only the

Table 5:. Comparison of the degree of luminal stenosis with the severity of the plaque ulceration in 109 ulcerated plaques without thrombosis. Severity of UP (Grade)

<50

50–59

I II III IV Total

7 9 5 6 27

6 5 2 5 18

Degree of Luminal Stenosis (%) 60–69 70–79 80–89 4 4 6 10 24

6 7 6 9 28

Totals 90–99

1 1 1 4 7

1 3

25 29 20 35 109

1 5

UP = Ulcerated plaque without thrombosis. (Reprinted from reference 57 with permission)

tion, calcification, and a necrotic core, and all are

Table 6 compares the frequency of atheromas,

objective signs of active atherosclerotic disease

adventitial inflammation, and calcification at the site

(See Chapters 3,4,5) [57]. We conclude inflamma-

of all UPs associated with or without luminal throm-

tion and calcification play some role in ulceration of

bosis in the 83 patients presented in Tables 4 and

the necrotic core (53,63).

5. Virtually all UPs are associated with inflamma-

Table 6:. Comparison of the incidence of inflammatory cell infiltrates, calcification, and necrotic plaques at the site of ulcerated plaques without thrombosis and with occlusive thrombotic lesions. The incidence of these three lesions in the entire group and in the control patients is also provided for comparison. CATEGORY

# of lesions

IC

%

CA

%

NP

%

Ulcerated Plaques < 50% Stenosis > 50% Stenosis Total UP

27 82 109

26 78 104

96 95 95

26 70 96

96 85 88

25 82* 107

93 100 98

Thrombotic Lesions

102

93

91

95

93

94

92

10-3

Atherosclerosis

Chronic Ulcerated Plaques

CATEGORY

# of lesions

IC

%

CA

%

NP

%

Entire Group 83 patients 22 controls

# of Sections 7036 1789

IC 3597 295

% 51 16

CA 2654 221

% 38 12

NP 2195 101

% 31 6

IC = Inflammatory cell infiltrates; CA = Calcification; NP = Necrotic plaque; UP = Ulcerated plaque without thrombosis; * = (p = <0.02) (Reprinted from reference 57 with permission)

UPs and Fibrotic Luminal Stenosis

To continue this hypothesis, the 28 UPs without thrombosis in the 70–79% stenosis range (Table 5), have been present longest and have resulted in the

If UPs without thrombosis persist indefinitely as

most severe stenosis. The 27 UPs associated with

chronic lesions, what is the relationship between

<50% stenosis are the most recently formed, with

chronic UPs and the development of fibrotic, non-

the least fibrotic stenosis. The age of the 18 UPs

atheromatous, luminal stenosis? Chronic inflamma-

associated with 50–59% stenosis and the 24 asso-

tory diseases, in general, are characterized by

ciated with the 60–69% stenosis would be interme-

inflammation, tissue destruction, and a FP response

diate in age. These UPs without thrombosis may

at the site of the inflammation [47]. We postulate

then persist as chronic UPs until the underlying

that the bodily response to chronic inflammation in

stenosis approaches 80% of the cross sectional

the coronary artery will be the same or very similar

area, at which point the conditions and substrate

to the bodily response to chronic inflammation

favoring thrombosis are present.

elsewhere in the body, i.e., fibrous tissue will form around the lesion. We would expect an FP

Sudden development of acute coronary syndromes

response at the site of a chronic, persistent UP to

as a result of thrombotic occlusion naturally leads to

be manifested by the development of progressive,

the assumption that the UP beneath the acute

fibrotic, luminal stenosis. Chronic UPs and the

thrombus is also an acute event. This assumption is

associated release of growth factors could account

unproved. We know of no way in which the age of

for rapid progression of insignificant lesions [154].

an UP can be accurately determined histologically.

Luminal stenosis, in this circumstance, would be

The chronic UP and the chronic FP response may

directly related to the inflammatory activity surround-

have been actively brewing for months or years

ing the chronic UP. It would be the result of and

without overt symptoms, before the acute event.

would develop after, not before, the plaque ulcerates. The luminal stenosis would then increase, not

Component versus Complication

because the plaque reseals but because the UP stimulates a FP response similar to healing by sec-

The well known association between UPs, thrombus

ondary intention [57,148,150]. The UP does not

formation, and acute coronary events does not

resolve, but persists as luminal stenosis increases.

show PU to be a complication of the disease pro-

The longer the UP persists, the greater will be the

cess, but only shows that it has the potential to

fibrotic luminal stenosis. More specifically, the

develop into a complication under certain circum-

severity of fibrotic luminal stenosis found in associa-

stances. A complication, strictly speaking, describes

tion with an UP may reflect the age of the UP.

a secondary disease that aggravates a previous one. Since there are many UPs not associated with thrombosis or other pathologic sequelae, and with10-4

Atherosclerosis

Chronic Ulcerated Plaques

out apparent clinical effects [63,120], PU, per se,

sense that it is an efficient method of reverse trans-

cannot technically be called a complication. If PU is

port, debulking and decompressing the plaque core

a component of active disease, it is not necessarily

by extruding plaque contents [15,57].

a pathologic event that must be, should be, or even can be prevented [155].

Absence of Thrombus

The distinction between PU as a component rather

Table 4 shows that, consistent with previous inves-

than a complication gives us insight into the nature

tigations [57,120,156,157], many UPs within the

of the IA and the disease process. It also alters our

coronary tree of patients who have died of ACD are

approach to the prevention and treatment of the

not associated with luminal thrombosis. The obser-

disease. If PU is a component of active atheroscle-

vations in this study of 83 patients demonstrate that

rotic disease, then all inflammatory atheromas may

PU is not synonymous with thrombosis, and throm-

be expected to ulcerate at some point, the large

bosis does not automatically form on exposed

vulnerable plaques [150,153] posing the greatest

subendothelial tissue [158]. Exposed subendothelial

danger. If such vulnerable plaques could be identi-

tissue, including collagen, may not be as thrombo-

fied in vivo, before ulceration, it may be possible to

genic as previously believed [159].

devise an intervention to control or remove the plaque before ulceration develops, or devise a

Figure 22 shows several examples of UPs without

treatment to neutralize the effects of ulceration

associated thrombosis. They appear to be open,

when it does occur.

draining, abscess-like cavities. The fibrous cap has been breached, exposing plaque contents and

If PU is a complication of active disease, then it is a

subendothelial collagen to flowing blood, with result-

random event, possibly affecting only a few

ing extrusion and partial emptying of the necrotic

plaques, with many atheromas never undergoing

core (Figures 22A-C). Why is thrombus absent if

ulceration. Identifying vulnerable plaques and per-

plaque contents or exposed subendothelial tissue

forming an intervention, may not be appropriate

are so thrombogenic [160]?

because not all such plaques may require intervention. Devising treatments to manage plaques that

The absence of thrombus in some UPs may be

will spontaneously ulcerate and drain at some point

explained by the presence of MMPs and other pro-

in the future is decidedly different than devising

teases within atheromas. These enzymes dissolve

treatments for complications that may or may not

tissue proteins, play an important role in degrading

develop.

and digesting dead or damaged tissue within the necrotic core, and erode the surrounding fibrous

Early PU Is the Norm

tissue, including the fibrous cap [114,161]. T cells trigger macrophages to secrete MMPs, showing

We believe that the norm is for plaques to ulcerate

these MMPs also to be a component of active,

early in their development. It is their failure to do so

atherosclerotic disease [53,162,163]. MMPs are

that is abnormal and pathologic in nature. Failure of

also capable of digesting fibrin and of preventing

a plaque to ulcerate early leads to the formation of a

thrombus formation and have also been shown to

large necrotic core, such as illustrated in Figure 6,

negatively affect platelet adhesion and aggregation

and to the large vulnerable plaque. Early ulceration

[163]. Thus, the pro-teases contained within

of a necrotic core may be quite beneficial in the

atheromas may prevent thrombus formation in UPs, 10-5

Atherosclerosis

Chronic Ulcerated Plaques

particularly those associated with insignificant luminal stenosis, by contributing to inherent thrombolysis. Figures 22D, E illustrates a large UP with a thrombus formed on a remnant of a fibrous cap, with no evidence of thrombus on the exposed surface at the base of the core. Most plaque contents are gone from this plaque with only residual degenerative tissue lining the margins of the core. In this situation, a thrombus has formed on one portion of an UP, but not on other exposed areas of the extensive ulceration, revealing a difference in the thrombogenicity of these two adjacent tissues [160]. If the tissue around the periphery of the necrotic core is markedly thrombogenic, we would expect histologic evidence of platelet aggregation and/or mural thrombosis in the area. The difference in thrombogenicity may be related to the MMPs and other enzymes that continue to be present at the margins of the necrotic core, even after the core contents have been removed [63,114].

In Review PU is a natural component, not a complication, of active progressive atherosclerotic disease, often occurring early in plaque development. PU is related to the activity of the IA, and it results in the debulking and decompression of atheromas. It is an efficient form of reverse transport. The UP without thrombosis provides insight into the nature of the IA and to the pathogenesis of PU, fundamental to our understanding of active atherosclerotic disease. UPs may persist as chronic festering, inflammatory lesions for indefinite periods, giving rise to fibrotic luminal stenosis and ultimately to occlusive thrombosis.

10-6

Atherosclerosis

What Is the Injurious Agent?

11. What Is the Injurious Agent? the primary IA because all patients who have them “To survive, a virus infects a cell and forces it to replicate; the

do not develop atherosclerosis and many patients

virus uses the cell’s replicative machinery to drive its own

without them do develop atherosclerosis. We must

replication.” K Tanaka, et al., [164]

look beyond cardiovascular risk factors for the primary IA.

Single versus Multiple IAs Atherosclerosis may be caused by a single agent, a Atherosclerosis is a complex, chronic, inflammatory disease, characterized by a series of highly specific cellular and molecular responses, believed to be caused by multiple IAs [18,165]. Atherosclerotic lesions, however, are non-specific. There are no pathognomonic histologic features that distinguish one IA from another. If atherosclerosis is caused by multiple agents, then many different agents produce identical, highly specific, cellular and molecular responses. The development of identical or similar lesions in response to multiple different IAs can only be explained if arterial cells and tissue respond in

single mutation, a family of similar agents, or possibly through molecular mimicry [166,167]. For example, Benditt has shown the SMCs of atherosclerotic plaques to be monoclonal in origin and suggested that these monoclonal cells arose out of a single SMC mutation, possibly caused by chemical mutagens or viruses [12,168]. Monoclonality of SMC in atherosclerotic plaques serves to focus attention on the possibility that atherosclerosis may be caused by single rather than by multiple IAs.

