Chapter 134: Atherothrombosis: Disease Initiation, Progression, and Treatment

Atherothrombosis describes a disease process that begins with atherosclerosis and predisposes to thrombosis in the artery. In the 1850s, Virchow¹ described atherosclerosis as an inflammatory and prothrombotic process. Rokitansky, and later Duguid, posited that atherosclerotic lesions are initiated by incorporation of platelet lipids into the vessel wall (“encrustation”) following thrombosis. It was subsequently demonstrated that insudation of plasma lipoproteins is responsible for most of the lipid content of the atherosclerotic lesions. In 1913, Anitschkow noted atherosclerosis developing in rabbits fed a relatively high cholesterol diet. Although the involvement of inflammation in atherosclerosis has been known for more than 100 years, the molecular mechanisms of atherosclerotic disease initiation and progression have become clearer only in the recent past.² It is now understood that the classical disagreement between the “lipid” hypothesis and the “inflammation” hypothesis of atherogenesis can be reconciled because there is direct linkage between cholesterol deposition and arterial inflammation in the process.^3,4

Lipid accumulation in the arterial intima, termed a fatty streak, can occur in adolescents and may progress in paroxysmal fashion to a hemodynamically significant lesion causing arterial insufficiency. Autopsy studies of young soldiers and young trauma victims indicated that occult coronary atherosclerotic plaques are commonly present in healthy individuals in their teens and twenties.^5,6 In addition, intracoronary ultrasonograph studies demonstrated the presence of coronary atherosclerosis in 37 percent of healthy heart donors age 20 to 29 years, 60 percent of those age 30 to 39 years, and 85 percent of those older than age 50 years.⁷ Several theories are espoused for this propitious condition. One well-recognized theory is the response to injury hypothesis whereby the inciting event that predisposes to atherosclerosis is injury to the endothelial lining of the artery. This hypothesis was formulated in animal studies that showed vessel narrowing and intimal thickening after endothelial denudation with angioplasty.^8,9 However, human pathologic studies of early atherosclerotic plaques indicate that endothelium is structurally present but is dysfunctional. The dysfunctional state of endothelium induces abnormalities in vascular tone, inflammation, growth, and thrombosis. Atherosclerotic risk factors contribute to endothelial dysfunction and promote atherosclerosis. This section describes the mechanisms responsible for endothelial dysfunction and the impact of atherosclerotic risk factors.

ATHEROSCLEROTIC RISK FACTORS

Increasing age, male gender, and heredity are the major atherosclerotic cardiovascular disease risk factors that cannot be modified (Table 134–1). Abnormal lipids, smoking, improperly controlled hypertension, improperly controlled diabetes mellitus, abdominal obesity, physical inactivity, and psychosocial factors are established risk factors that can be modified, accounting for most of the risk of myocardial infarction worldwide in both sexes and at all ages.¹⁰

In addition to these traditional risk factors, newer risk factors have been recognized.¹¹ With the use of highly active antiretroviral therapy (HAART), HIV-infected patients have demonstrated a dramatic overall increase in life expectancy.¹² At the same time, HAART-treated HIV patients also have an increased risk of developing premature cardiovascular disease over time. Both HIV viral proteins and the antiretroviral drugs themselves cause endothelial dysfunction. They activate cell signaling cascades, induce oxidative stress, disturb mitochondrial function, alter gene expression, and impair lipid metabolism in vascular cells, macrophages, and adipocytes.^13,14

Cardiovascular morbidity and mortality is also recognized to be exceedingly high in patients with chronic renal failure.^15,16 Increased risk of premature atherosclerotic cardiovascular disease in patients on chronic hemodialysis has been known for many years, but recent studies point to an increased risk even at early stages of chronic kidney diseases. Low glomerular filtration rates and/or proteinuria are independently associated with increased rates of cardiovascular disease.¹⁷ Other factors, such as sympathetic overactivity,¹⁸ are likely to contribute to the pathophysiology of cardiac risk in these patients. Among other emerging risk factors is obstructive sleep apnea, in which treatment may improve cardiovascular outcomes.¹⁹