Mechanism of Progression

the same way, a non-specific way, to all IAs, whether that be hypoxia, chemical agents, physical agents, infectious agents, immune responses, genetic abnormalities, or nutritional injury [47]. This concept is not only questionable, it is not consistent with the pathogenesis of a highly specific, complex disease. We believe atherosclerosis is too complex to be caused by multiple agents.

An understanding of how the IA spreads, expands, and progresses is fundamental to characterizing it. What is the driving force behind the growth and expansion of atherosclerotic plaques and the accumulation of excessive numbers of SMCs [12]? What is the source of energy that sustains and/or replenishes the IA? Does the initiating IA continue to be present and active, increasing in number, amount,

Chapter 2 showed marked differences in the histol-

and concentration as the disease progresses? If so,

ogy of arteries injured by PTCA compared with

by what sustaining mechanism? What is the mech-

recently placed coronary bypass vein grafts, or to

anism of continuing injury? Is it caused by the same

atherosclerotic fibrous tissue (Figures A-E). The

IA responsible for the initial injury? Is continuing

artery wall does not respond in a non-specific way

injury due to the presence of cellular toxins, such as

to all IAs. Risk factors such as hyperlipidemia,

oxidized LDL, generated by a series of metabolic

smoking, and high blood pressure contribute to,

and biological reactions set in place by the original

accelerate, or aggravate the growth and expansion

injury, which then become self-perpetuating [169]?

of the primary IA, but they are not the primary IA

If so, it is akin to a vicious cycle that once set in

itself. These cardiovascular risk factors cannot be

motion is difficult or impossible to stop or interrupt 11-1

Atherosclerosis [18]. Does oxidized LDL beget oxidized LDL, injur-

What Is the Injurious Agent?

Evolutionary Purpose

ing more and more tissue, and causing growth and expansion of plaques?

What is the evolutionary purpose of lipid retention in atherosclerosis? In Figures 1–4, Chapters 1 and 2, we demonstrated the appearance of lipid-laden

Atherosclerosis is a progressive disease, but not necessarily steadily progressive. It is marked by exacerbations and remissions that can be greatly influenced by the control of risk factors, particularly the reduction of blood lipids [170–172]. A disease subject to exacerbations and remissions is not consistent with a disease driven in a relentless, selfperpetuating circle. If this reasoning is correct, the progression of atherosclerotic lesions is not due to a self-perpetuating chemical or metabolic reaction. It is due to a series of highly specific cellular and molecular responses, caused by an IA that may be considerably influenced by external factors.

SMC in early atherosclerotic lesions, and also noted lipid-laden SMC are not present in normal, unaffected intima. Lipid-laden SMCs must have been altered or affected in some specific way, presumably by the IA, to cause them to take up excessive amounts of lipid. In vitro studies show infectious agents can alter biological processes in the artery wall and predispose to atherosclerosis [165]. The IA has either entered the SMC and altered intracellular mechanisms concerned with lipid regulation, or factors external to the cell, associated with the IA, have altered these mechanisms and produced a dysfunctional but still viable cell [175,176]. If the IA were an extracellular toxin, such as oxidized LDL, we would

Progressive growth and expansion of atherosclerotic injury can only be explained and can only occur if the causative IA is able to replicate and/or be continually replenished. Lee, et al, notes lipids may become “biologically active,” suggesting lipids have a life of their own, are capable of replication, or are self- replenishing [63]. If the IA is not continually replenished by some metabolic mechanism, such as occurs in a vicious circle, but expands by virtue of replication of the IA, then the IA may be an infectious organism. Furthermore, if the growth and progression of atherosclerotic lesions are decreased or reduced by reducing circulating lipid, and if the IA responsible for this growth is an infectious organism, then the infectious organism may require lipid for survival, growth, expansion, and

expect the cell to be destroyed, not to be rendered partially dysfunctional. The same holds true for cellular injury caused by other classes of IAs, including hypoxia, physical agents, and nutritional injury, which are unlikely to alter intracellular function in such a specific way without killing the cell. Furthermore, there is no reason for chemical or physical agents, hypoxia, or nutritional injury to promote the retention of lipid or the uptake of lipid by the SMC because the retention of lipid has no effect on the action of these agents. For these reasons, we do not believe the IA causing atherosclerosis is an extra-cellular toxin, chemical, or metabolite, but is an intracellular infectious organism that is able to alter intracellular mechanisms to suit its own intended purposes.

replication. Pathogen-infected cells may alter membrane traffic for nutrient acquisition or act as a

Herpes simplex virus (HSV), Cytomegalovirus

cofactor to lipids in atherosclerosis [173,174].

(CMV), Chlamydia Pneumoniae (Cp), and Heliobacter Pylori (H. Pylori) are four organisms currently being considered in the pathogenesis of atherosclerosis. Studies show these four organisms can target and infect SMC, macrophages, and endothelial 11-2

Atherosclerosis

What Is the Injurious Agent?

cells, and can alter intracellular mechanisms

macrophages, and endothelial cells, causing the

involved in the uptake, metabolism, and degradation

same intracellular abnormalities in each infected

of cholesterol, with resulting lipid-laden cells [177–

cell.

179]. The lipid-laden SMCs in Figures 1–4 may all be infected with an infectious organism that specifically alters intracellular metabolism concerned with the uptake of lipid, presumably for some purpose beneficial to the IA. For example, Cp is an energy parasite that utilizes host cell mechanisms to supply adenosine triphosphate [174]. The uptake of lipid by an SMC infected with Cp may be caused by or promoted by the organism to secure an energy supply from the ingested lipid [174,177]. Cp may stimulate the expression of scavenger receptors by the SMC to take up oxidized LDL, in the same way H. Pylori stimulates the formation of iron-scavenging systems [180]. Lipid retention in the ECM, and the excessive uptake of lipid by the SMCs may be orchestrated by Cp or other organisms for their own benefit [174].

Single versus Multiple Infectious Agents If the IA responsible for initiating atherosclerosis is an infectious organism, is it a single infectious agent, a single family of agents, or multiple, different, infectious agents that are able, through molecular mimicry, to produce similar types of injury (166,167)? The objections raised earlier in this chapter to multiple IAs causing the same series of complex cellular and molecular changes in the artery wall also apply to multiple and different infectious agents. It is unlikely that different types of infectious agents, such as Cp, HSV, CMV, H. Pylori, or other unknown infectious agents, could all produce exactly the same cellular changes and lead to

The driving force behind the growth and expansion

the same atherosclerotic lesions in all patients. The

of the atherosclerotic lesion may be a replicating,

possibility exists atherosclerosis may be caused not

expanding, growing, infectious organism. We specu-

only by a single injurious agent, but a single infec-

late the infectious organism continues to be present,

tious organism [181].

active, and replicating as long as an adequate supply of the necessary lipid is available to it. Restriction or lowering of blood lipid may affect lipid metabolism within the plaque as well as the availability of the type of lipid required by the organism. This could explain why lesion growth is retarded and acute events decreased in those patients whose serum lipids are reduced [170–172].

A single infectious organism entering the SMC and altering specific intracellular functions could be responsible for the highly specific cellular and molecular re sponses of plaque formation and the uniformity of atherosclerotic lesions [18]. A single infectious organism, circulating in the blood, entering the artery wall at any vulnerable or injured site, could explain the multicentric origin (Chapters 1 and

Based on the evidence presented, we hypothesize

5) and the histologic similarity of widely separated

that atherosclerosis is caused by an infectious

lesions. The growth, contiguous expansion and

organism that alters intracellular functions and

spread of a single, replicating, infectious organism

mechanisms, creating an increase in lipid uptake

could explain the circumferential and longitudinal

and retention by SMCs. The intracellular infectious

spread of the disease, the findings of infectious

organism then utilizes the retained lipid, either

organisms within plaques [182], and the fusion of

directly or indirectly, as a source of energy to fuel

adjacent plaques. An infectious cause of atheroscle-

replication, growth, and expansion of atherosclerotic

rosis could explain the consistent presence of

lesions. Eventually it infects other SMCs,

inflammatory cells surrounding plaques, and the 11-3

Atherosclerosis

What Is the Injurious Agent?

reason why atherosclerosis is a chronic inflamma-

without killing the cell. Intracellular mechanisms are

tory disease. A single infectious organism that

altered in ways that foster the replication, expan-

infects and alters SMC function to such an extent

sion, and spread of the infectious organism [183].

that the cell ultimately dies could explain why

This particular mechanism of injury appears to be

plaque tissue degenerates and a necrotic core is

repeated over and over again, and is similar to the

formed. These observations provide insight into the

way a virus utilizes host cell mechanisms for replica-

nature of the IA, and lead us to advance the follow-

tion [164,192]. The affected and infected cell then

ing hypothesis:

becomes a tool of the infectious organism, a component and participant in the disease process,

Hypothesis

rather than a defender against the infectious organ-

We hypothesize that atherosclerosis is caused by a

include, but are not limited to, the following:

ism. These subverted intracellular mechanisms

single infectious organism, endemic throughout the world (183–186]. Atherosclerosis is a world-wide

1. Subversion of macrophage functions, resulting in

disease, and its lesions are similar in all peoples, as

failure of the macrophage to recognize, kill, and

with tuberculosis. We hypothesize that the infec-

remove the infectious organism at the time of initial

tious organism resides in a dormant state within a

injury when the organism is present in small

circulating cell, probably the monocyte exemplified

amounts and localized to a small area of the artery

by HSV, CMV, and Cp, as an obligate intracellular

wall. The responsible intracellular mechanisms are

pathogen (187–189]. The organism is activated

not presently known, but defective killing of phago-

when the monocyte, acting as a vector, is attracted

cytosed organisms is known to occur in chronic

to sites of endothelial injury and carries the infec-

granulomatous disease [193]. This initial failure of

tious organism into the artery intima (187,188,190].

macrophage function may explain how the infectious organism is able to establish a foothold, and

We further hypothesize that the infectious organism,

“set up shop,” and create atherosclerotic lesions in

after entering the artery wall, is released from the

multiple, separate areas of the artery wall. In con-

monocyte enters and “infects” resident, intimal

junction with the failure to neutralize and kill the

SMCs, and endothelial cells which now become an

infectious organism, the macrophages also fail to

additional host cells [187,191]. The activated infec-

phagocytose and remove dead and damaged tis-

tious organism is now present in monocyte derived

sue, as well as fail to participate in the repair and

macrophages, intimal SMCs, and endothelial cells

healing of the injured tissue [194]. Subversion of

[177], causing intracellular injury [183]. Considering

these 2 basic defensive responses, failure to

the possibility that the various histologic features of

phagocytose and kill the infectious organism, and

atherosclerosis are infectious in origin raises ques-

failure to remove damaged tissue at the time of ini-

tions about the mechanisms of injury.

tial infection, is pivotal in the initiation of atherosclerosis.

Mechanisms of Injury 2. Subversion of normal intracellular functions of the We postulate that the basic method or mechanism

SMC results in the secretion of an abnormal form of

of injury utilized by the infectious organism is to

CSPG that reacts with and retains lipid within the

enter and infect an intimal cell, injuring the cell and

interstices of the extracellular matrix [10]. This sub-

altering or subverting normal intracellular functions,

version may explain why lipid is a major component 11-4

Atherosclerosis

What Is the Injurious Agent?

of atherosclerotic tissue [35]. The retention of lipid is

the immune system and the proliferation of B and T

a component, not a complication of atherosclerotic

lymphocytes, commonly associated with later, more

disease, and all plaque tissue containing such lipid

advanced stages of atherosclerosis.

is not normal tissue. It is diseased and pathologic. Early plaque growth is associated with proliferation

5. In some way, the infectious organism is able to

of SMCs, not normal SMCs but SMCs altered in

subvert or to avoid the killing immune responses

some way, inhibited from healing the area of injury

employed by T and B lymphocytes after the immune

[12].

system is activated. As evidence in Chapter 3 showed, the number of T cells in the adventitia increased in direct proportion to plaque size, indicat-

3. The infectious organism subverts the intracellular mechanisms of the SMC, monocyte-derived macrophages, and endothelial cells governing the uptake, metabolism, and degradation of extracellular lipid, particularly the expression of scavenger receptors [167,175]. Disturbing, altering, or subverting these lipid regulatory mechanisms results in

ing active, continuing injury by the IA, along with generation of more and more antigen. Activation of the immune system leads to the proliferation of antigen-specific T lymphocytes, the number of T and B cells being directly related to the amount of the antigen produced or presented to the T cells [192,193].

excessive uptake of lipid and results in the formation of lipid-laden SMC, macrophages, and endothelial cells. The purpose behind this lipid up take may be to allow the host cell, directly or indirectly, to metabolize the lipid into a form that can be utilized, such as oxidation of LDL and/or the esterification of cholesterol, by the infectious organism (195). Oxidized LDL may be one form of lipid required by the infectious organism [175].

However, activation of the immune system and the associated activation of T lymphocytes, although quite vigorous and progressive to judge by the number of T cells, is not effective in neutralizing the IA, or in halting the growth and spread of the disease. Plaques continue to grow and expand in spite of a vigorous immune response. If these immune responses were effective, growth and spread of the infectious organism would be stopped, plaque

4. The ingestion of excess lipid by intimal SMCs is

growth would cease, and the number of T cells

associated with loss of cellular mobility within the

would decrease. Although the T cell and other

tissue, limiting the ability to migrate and transport

immune responses appear to be intact and function-

lipid back into the circulation [37]. Continued lipid

ing normally, the IA is able to avoid, subvert, or

ingestion, over and above that required by inherent

overcome these various defenses and survive,

cellular metabolism and/or the infectious organism,

replicate, grow, and expand.

ultimately results in over-distention, rupture, and death of the cell. Death of the cell releases the

We gain some insight into this issue from the study

retained lipid, and presumably the infectious organ-

of H. Pylori. H. Pylori is a well-adjusted parasite that

ism itself, into the extracellular space. The infectious

survives in a hostile environment despite vigorous

organism is now presumably free to infect other

humoral and cellular immune responses against it

cells, but it is also exposed to the B and T lympho-

[180]. The enzymatic pathways it needs for survival

cytes of the immune system (192). The release of

are continually switched on, and the presence of

the infectious organism upon the death of the inti-

variable regions in gene coding for surface structure

mal cells may be the event that triggers activation of

allows organisms to evade immune responses by 11-5

Atherosclerosis

What Is the Injurious Agent?

altering their surface antigens. This could be true in

the growth and replication of the organism, possibly

atherosclerosis if the infectious organism is able to

through increased or more readily available lipid

subvert the immune system.

[140].

What immune mechanisms are subverted or altered to allow an infectious organism survive in atherosclerotic plaques? Insight is gained from studies of the Acquired Immunodeficiecy Syndrome (AIDS) virus. The AIDS virus enters, subverts, and eventually destroys the helper T lymphocytes that play a key role in presenting viral antigens to the killer T lymphocytes [192,193,196]. Helper T lymphocytes secrete 2 cytokines, Interluken-2 and Interferon-gamma, essential for the immune system to function normally and effectively [193,196]. If these two cytokines are absent, the immune response is blunted, killer mechanisms are compromised, infectious organisms are not killed, and the infectious organism continues to replicate, grow, and spread [196]. Something similar may happen in atherosclerosis, preventing the infectious organism from being neutralized and eliminated by the immune system.

Autoimmunity Perhaps atherosclerosis is an autoimmune disease, and arterial wall injury is caused by immune complexes [166]. We visualize autoimmune disease as similar to a vicious circle. Once established, it is very difficult to stop, and it is not subject to the exacerbations and remissions seen with atherosclerotic disease. We would also visualize an autoimmune disease, caused by circulating immune complexes, to be a diffuse disease, not a multi centric, focal disease. Further, we would expect an autoimmune disease to respond to the administration of corticosteroids, but steroids have not been shown to affect the course of human atherosclerosis. It is possible that autoimmune mechanisms may play a role in contributing to the cellular injury and cell death within a plaque and in this way contribute to the expansion and spread of the IA.

In Review 6. An infectious organism subverts normal defensive responses by altering the hemostatic response

Based on the evidence presented, we believe the

to injury. Hemostatic abnormalities are produced by

primary IA causing atherosclerosis is a single infec-

altering the phenotype of the endothelial cells from

tious organism, an obligate intracellular pathogen

anticoagulant to procoagulant, generating the for-

residing in a circulating cell, probably the monocyte.

mation of thrombin [181]. SMCs and endothelial

Whether the organism is a bacterium or a virus is

cells infected with Cp express PAI-1, T factors, and

unknown. Both classes of agents appear to infect

Interlueken-6, all procoagulants [197]. Excessively

intimal cells and to alter intracellular mechanisms

high levels of PAI-1 disturb the thrombo-regulatory

with the potential to produce atherosclerotic lesions.

balance between TPA and PAI-1, producing a pro-

The infectious organism is activated after entry into

thrombotic state. The result is an increase in hemo-

the artery wall, infects intimal cells, and establishes

static responses to endothelial injury over and

a foothold within the wall. The organism proceeds to

above that required for injury repair. If this action of

replicate, expand, and grow in all directions from a

the infectious organism does produce a prothrom-

central focus. We speculate that the organism

botic state, to what purpose? The most logical

requires lipid, probably oxidized LDL, to fuel replica-

explanation is that thrombus in some way facilitates

tion and growth, and that it subverts normal cellular functions and defense mechanisms to procure this 11-6

Atherosclerosis

What Is the Injurious Agent?

lipid without killing the cell. We believe all of the abnormal cellular responses, listed above, are orchestrated by the infectious organism to procure the energy required for replication and growth.

11-7

Atherosclerosis

The Toxic Atheroma

12. The Toxic Atheroma [200]. A necrotic focus will spontaneously ulcerate “Fragmentation and embolization of platelet-rich mural thrombi

and drain the necrotic material into a body cavity,

at the site of vessel injury could cause damage by obstructing

onto the body surface, or be resorbed and undergo

myocardial microcirculation.” JF Mustard, [128]

The Fundamental Acute Lesion

fibrotic replacement.

Some of the chemical toxins associated with atheromas are products of lipid metabolism [61], acids generated as part of cell necrosis [200],

All acute coronary syndromes (ACS) can be traced,

inflammatory cytokines [18], immune complexes

in one way or another, to UPs and the resulting

[201], and other unknown factors. Oxidized LDL,

sequelae [57,148]. The UP, therefore, is the funda-

one of the most well known IAs, is believed to play a

mental lesion underlying the development of ACD

major role in the formation of the necrotic core [61].

and is the initial precipitating event leading to these

Reactive oxygen species are known to be highly

syndromes [57]. The many different ACSs that

toxic metabolites, causing immediate injury to cells,

develop as a result of PU are due to the many inter-

but the source of these free radicals has not been

acting factors triggered by this breach in endothelial

fully determined [202]. Atheromas contain reactive

integrity, including the size, location, number, and

oxygen species and may serve as a source of these

specific types of PU [57,148,154,198,199].

toxic metabolites at the time of PU [203–205]. Based on our observations of resorption and possi-

The primary aim of this chapter is to make clinical-

ble reversal of vascular calcification (Chapter 5), the

pathologic correlations in support of our hypothesis

necrotic core may also contain products of bone

that the initial symptoms and signs associated with

metabolism, such as HA, a particularly irritating

the onset of many ACSs are caused by the dis-

compound [96]. HA may produce similar effects

charge of plaque contents, particularly plaque toxins

when discharged directly into the coronary circula-

from UPs, NOT by ischemia. Plaque contents

tion. Atherosclerosis is the only disease where

include any chemical agents, particulate matter, or

necrotic material and all associated chemical toxins,

tissue fragment microemboli discharged from UPs.

are consistently discharged directly into the coro-

A secondary aim, in Chapter 13, is to discuss the

nary circulation, the most important blood supply in

potential role of plaque toxins in the subsequent

the body.

manifestations of the different ACSs.

The Toxic Atheroma

How toxic are the contents of an atheromatous core? Our reference to “plaque toxins” in this chapter is meant to include any and all non-physiologic

Necrotic tissue stimulates bodily responses to

chemical agents, present in the necrotic core, that

remove it because it contains chemical agents that

could produce direct stimulation and/or injury to

trigger inflammatory mediators, injuring surrounding

cardiac tissues when released into the coronary

viable tissue [18]. Necrotic tissue becomes increas-

circulation. Lyford and Connor, et al. [206], using

ingly acidotic as cells undergo degeneration and

extracts made from human atheromas, injected the

destruction, and as cellular proteins are denatured

extracts intravenously into 10 rats. Six of the 10 12-1

Atherosclerosis

The Toxic Atheroma

died of cardiac causes within minutes of the injec-

taneous discharge of these toxins at the time of PU

tion. These sudden cardiac deaths were believed to

may be expected to produce immediate symptoms

be due to direct, toxic effects of the chemical agents

and injury when the toxins circulate to downstream

contained in these extracts on the heart [Connor,

structures [202]. The effect of these plaque toxins

personal communication]. Nine of these rats also

discharging directly into the coronary circulation

showed pulmonary emboli, indicating these toxins

may be similar to infusing absolute alcohol directly

and/or plaque contents are quite thrombogenic

into the first septal branch of the LAD to produce a

[198]. The possibility that chemical agents con-

focal, controlled, myocardial infarction in the treat-

tained within the necrotic core are potent cellular

ment of hypertrophic cardiomyopathy [210,211].

toxins that could cause sudden and immediate direct injury to cardiac tissues when released into the coronary circulation must be considered in the pathogenesis of ACD [207].

The quantity of plaque toxins and other necrotic material discharged at the time of PU will depend on the anatomic features associated with each plaque, the location of the plaque within the coronary tree, the size of the plaque, and the size of the PU [57]. No two PUs are identical. Figures 8, 17–20, 22, 24, and 26, illustrate various types of PUs, ranging from tiny shoulder ulcerations to completely shelled-out plaques. The amount of plaque contents, and, therefore, the amount of plaque toxins released at the time of PU may be expected to vary widely from plaque to plaque. We assume the effects of these plaque toxins will depend upon the concentration, dose, the speed of release, the duration of the discharge, the inherent toxicity of the agents, and how quickly these agents can be neutralized and removed. Consideration must also be given to the number and age of the UPs in any given patient because multiple, chronic UPs are the rule in patients with acute coronary disease [57,148,150,156,208,209].

For example, a small amount of toxin discharged intermittently from a tiny shoulder ulceration (Figure 8) may produce no clinical symptoms, whereas a large bolus of toxin from a shelled-out plaque (Figures 18, 24), may precipitate a sudden, acute coronary event. Furthermore, the sudden and spon12-2

Atherosclerosis

The Toxic Atheroma

Figure 24: Pathologic lesions of a 39-year-old white male who died SCD outside the hospital. A, Dissected coronary artery showing moderate stenosis in the mid-LAD branch. Asterisk = main left coronary artery. B, Gross view of the narrowing in A showing 80% luminal stenosis with an intact fibrous cap (white arrow) overlying an empty, shelled out, necrotic core (white asterisk). No occlusive thrombus is present. Black asterisk = normal lumen. C, Histologic section of the UP in B, showing a remnant of the fibrous cap (white arrow) and an empty necrotic core (white asterisk) without evidence of thrombosis. D, Cholesterol crystals and other plaque debris in a small branch of the vasa vasorum supplying the artery wall distal to the UP in B.

12-3

Atherosclerosis

The Toxic Atheroma epicardial arteries and passed to the distal microcirculation. If embolic material circulates to these structures, then so do any associated plaque toxins.

If the embolic plaque tissue is friable, as might be expected of necrotic tissue (Figures 6,8), and undergoes easy dispersion, the obstruction to flow in the microcirculation may be insignificant. Large tissue fragments (Figure 6F), may be more serious Figure 26: A, X-ray of proximal left coronary artery in a 38-yearold male who died SCD outside the hospital. Asterisk = Left main coronary artery. White arrow = Acute thrombotic obstruction of

because such fragments will not be easily dispersed and could cause prolonged obstruction of relatively

the LAD coronary artery proximal to a bifurcation. C = CIRC

large arteriolar branches. The result may be the

artery. I = Intermedius branch. B, Microscopic view of the

production of focal ischemia or even infarction of the

thrombus in A showing plaque contents and cholesterol crystals

myocardium [64,128,212,214]. Obstruction of the

(long black arrows) mixed with thrombus (short black arrows). C, X-ray of proximal RCA in an 86-year-old female who died of

microcirculation by embolic plaque material, even

myocardial rupture several hours after receiving streptokinase for

transient obstruction, may delay the dilution, neutral-

acute anterior myocardial infarction. White arrow = Site of

ization, or washout of any associated plaque toxins,

intraintimal thrombus. D, Microscopic view of intraintimal

resulting in greater toxic injury.

thrombus shown in C. Black arrows identify thrombus mixed with blue injection mass indicating thrombus formed within a nonobstructing UP. E, X-ray of proximal RCA of 37-year-old male who died SCD outside the hospital. White arrow = Site of UP with intraintimal thrombus shown in F. F, Note injection mass has replaced plaque contents and fibrin strands have formed (black arrows). B, D & E all H & E stain.

Embolization of Plaque Tissue Embolic particulate matter, tissue fragments, and platelet microemboli from UPs are found in the majority of patients who died of ACD [64,128,212– 216]. The effect of this embolic material will depend upon such factors as consistency, number, size, the speed with which they are released, the speed with which they are removed, and the specific cardiac structures affected. Figure 23 shows several examples of embolic material in the lumen of the epicardial arteries, the vasa vasorum, the myocardial arterioles, and in the nutrient arteries to the conduction system of different patients. All emboli shown in Figures 23–25 were located distal to UPs in the

12-4

Atherosclerosis

The Toxic Atheroma relieve thrombotic occlusion causing acute inferior myocardial infarction. C, Microembolus in an arteriole (long arrow) lying adjacent to the proximal left bundle branch (short arrows). D, Small myocardial arteriole, adjacent to left ventricular endocardium and distal left bundle branch (black arrows), containing a small microembolus (white arrows). E, Embolic plaque contents in the upper interventricular septum of an 82year-old white female who died of cardiogenic shock in the hospital. F, Large intramyocardial microembolus (white arrow) of a 34-year-old white male with a history of chest pains who died SCD outside the hospital. This microembolus was distal to a nonocclusive thrombus in the LAD artery.

Figure 23: A, Low-power and B, high-power view of microembolus in the vasa vasorum (arrow) in the mid-CIRC artery of a 39-year-old white male who received streptokinase to

12-5

Atherosclerosis

The Toxic Atheroma

Downstream Structures If the initial symptoms of ACD are caused primarily by the discharge of plaque toxins, we should see histological evidence consistent with such toxic injury by examining downstream structures. The first tissue to be potentially affected by plaque toxins is the endothelium of the artery containing the UP and the endothelium of the microcirculation fed by that artery. Recent studies show microparticles of Figure 25: The photographs shown here are from the same patient in Figure 24 and distal to the UP in Fig. 24B. A, Plaque

endothelial origin are present in the circulating blood

contents and debris (white arrow) in the LAD distal to the UP. B,

of patients with ACD, indicating endothelial injury

Small microembolus (arrows) in a myocardial arteriole. C,

somewhere in the vascular system [220]. We have

Section of the myocardium showing contraction bands (arrows).

observed extensive, circumferential, endothelial

D, Section showing contraction bands (black arrows) in the left bundle branch.

erosions extending over a distance of several cm (Figure 17D and E), consistent with injury by an

Initial Symptoms

erosive toxin. Such erosions, localized to one coronary artery and distal to an UP, provide evidence the endothelium and the subendothelial tissue have

By “initial symptoms” we mean those symptoms and

been injured by local substances, presumably origi-

signs occurring within minutes after the onset of an

nating from the UP. If the offending agent causing

ACS. If initial symptoms of ACD are caused by

this erosion were a circulating systemic toxin, then

direct stimulation and/or injury of cardiac tissues by

we would expect all arteries, not selected arteries,

plaque toxins discharged from UPs, and the effects

to show similar erosions.

are immediate [202], then the initial symptoms should coincide with the moment of PU or the moment the toxins are released.

Endothelial injury by plaque toxins from UPs has implications for the pathogenesis of new atherosclerotic lesions because any endothelial injury may

The onset of symptoms should mark the moment

provide a point of entry for the IA [1]. Endothelial

when plaque toxins reach the myocardium or other

injury produced by plaque toxins may be important

structures. The initial symptoms, then, could reflect

in the pathogenesis of multiple, separate plaques in

toxic stimulation or injury, rather than ischemic

a single coronary artery (Figures 1 and 2), produc-

injury. Time is required for ischemic injury to

ing multicentric lesions (Chapters 1 and 5). Further,

develop and for ischemic metabolites to form [217].

these injurious chemical agents may injure and

Ischemic metabolites would not be present in suffi-

weaken the fibrous cap of a distal plaque (Figures

cient amounts to cause immediate stimulation or

17B,C, and F) and may contrib ute to the formation

injury of cardiac structures in the first few minutes

of a second UP within the same coronary artery. We

after the onset of symptoms, even when coronary

have found as many as 3 or more widely separate

flow is acutely reduced [218,219]. We postulate the

UPs within the same coronary artery in 13 of 83

initial symptoms associated with ACSs are due to

(16%) patients who died of ACD [57].

toxic rather than ischemic injury. 12-6

Atherosclerosis In addition, the circulation of large amounts of potent toxins to the distal myocardial microcirculation may severely injure or destroy the capillary endothelium and thus obstruct blood flow through the affected area, even with adequate antegrade flow. This mechanism may be one explanation for the no-reflow phenomena observed in some ACSs [221,222], discussed in Chapter 13 in the section on acute myocardial infarction (AMI). The release of

The Toxic Atheroma

In Review Considerable histologic and clinical evidence exists that plaque toxins, originating from UPs, have the potential to produce the initial symptoms by stimulating or injuring many cardiac structures. These toxins may play an important role, in both the initial clinical and subsequent manifestations as well as the prognosis of various ACSs.

plaque toxins from UPs may affect other cardiac structures, including the SMC and nerves of the media and adventitia of the epicardial artery, the conduction system, and the cardiac chemoreceptors.

The artery wall overlying an atherosclerotic plaque often has a highly developed vasa vasorum [223] that could carry any circulating plaque toxins quickly to the adventitia. Injury and/or stimulation of the media and/or adventitial nerves by plaque toxins could potentially produce vasoconstriction of a focal portion of the coronary artery, as well as chest pain, by stimulating sympathetic nerves.

If the artery containing an UP supplies a portion of the conduction system, these toxins may cause direct injury or dysfunction of the pacemakers and/ or the peripheral conduction system, leading to various arrhythmias in the absence of ischemia [218,224].

Plaque toxins may also stimulate the cardiac chemoreceptors, activating reflexes such as the Von Bezold Jarish (VBJ) reflex [225], producing the bradycardia and hypotension often observed when this reflex is activated during acute coronary events. By definition, the VBJ reflex is produced by chemical stimulation of the cardiac chemoreceptors located throughout the heart, further implicating a chemical agent in the pathogenesis of the initial symptoms. 12-7

Atherosclerosis

Plaque Toxins and Clinical Coronary Syndromes

13. Plaque Toxins and Clinical Coronary Syndromes the adventitia of the epicardial arteries. Chemical “…Different morphologic patterns of myocardial necrosis

agents derived by the second mechanism are chem-

suggest that various pathogenetic mechanisms with different

ical toxins discharged from UPs (discussed above),

biochemical derangements may interact in the natural history of the coronary heart disease.” G Baroldi [251]

and circulate to all distal cardiac structures, including the adventitial nerves distal to the UP.

The Pathogenesis of Angina Pectoris Pain

These two different mechanisms, each associated

The pathogenesis of the pain of angina pectoris

anginal-type pains, could explain some of the chest

remains in doubt [217,226]. Lewis advanced the

pain variations in angina pectoris. For example,

theory that the “P” (pain) factor, a chemical agent

direct stimulation of adventitial sympathetic nerves

derived from ischemic cells, stimulates sympathetic

by plaque toxins may produce different anginal

nerves, producing the pains of angina pectoris

pains from ischemic metabolites that stimulate

[227]. The specific chemical agent(s) were not iden-

intramyocardial sympathetic nerves. Similarly, stim-

tified. Lewis attemped to explain the wide variety of

ulation of adventitial nerves by plaque toxins in the

anginal pains on the basis of one mechanism,

right coronary artery may produce a different type

myocardial ischemia. He did not consider the possi-

and distribution of chest pains than the same toxic

bility that two different mechanisms might produce

stimulation of adventitial nerves in the left coronary

the pains. Many investigators, including Lewis,

artery. The same reasoning applies to the toxic

believe cardiac sympathetic nerves within the

stimulation of adventitial nerves in the proximal

myocardium are stimulated by chemical metabolites

compared with the distal portion of the artery [216].

with different chemical agents that stimulate sympathetic nerves in different locations, both producing

derived from ischemic myocardium [217,227], but this does not explain many features of angina pec-

If this hypothesis is correct, it may be possible to

toris.

distinguish these two different mechanisms clinically. For example, the anginal pains produced by

We hypothesize the pain of angina pectoris is

ischemic metabolites from myocardial cells may

caused by the stimulation of cardiac sympathetic

produce the typical substernal tightness, heaviness,

nerves by two different, broad classes of chemical

and pressure associated with stable angina pec-

agents, derived by two separate and distinct mech-

toris. Chest pains of this type are commonly precipi-

anisms. Chemical agents derived by the first mech-

tated by exertion, relieved by rest, and are fre-

anism are those metabolites produced by ischemic

quently associated with one or more significant,

cells and stimulate intramyocardial sympathetic

fixed, coronary stenoses. The relationship between

nerves, producing typical anginal chest pain. These

exertion and the chest pain provides strong evi-

metabolites, localized to the area of myocardial

dence in favor of ischemia. These chest pains,

ischemia, are subsequently neutralized and/or

which are usually gradual, not sudden or dramatic in

removed by flowing blood (227), and do not reach

onset, and which are directly related to the amount 13-1

Atherosclerosis

Plaque Toxins and Clinical Coronary Syndromes

of exertion, suggest it takes time for cells to become

the amount, potency, and speed of release of the

sufficiently ischemic to produce the metabolites that

toxin, and the specific nerves stimulated. The sec-

ultimately stimulate the nerves and produce the

ond mechanism would be expected to produce a

chest pain.

wide range and variety of chest pains because there are so many variable precipitating factors.

The anginal pains produced by chemical toxins discharged from UPs may be expected to be sudden in onset when the toxin, discharged from the UPs, strikes distal sympathetic nerves [202]. Chest pains produced by this second mechanism would be expected to come on at any time, at rest or during sleep, without exertion or other obvious precipitating

The possibility that two different broad classes of chemical agents with different chemical characteristics, originated by two mechanisms that stimulate different sympathetic nerves differently, must be considered in the pathogenesis of the pain of angina pectoris (228).

cause, whenever the plaque ulcerates and the discharge of toxins occurs.

These two mechanisms may also be distinguished

Variant (Prinzmetals) Angina (VA)

by the mode of relief. Chest pain caused by the first is relieved by rest, presumably because the production of ischemic metabolites ceases when exercise stops. The administration of vasodilators, such as nitroglycerin, may assist in neutralizing or removing these metabolites through increased blood flow (227). Chest pains produced by the second mechanism are similar to those described for unstable angina, often subsiding spontaneously without specific treatment, suggesting that the offending chemical toxin is no longer present, presumably neutralized or removed by flowing blood. Such spontaneous relief of chest pains suggests coronary blood flow was not significantly obstructed by a fixed stenosis.

The VA syndrome is the prototype we will use to illustrate how plaque toxins and embolic plaque contents may be involved in the pathogenesis of the initial symptoms in ACSs. In patients with VA, the sudden onset of severe anginal chest pain, S-T segment elevation, cardiac arrhythmias, and myocardial perfusion defects, all developing simultaneously, are consistent with spontaneous PU and discharge of plaque toxins into the coronary circulation. The quick reversal of these symptoms and signs, with the exception of the myocardial perfusion defects [229–231], is consistent with prompt neutralization, dilution, and/or washout of these same toxins. Prompt reversal of the entire syndrome is also consistent with unobstructed coronary flow.

The third possible way the two mechanisms could be distinguished is by the overall character, somatic distribution, and reproducibility of the anginal chest

Many investigators attribute VA to acute ischemia

pain. We would expect anginal chest pains associ-

caused by reversible coronary vasospasm

ated with the first mechanism to be reproducible,

[229,231]. The triggering agent responsible for pro-

non-progressive, and localized to the same somatic

ducing this spasm has not been identified

distribution with each episode. The character and

[69,232,233]. Until it is, it cannot be stated with cer-

distribution of the chest pains produced by the sec-

tainty that VA is caused by coronary vasospasm

ond mechanism would be expected to vary in

because the triggering agent may have a number of

intensity, distribution, and duration, depending on

effects including, but not limited to, coronary spasm. 13-2

Atherosclerosis Angiographic, intravascular ultrasound, and postmortem examinations of the area of coronary spasm commonly show the presence of atherosclerotic plaques and, in a few cases, the presence of acute

Plaque Toxins and Clinical Coronary Syndromes effect of the toxins. This direct toxic suppression of ventricular function could explain why some VA episodes are prolonged and unresponsive to treatment [231].

lesions [69,229,234–237]. The triggering agent that precipitates spasm appears to be related in some way to active atherosclerotic disease, possibly an acute lesion at or near the site of spasm [229]. Our pathologic studies of patients who died as a result of ACD show the presence of one or more UPs, many of them unrecognized on angiography [57,147]. Because patients with VA are at risk of developing acute coronary events, we postulate that patients with VA also have one or more unrecognized, chronic UPs in their coronary tree. If this is correct, patients with VA have a source of plaque toxins and plaque contents with the potential to discharge plaque toxins into the coronary circulation and produce, directly or indirectly, the VA syndrome. Plaque toxins may be the “local factor” mentioned by Maseri as playing a role in coronary spasm and the VA syndrome [234].

Figure 8 illustrates several UPs discharging plaque contents into the lumen. We have suggested the plaque contents from plaques such as these are intermittently “milked” into the lumen by pulsatile arterial pressure. The intermittent discharge of embolic plaque contents and toxins may be expected to produce intermittent symptoms. The VA syndrome could be caused by the discharge of toxins from plaques with this configuration, triggering not only intermittent coronary spasm, but also injury or dysfunction of the distal myocardium and/or the conduction system. Although the prognosis is generally good for patients with VA, an sudden cardiac death rate of 3.6% and an AMI rate of 6.5% indicate VA is not a benign condition but is another manifestation of active, ulcerative, coronary disease [231].

The development of myocardial perfusion defects

The presence of one or more chronic UPs that

following an episode of VA indicates some type of

intermittently discharge plaque toxins could explain

myocardial injury has occurred [229–231]. Recovery

many of the following observations and findings in

of myocardial perfusion can occur in a matter of

the patient with VA. For example, the increase in

hours or can be prolonged and require several

heart rate following an episode of VA [231] could be

days, indicating the injury was temporary but was

due to the release of catecholamines secondary to

severe enough to cause ventricular dysfunction

toxic stimulation and/or injury of the myocardium.

[229–231]. Had myocardial injury been caused

Multiple sites of coronary spasm [238] could be due

solely by vasoconstriction or spasm, we would

to the presence of multiple, actively discharging

expect the spasm to subside with the symptoms,

UPs [57]. The coronary spasm associated with

and the ventricular dysfunction to reverse in a mat-

PTCA [203,239] could be caused by the release of

ter of minutes. Prolonged ventricular dysfunction

plaque toxins when the plaque is split by balloon

suggests additional factors, over and above spasm

inflation, its contents discharged. The S-T elevation

of the epicardial artery or the myocardial arterioles,

characterizing an episode of VA, mimicking AMI,

caused the perfusion defects. We believe those

and with associated perfusion defects [231], can be

additional factors are primarily plaque toxins origi-

explained by the release of a large amount of highly

nating from UPs that supply that segment of

potent toxins that cause temporary transmural dys-

myocardium. Embolic material [64,128,212–216], as

function, similar to that of a transmural infarction.

discussed above, would add to and aggravate the 13-3

Atherosclerosis We believe coronary spasm is not the cause, but is one of many components of the VA syndrome triggered by plaque toxins.

Sudden Cardiac Death (SCD)

Plaque Toxins and Clinical Coronary Syndromes Third, careful pathologic studies show the majority of SCD patients have one or more UPs in their coronary arteries and, therefore, have the lesions that could serve as a source of plaque toxins [57,64,150,156,245–247]. In addition, immediate coronary angiograms of SCD patients who are suc-

We hypothesize that SCD, an ACS whose initial symptom is a sudden lethal arrhythmia, is caused by the sudden dramatic arrest or disruption of cardiac conduction by plaque toxins, not by acute ischemia. SCD, as discussed here, refers to those patients with apparently normal hearts, and no symptoms of coronary heart disease, who collapse

cessfully resuscitated show the presence of complex UPs similar to those seen on postmortem examination [247–249]. Thus, the majority of SCD patients have acute structural abnormalities in the coronary arteries, along with the sudden, lethal, disruption of cardiac rhythm [57,139,156]. Abnormal structure correlates with abnormal function.

and die suddenly without warning [240]. The majority of these patients have unrecognized coronary

Fourth, contraction bands or myofibrillar degenera-

atherosclerosis. The clinical and pathologic evi-

tion, an early histologic sign of injury, have been

dence supporting this hypothesis are as follows:

found in the myocardium and in the conduction system of many SCD patients [250–252]. Contraction bands in the conduction system suggest injury

First, SCD in these patients occurs much too rapidly

occurred at approximately the same time as the

to be caused by acute myocardial or conduction

onset of the lethal arrhythmia (Figures 24 and 25).

system ischemia [207,218]. Ischemic injury and

The effect of plaque toxins on the myocardium and

occlusive thrombosis take time to develop [241].

the conduction system may be quite similar to the

Even acute total occlusion of a coronary artery, as

immediate effects of absolute alcohol [210,211]. We

with a balloon during PTCA, does not produce sud-

propose that plaque toxins caused the contraction

den lethal arrhythmias [219].

bands in the conduction system and are responsible for causing the lethal arrhythmia through sudden, direct injury to the conduction system [252].

Second, Holter monitor recordings of patients taken at the time of SCD, frequently do not show S-T

Fifth, the lethal arrhythmia causing SCD is poten-

changes to support a sudden onset of acute

tially reversible. Successful defibrillation and resus-

ischemia, certainly not the massive ischemia, to be

citation of the SCD patient are not necessarily

expected if that was the cause [242,243]. Holter

followed by an immediate recurrence of the original

recordings do, however, often show an increase in

episode, indicating the triggering agent is transiently

heart rate, plus the development of complex ven-

present. This observation, not consistent with a

tricular ectopy just before the onset of the lethal

fixed coronary obstruction or an occlusive thrombus

arrhythmia [242,243]. The increase in heart rate

causing severe ischemia, is consistent with rapid

may be due to the release of catecholamines by

washout of a plaque toxin. Furthermore, SCD sur-

plaque toxins (244), postulated earlier regarding the

vivors do not necessarily go on to develop occlusive

VA patient. Complex arrhythmias indicate a serious

thrombosis and/or AMI, so the arrhythmia was not a

disturbance of the conduction system.

precursor to these events [245,248,253]. AMI is found in a minority of SCD patients, providing addi13-4

Atherosclerosis

Plaque Toxins and Clinical Coronary Syndromes

tional evidence the arrhythmia was not related to a

determined to be approximately 80% of the cross-

prior, unrecognized, or silent AMI [57,240,244,253–

sectional area, was evident on the post-mortem

255]. Our pathologic studies of 26 SCD patients

angiogram (Figure 24A). Gross and microscopic

without AMI showed 59 UPs, but only 23 (39%)

examination of the stenotic area showed a shelled

were associated with luminal thrombosis. Of these

out plaque, with a small false channel (Figures 24B,

only 9 thrombi (39%) were totally occlusive [57].

C), but no evidence of thrombosis. No other acute coronary lesions were present in this patient.

Sixth, infarction of the conduction system is rarely found at post-mortem examination, even with large

Large amounts of plaque contents were present in

trans-mural infarctions. The conduction system

the lumen distal to the UP (Figure 25A), as well as

appears to be relatively resistant to ischemia and,

microemboli in the vasa vasorum (Figure 24D), and

presumably, relatively resistant to developing lethal

in the microcirculation of the distal myocardium

arrhythmias caused by ischemia [252,256]. This

(Figure 25B). Contraction bands were found in the

inherent resistance is probably related to the rich

myocardium and in the peripheral conduction sys-

collateral and anastomotic blood supply to the con-

tem (Figures 25C,D), indicating recent injury. The

duction system [257], and the rich and preferential

clinical history plus the histologic evidence are con-

blood supply to the peripheral conduction system

sistent with sudden PU, the discharge of a large

via the left ventricular subendocardium [258]. In

amount of plaque contents and plaque toxins, caus-

addition, since conducting fibers do not contract and

ing direct injury to the conduction system and the

may not have the same energy requirement as reg-

adjacent myocardium. The findings in this patient

ular myocardium, they may be able to tolerate

illustrate a common mechanism that could be

ischemia without serious injury or malfunction [252].

responsible for SCD.

A serious insult is required to injure the conduction system, and to disrupt normal cardiac conduction, and to produce lethal arrhythmias.

Unstable Angina (UA) and Non S-T Segment Elevation Myocardial Infarction (NSTEMI)

Seventh, such interventions as coronary artery bypass surgery, PTCA with or without stents, aimed

The prevailing opinion is that the UA syndrome and

at relieving presumed myocardial ischemia in SCD

closely related NSTEMI are caused by acute

survivors, have been generally unsuccessful [249],

myocardial ischemia secondary to plaque rupture

providing further evidence the lethal arrhythmia may

[69], thrombus formation, and reduced coronary

not be caused by ischemia. We believe ischemia is

flow [259]. The diagnosis of myocardial ischemia is

an infrequent and remote pathogenetic factor in

based on anginal chest pain, S-T segment depres-

precipitating the lethal arrhythmia responsible for

sion, and, when present, the elevation of cardiac

SCD.

enzymes [69]. However, angiographic and pathologic studies often show one or more acute lesions,

Figures 24 and 25 show the pathologic findings in a

in the form of complex UPs, in the coronary arteries

39-year-old white male who died SCD outside the

[57,69], but not necessarily thrombus formation or

hospital, shortly after complaining of chest pains.

obstruction to coronary flow. Therefore, the UA

The patient had no prior history of heart disease,

patient, like the patient with SCD, has UPs that

and the postmortem examination showed no evi-

could serve as a source of plaque toxins. If VA and

dence of AMI. Luminal stenosis, subsequently

SCD are caused by plaque toxins, not by ischemia, 13-5

Atherosclerosis it is reasonable to postulate UA and NSTEMI are also produced by the same mechanism. What evidence supports this hypothesis?

Plaque Toxins and Clinical Coronary Syndromes Many of the multiple, chronic UPs found in patients with ACD contain intraintimal and mural thrombi, with plaque contents incorporated into the body of the thrombus (Figure 26) [198]. An intraintimal

The sudden onset of rest angina, nocturnal angina,

thrombus acts to seal the UP [148] and to prevent

or new onset severe angina [259], like SCD, occurs

the discharge of plaque contents and plaque toxins.

too rapidly for either thrombosis or ischemia to

A thrombolytic drug, by causing lysis of intraintimal

develop and precipitate these symptoms [241].

thrombus, reverses the normal repair responses

Moreover, anginal-type chest pain and S-T depres-

(Chapter 9), and reopens what is essentially a

sion are not specific for ischemia and can be pro-

sealed UP [148,260]. Multiple UPs may be present

duced by other agents, particularly by toxic chemi-

in all 3 major coronary arteries [57], and may be

cals [210,211]. Anginal chest pain and S-T

reopened by these drugs and discharge plaque tox-

depression represent presumptive evidence, not

ins to the entire myocardium. This could result in

proof, of ishemia. Thrombolytic drugs, intended to

global myocardial dysfunction in spite of adequate

relieve ischemia, have not only failed to help the

coronary blood flow [222,261]. In addition to reopen-

patient, they have led to an increase in fatal and

ing the core, the lysis of thrombus also releases any

nonfatal AMI [69]. As a result, these drugs are no

plaque contents and toxins contained within the

longer recommended for the treatment of UA and

body of the thrombus, adding to the toxins liberated

NSTEMI [69]. There is reason to question ischemia

from the reopened core itself. Microembolic obstruc-

in the pathogenesis of UA and NSTEMI syndromes.

tion of the microcirculation could also contribute to toxic injury by obstructing antegrade, collateral, and

If plaque toxins can stimulate sympathetic nerves in

anastomotic flow, as discussed above [128].

the heart to produce angina pectoris and are sufficiently potent to cause S-T changes and myocardial injury, then toxic injury mimics, and is indistinguishable, from ischemic injury. We hypothesize UA and NSTEMI syndromes are not due to ischemia at all, but are caused primarily by plaque toxins. Furthermore we believe that plaque toxins are sufficiently potent in themselves to cause myocardial injury and NSTEMI, without implicating ischemia, thrombosis, or obstruction of coronary flow. The subsequent development of thrombotic occlusion, ischemia, and S-T Elevation Myocardial Infarction (STEMI) in some UA or NSTEMI patients does not nullify our hypothesis. Once thrombotic occlusion develops, ischemic injury becomes superimposed on toxic injury, and an entirely different syndrome is present. This hypothesis could explain why thrombolytic drugs are contraindicated in the patient with UA and NSTEMI and sheds light on the mechanisms

The lysis of intraintimal thrombus also allows blood to re-enter the now empty, or partially empty, necrotic core [262]. This reentry may change the dynamics within the core. For example, the reentry of blood could lead to dissection of blood along cleavage planes (Figure 7, Chapter 4), the formation of blind pockets (Figure 20), or the disruption and/or dislodging of the fibrous cap to form an occluding flap of tissue (Figure 20). Administering a thrombolytic drug is also associated with the activation of platelets [263,264]. Activation of platelets may lead to the formation of a new intraintimal thrombus in a newly reopened necrotic core, and lead to an occlusive thrombus and acute events. Therefore, thrombolytic drugs may convert a relatively stable UP into an unstable UP, resulting in fatal and non-fatal AMI. The mechanisms described

involved. 13-6

Atherosclerosis

Plaque Toxins and Clinical Coronary Syndromes

here could explain adverse clinical events, and the

Thrombolytic drugs are contraindicated because

paradoxical response that follows thrombolysis in

they dissolve intraintimal thrombus, reopen sealed

patients with UA and NSTEMI [264].

UPs (Figure 26), and lead to the discharge of more plaque toxins, and activate platelets leading to

Aiming therapy at ischemia and thrombosis with

acute events. Therapeutic efforts aimed at stabiliz-

thrombolytic drugs in patients with UA and NSTEMI

ing the UP and neutralizing plaque toxins would

fail [69] because ischemia and thrombosis are the

focus on the right target, in the right patient, for the

wrong targets. The target is the UP and the associ-

right reasons.

ated discharge of plaque contents and plaque toxins. Therapeutic efforts should be aimed at the identification and elimination of the culprit UP

S-T Elevation Myocardial Infarction (STEMI)

responsible for discharging the plaque toxins and other material. How is this best accomplished? Why are thrombolytic drugs recommended for the treatment of STEMI [122,269], but not for UA and At the present time, the approach offering the best chance of eliminating an actively draining UP is to intervene with a cardiac catheterization, identify the culprit UP, perform PTCA and place a stent in the area (Chapter 4). This approach would stabilize the UP by doing two things. First, the plaque would be completely drained by splitting the fibrous cap with a balloon, allowing coronary blood flow to carry away any remaining plaque contents and toxins. Second, the placement of a stent would stabilize the plaque by occluding any cleavage planes, would close any blind pockets, tack up any loose fragments of fibrous cap, and would produce unobstructed coro-

NSTEMI [69], if all have the same pathologic substrate in terms of UPs and the same overall plaque burden? The major difference is STEMI IS caused by ischemia secondary to acute obstruction of coronary flow, commonly by occlusive thrombosis, but UA and NSTEMI are not. The administration of a thrombolytic drug to the patient with STEMI is appropriate because this treatment is aimed at the right target, occlusive thrombosis, for the right reason, to restore coronary blood flow. Relieving the thrombotic occlusion with a thrombolytic agent relieves the ischemia, reduces the size of the infarction, and improves the prognosis [122,269].

nary flow across the lesion. Any complications, such as spasm or sudden ventricular arrhythmias resulting from the liberation of toxins during PTCA could

Since the majority of occlusive thrombi are super-

be handled at the catheterization table. This view is

imposed on a UP, we assume plaque toxins are

supported by clinical studies that show early inter-

discharged from the core prior to the formation of

vention in the patient with UA and NSTEMI results

the thrombus [262], producing injury, initial enzyme

in a decrease of recurrent events and improvement

elevation, and symptoms, before thrombosis and

in the long-term prognosis [265–268]. This

ischemia develops (Figure 8). After the formation of

approach would not disturb or affect intraintimal

the occlusive thrombus, subsequent enzyme eleva-

thrombi in UP elsewhere in the coronary tree or

tion is due to ischemic injury and infarction, super-

create more unstable lesions.

imposed on the initial toxic myocardial injury or infarction. These two different mechanisms for pro-

We believe UA and NSTEMI syndromes are caused

ducing STEMI could explain the two different histo-

by plaque toxins released from chronic UPs, not by

logic types of myocardial infarction described by

ischemia secondary to thrombotic obstruction. 13-7

Atherosclerosis

Plaque Toxins and Clinical Coronary Syndromes

Baroldi [251]. It is possible that we are dealing with

the infarction, but also causing post-infarction

two entirely different mechanisms in the pathogene-

angina despite adequate coronary flow [222]. Tox-

sis of STEMI.

ins may also injure the peri-infarction tissue, causing re-entrant arrhythmias, and/or they may directly

If two mechanisms are present, one ischemic and

injure the conduction system and precipitate the

one toxic, they might offer an explanation of some

various arrhythmias often observed following

of the observations, clinical findings, and complica-

thrombolysis [270,271]. These observations may

tions following the administration of thrombolytic

explain why a bigger, more wide open artery is not

agents to the patient with STEMI [222]. The most

necessarily better if this flow brings with it more tox-

important observation is that thrombolytic drugs are

ins or more emboli [222]. These toxins may be the

not 100% effective in opening acutely occluded

“untreated factor” or unrecognized factor responsi-

coronary arteries, as might be expected if the occlu-

ble for the no-reflow phenomena referred to by

sion were pure thrombus. We believe that the failure

Gibson [126].

of thrombolytic agents to reperfuse an acute occlusion is due to the presence of a blind pocket and/or

Thrombosis versus Plaque Toxins

a flap of fibrous cap that do not respond to the lytic agent (Figure 20). Perforating this blind pocket or flap with a guide wire may be all that is necessary for the thrombolytic drug to perfuse the artery and lyse any associated thrombus [123].

Although thrombolytic drugs relieve thrombotic occlusion and improve survival in patients with STEMI, it is becoming increasingly apparent that thrombosis and resultant ischemia are not the primary cause nor the primary pathologic lesion in

Reperfusion of an obstructed artery with a thrombolytic agent or with PTCA may be followed by reperfusion injury, manifested by failure of the injured myocardium to recover contractility despite adequate coronary flow, the no-reflow phenomena [222]. It is our theory that reperfusion injury and the no-reflow phenomena are caused by plaque toxins liberated from UPs, both before the thrombotic occlusion and then again after reperfusion. Thrombolysis restores coronary flow, but it also liberates any toxins contained within the thrombus, reopens the necrotic core, and releases more plaque toxins.

many other ACSs. In other words, “Ischemic Heart Disease” may not be ischemic at all, in many patients, but “Toxin Induced Heart Disease.” It is a mistake to attempt to explain all the facets of ACD on the basis of ischemia and/or occlusive thrombosis. If the fundamental lesion underlying all ACS is the UP, and UPs discharge plaque toxins capable of causing direct injury to cardiac tissues, then it is reasonable to consider plaque toxins as a major additional etiologic factor in the pathogenesis of ACD. Thrombosis is but one of the many components, reactions, and responses that characterize ACD. Thrombosis is not inherently pathogenic, but

No-reflow is believed to be due to severe injury to

becomes pathologic only in certain circumstances

the microcirculation, particularly the endothelial lin-

(Chapter 9). Therefore, preventing or treating

ing of these vessels, obstructing flow. Toxic injury to

thrombogenic responses per se is not appropriate

the microcirculation may be so severe that angio-

for all ACSs and correcting thrombosis corrects only

graphic dye leaks into the myocardium and results

one component of ACD. There are many other

in the myocardial staining or blush often observed at

components to consider.

angiography [222]. Further, these toxins may circulate to the peri-infarction zone, not only enlarging 13-8

Atherosclerosis

Myocardial Rupture and Thrombolytic Drugs

Plaque Toxins and Clinical Coronary Syndromes these toxins. The trapped toxin could cause excessive softening, liquefaction, and, if not neutralized, rupture of the myocardium. This could explain

Myocardial rupture is a well-recognized complica-

myocardial rupture in patients with small infarctions

tion of thrombolytic therapy for STEMI, occurring

involving secondary branches [276], probably with

much more frequently in these patients than in

limited collateral and/or anastomotic flow where a

patients not receiving a thrombolytic drug [272–

toxin could be highly concentrated.

274]. Myocardial rupture occurs only in patients with transmural infarctions [273] and appears to be related to excessive softening and liquefaction of

The use of primary PTCA and stents without throm-

infarcted tissue, beyond what is observed in the

bolytic drugs for the treatment of STEMI has

majority of transmural infarctions. Infarction and

resulted in a decrease in myocardial rupture com-

necrosis are not synonymous terms [200] because

pared with results following the use of thrombolytic

fully necrotic tissue has no structure while infarcted

drugs [277]. The reduction of myocardial rupture

myocardium still has many structural elements

following PTCA may be explained by the removal of

intact. Therefore, when the infarcted tissue

the obstructing luminal thrombus, restoring blood

becomes excessively necrotic and is without any

flow without disturbing intraintimal thrombus, not

residual fibrous structure, it is prone to rupture.

reopening sealed UPs, and the washout of any retained toxins by restored blood flow.

What is the pathogenesis of myocardial rupture in general, and after thrombolysis in particular? Why

Stunned Myocardium

does excessive liquefaction occur, and why does it occur more frequently in the patient receiving thrombolytic drugs? Myocardial rupture has been attributed to a lack of collateral and/or anastomotic blood flow to the infarcted area [275]. Embolic obstruction of these alternate channels could occur as a result of emboli originating from the culprit occlusive thrombus as it undergoes lysis, or from emboli originating from mural or intraintimal thrombi in UPs in uninvolved coronary arteries [64] that supply collateral flow to the obstructed, culprit artery. Lack of well developed collateral flow in patients who develop their first infarction may also be a factor in rupture as well [275].

Stunned myocardium is defined as viable but dysfunctional, akinetic, dyskinetic, noncontractile myocardium in the presence of adequate coronary blood flow. Myocardial stunning is usually a temporary phenomenon, lasting only hours to days, often associated with ACSs including VA, SCD, UA and NSTEMI [278,279]. Most investigators believe stunning is related in some way to ischemia produced by obstruction or partial obstruction of blood flow, sufficient to cause cellular dysfunction and loss of contractility, but not of viability [201,279]. However, stunned myocardium is non-contractile, yet it has normal perfusion on thallium scintigraphy, resulting in a “perfusion-contraction” mismatch [279]. If the

We hypothesize the liquefaction of infarcted

stunning were due to ischemia, the perfusion scan

myocardium is caused primarily by excess plaque

should show a decreased uptake of thallium to go

toxins that become trapped or localized in the

along with the proposed decrease in coronary flow.

infarcted area. Obstruction of antegrade flow and

The failure to establish the presence of ischemia by

embolic obstruction of collateral or anastomotic flow

this objective method is strong evidence stunning is

may delay or prevent neutralization or washout of

not caused by ischemia. 13-9

Atherosclerosis We believe myocardial stunning is another ACS, which could be explained on the basis of plaque toxins liberated from unrecognized UPs. Myocardial injury caused by a plaque toxin may be temporary and reversible, depending upon the amount of toxin discharged from the necrotic core and the speed with which it is neutralized or washed away by flowing blood. If stunning is caused primarily by plaque toxins and not by obstructed blood flow, this could explain the quick reversal of stunning after an episode of VA, UA, or focal myocardial dysfunction in patients with unobstructed coronary arteries [280,281]. The discharge of plaque toxins from a non-obstructive UP during exercise could also explain stunning associated with exercise testing and its quick reversal in the post-exercise period [280]. Furthermore, stunning is attenuated by the use of superoxide dismutase, an enzyme that tends

Plaque Toxins and Clinical Coronary Syndromes temporary injury caused by toxins and quick washout of these toxins by circulating blood, as in myocardial stunning [284]. We hypothesize that patients with SI develop ACSs because the plaque discharging the toxins develops a larger ulceration and discharges sufficient toxins to produce clinical syndromes, such as UA or NSTEMI, or proceed on to develop coronary thrombosis and STEMI [286,287]. If this is correct, identifying patients with silent and reversible S-T depression on Holter monitors may identify patients with active, open, unstable UPs in whom an intervention could prevent an acute event. Our hypothesis is supported by recent studies confirming early intervention with PTCA and stent placement reduces the acute event rate in SI patients [287].

Oral IIB/IIIA Inhibitors

to neutralize free radicals, which offers further evidence that stunning may be related to toxic injury [201,282,283]. It is interesting that Kloner, et al., concluded that myocardial stunning consists of two components, an ischemic component and a toxic component caused by reactive oxygen species, with the toxic component appearing to be larger than the ischemic component [201]. We believe stunned myocardium is caused primarily by plaque toxins from UPs, quickly washed away before permanent injury or infarction can occur, not by ischemia.

Silent Ischemia (SI)

Oral IIB/IIIA inhibitors have been dropped from clinical testing because of an unexplained increase in adverse clinical events, including bleeding and fatal and non-fatal AMI [141,288,289]. The oral IIB/IIIA inhibitors were given on the assumption that acute coronary syndromes such as UA and NSTEMI, were caused by ischemia related to thrombus formation. As discussed above, we hypothesize these ACSs are NOT caused by thrombosis or ischemia, and this explains why the IIB/IIIA inhibitors did not benefit these patients. We believe the oral IIB/IIIA inhibitors were given to the wrong patients for the wrong reasons, causing harm rather than benefit

SI may not be caused by ischemia at all, but by the

[141].

recurrent release of subclinical amounts of plaque toxins, discharged from asymptomatic, unrecognized chronic UPs (Figure 8). The diagnosis of SI is

The failure of the oral IIB/IIIA inhibitors is the same

commonly based on S-T segment depression on a

as that postulated for thrombolytic drugs. These

Holter monitor recording or during exercise testing

agents cause excessive bleeding and fail to prevent

[284,285]. S-T depression is not specific for

ACSs in unstable coronary syndromes because

ischemia and could be caused by plaque toxins, as

they unseal UPs and prevent the normal hemostatic

already discussed. The prompt reversal of S-T

responses required for repair. IIB/IIIA inhibitors, like

depression in the patient with SI is consistent with 13-10

Atherosclerosis

Plaque Toxins and Clinical Coronary Syndromes

thrombolytic agents, transform stable UPs into

required for injury repair (Chapter 9). As a conse-

unstable plaques, leading to the discharge plaque

quence, the UPs are not reopened, plaque toxins

toxins that initiate acute coronary events.

are not released, an unstable lesion is avoided, and resolution and repair of an UP can occur. ASA may

Why are intravenous IIB/IIIA inhibitors successful in preventing many of the complications associated with coronary interventions, including stent placement in patients with STEMI, NSTEMI, and UA [290]? In our view the difference lies in the duration of treatment. The intravenous IIB/IIIA inhibitors are given for a very short period of time. They do not significantly disturb intraintimal thrombus in UPs,

also prevent the platelet aggregation response associated with the release of thrombogenic plaque contents [207], in this way preventing or reducing the number and size of platelet microemboli passing into the distal circulation during acute plaque rupture.

Conclusions

but they prevent new thrombus formation at the site of the PTCA and/or stent. In contrast oral agents

We believe, based on the foregoing evidence, that

are given for long periods of time and prevent nor-

atherosclerosis is caused by a single infectious

mal hemostatic responses of UPs, essential to

agent, or by multiple infectious agents through

control bleeding and resolution of injury. It is the

molecular mimicry, which enter the artery wall at

prevention of these normal hemostatic responses

any breach of endothelial integrity and establish a

that results in bleeding into the plaque core,

focus of infection. This organism then grows and

swelling of the core, the creation of an unstable

expands, causing first a proliferation of diseased

lesion, and the production of acute coronary events.

fibrous tissue, then destruction of this tissue to form atheromas. The organism grows and expands by

Aspirin (ASA), UPs, and Plaque Toxins Why is ASA, a platelet inhibitor, successful in preventing primary and secondary acute coronary events [291] when oral IIB/IIIA inhibitors, powerful anti-platelet agents, precipitate acute coronary events? What does ASA actually do, and what is

direct contiguity in both circumferential and longitudinal directions, fusing with adjacent atheromas. The organism grows and expands by subverting normal defensive responses to its own purposes. Lipid is retained in all plaques because the organism requires oxidized LDL, or some variant thereof, as a source of energy to fuel replication and expansion.

involved in the prevention of acute coronary events? ASA has no known effect on either the formation or

Control of risk factors, particularly the reduction of

prevention of PU, but appears to have significant

cholesterol and other lipids, may cause the organ-

effect on those developments that take place after

ism revert to a dormant state for an indefinite

PU [291,292]. We hypothesize ASA acts by prevent-

period. A chronic inflammatory response develops

ing the growth of thrombus, but has little or no effect

because the IA is a foreign organism, and inflamma-

on thrombus that is already present, whether occlu-

tory defenses mobilize against it. Calcification,

sive, mural, or intraintimal thrombus. If this is cor-

which occurs quickly on degenerated tissue and

rect, ASA prevents acute events by NOT disturbing

uses normal bone-forming mechanisms, is effective

the normal hemostatic and thrombogenic responses

in delaying the growth and expansion of the infec-

aimed at injury repair, but prevents the tendency for

tious organism. Adventitial tissue is inherently

a thrombus to grow above and beyond what is

resistant to the infectious agent, thickening to pre13-11

Atherosclerosis

Plaque Toxins and Clinical Coronary Syndromes

vent outward expansion into the pericardial space

successful and we are able to eradicate the infec-

and confining the organism to the intimal layer. The

tious organism, would this remove the necessity to

only outlet for an expanding atheroma is to ulcerate

stop smoking, reduce cholesterol, blood pressure,

and drain into the lumen of the coronary artery.

and other risk factors? Would preventive measures

Although the organism is usually most active deep

go by the board in favor of antibiotics, taken when-

within the intimal layer, it may also localize to the

ever the organism becomes active?

endothelial surface and spread, erysipelas-like, in all directions from a central focus.

We visualize that current preventive measures will continue to be necessary, even if an infectious

The natural course of events for any necrotic focus

organism is identified and we have a specific antibi-

is to spontaneously rupture and drain into a body

otic or a vaccine. We know organisms that give rise

cavity, to the external surface of the body, or to be

to chronic infectious diseases may become resistant

replaced by fibrous tissue. We believe most

to antibiotics and emerge in even more resistant

atheromas ulcerate and drain at some time during

forms, as with tuberculosis. AIDS is treated with

their existence, usually when the plaque is relatively

drugs whose aim is to force the virus into remission,

small.

rather than an attempt to eradicate the organism from the body. The same may be true of atheroscle-

UPs frequently persist as chronic, indolent, fester-

rosis. If we can identify patients who are carriers of

ing, gradually progressive, inflammatory lesions, are

the organism, then we may be able to institute spe-

asymptomatic, but provide the substrate for rapid

cific measures to prevent transmission. Identifying

progression leading to acute coronary events. UPs

the organism may allow us to institute measures

are the fundamental lesion underlying all ACSs, and

that keep it in a state of remission, even if not eradi-

the variety of ulcerations is what gives rise to the

cated. Adding an antibiotic or antiviral treatment to

many different ACSs.

risk factor modification may accelerate a remission, forcing the organism into a dormant state. This

The atheromatous core contains many plaque toxins. These toxins, when discharged from an UP,

would have the same effect as eradication if the organism could be kept dormant indefinitely.

stimulate and/ or injure all cardiac structures. The effect of these toxins depends on the amount released, their potency, the structures affected and how quickly the toxins are removed by circulating blood. We believe the release of plaque toxins is primarily responsible for VA, SCD, UA, and NSTEMI, myocardial rupture after AMI, myocardial stunning, and silent ischemia.

Based on the observations put forth in these chapters, particularly the infectious organisms ability to subvert normal defense mechanisms, we believe it may be difficult to eradicate and will present a continuing challenge in the future. As with tuberculosis, the fight requires ongoing vigilance to diagnose, treat and limit the growth of this organism.

Future Directions If atheroscherosis is found to be caused by an infectious organism, we assume an antibiotic or antiviral method will be developed to treat the organism in order to eliminate the disease. If this approach is 13-12