ENDOTHELIAL DYSFUNCTION

Cardiovascular risk factors and abnormal blood rheology are thought to result in endothelial dysfunction that predisposes the aorta and arteries to atherosclerotic plaque development, sparing the arterioles and capillaries (Fig. 134–1). Endothelial dysfunction is a term that encompasses perturbations in the diverse physiologic functions of normal arteries, including regulation of vascular tone, inflammation, growth, and preservation of blood fluidity. Lipid accumulation²⁰ and endothelial dysfunction are intimately connected and seminal to the initiation and progression of atherosclerosis. Endothelial dysfunction occurs early in the development of plaque and is systemic in nature, afflicting vessels throughout the arterial circulation without gross evidence of atherosclerotic plaque formation. Emerging data indicate that proatherosclerotic genes are upregulated and antiatherosclerotic genes are downregulated in areas of turbulent blood flow, as seen at branch points of arteries,²¹ resulting in vascular adhesion molecule expression and recruitment of monocytes.²² The atherosclerotic plaque initially may expand outward rather than inward into the vessel wall, making some significant lesions difficult to visualize by angiography. The components of the mature atherosclerotic lesion include smooth muscle cells, macrophages, T lymphocytes, and calcification, in addition to accumulation of lipoproteins.²³ Neutrophils and mast cells also are implicated in the atherosclerotic process.²² Later in the process, increased activity of matrix metalloproteinases in the atherosclerotic cap predisposes to plaque rupture or ulceration, resulting in tissue factor (TF) exposure and platelet adhesion, culminating in thrombus formation.²⁴ The thrombus may undergo endogenous fibrinolysis with plaque healing or become occlusive and produce organ damage (e.g., myocardial infarction [MI]). In severe lesions, lamellar bone, presumably from endochondral calcification, may appear.²⁵ There is evolving evidence that extracellular vesicles (EVs), also known as microparticles, are involved in the atherosclerotic process.²⁶ The following sections describe in detail the major manifestations of endothelial dysfunction that occur early in the atherosclerotic process.

Abnormal Vascular Tone

The importance of the endothelium in maintaining vascular tone was first recognized when endothelial cells of rabbit aorta were inadvertently removed and resulted in paradoxical vasoconstriction after administration of acetylcholine.²⁷ The major endothelium-dependent vasodilator normally produced was found to be nitric oxide (NO), a free radical gas with multiple physiologic properties,²⁸ including inhibition of platelet aggregation and inflammation and stimulation of angiogenesis. Numerous studies indicate that the endothelium does not vasodilate appropriately in the setting of traditional and emerging cardiovascular risk factors. Cardiovascular risk factors are thought to reduce NO availability through a variety of mechanisms, including increased oxidative stress, through generation of reactive oxygen species, and in so doing create an environment conducive to development of atherosclerosis.²⁹ Major sources of reactive oxygen species are nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs). The catalytic subunits of the NOXs are the NOX proteins and are found in atherosclerotic lesions.³⁰ A reduction in NO synthesis is thought to occur because of decreased availability of tetrahydrobiopterin, an essential cofactor for synthesis of NO.³¹ Administration of sepiapterin, a substrate for tetrahydrobiopterin, improves endothelial dysfunction.³² Also, recent evidence indicates that the transcription factor p53 and the adaptor protein Shc both play essential roles in impairing endothelium-dependent vascular relaxation.³³ High cholesterol levels are thought to produce oxygen free radicals that may inactivate NO. NO synthases are the enzymes responsible for converting l-arginine to NO (Fig. 134–2) (Chap. 115). The enzyme may be perturbed by modified low-density lipoprotein (LDL), resulting in decreased NO production. Supplementation of the diet with l-arginine leads to improvement in endothelial-dependent vasodilation.³⁴ Elevated levels of asymmetric dimethylarginine, an endogenous competitive inhibitor of NO synthase, found in patients with hypercholesterolemia and diabetes, also may result in decreased NO availability.³⁵ Oxidized LDL is thought to increase the elaboration of asymmetric dimethylarginine by endothelial cells and decrease its degradation by the enzyme dimethylarginine dimethylaminohydrolase.³⁶ Administration of acetylcholine to patients with elevated serum LDL³⁷ and relatively low high-density lipoprotein (HDL)³⁸ may result in abnormal vasoconstriction, which can be reversed with nitroglycerin (an endothelium-independent vasodilator).³⁹ Intravenous infusion of HDL improves endothelial-mediated vasodilation through improved NO availability.⁴⁰ The decreased vasodilatory capacity because of dyslipidemia may facilitate the development of coronary ischemia.

Impaired endothelial vasodilation is noted with advanced aging,⁴¹ when the hands are exposed acutely to cold, and during mental stress. The impairment may be mediated by increased production of endothelin, a potent vasoconstrictor.⁴² Sex differences are also seen in endothelial function as women in middle age tend to have more endothelial vasodilation than men at any age.⁴³ Proinflammatory cytokines may induce formation of endothelial-derived EVs making them a surrogate marker for endothelial dysfunction⁴⁴ and a biomarker for cardiovascular events.⁴⁵ Infection with concomitant inflammation is associated with impaired endothelial vasodilation. For example, repeated infection with Chlamydia pneumoniae results in endothelial dysfunction via impaired NO availability.⁴⁶ The combination of coronary artery disease and elevated serum levels of high-sensitivity C-reactive protein (hsCRP) is an independent predictor of abnormal endothelial vasoreactivity.⁴⁷ External radiation therapy also results in endothelial dysfunction and may explain the increased risk of atherosclerosis in patients receiving mantle irradiation for Hodgkin lymphoma.⁴⁸

Endothelial Inflammation

The endothelium does not routinely interact with inflammatory cells but is poised to express adhesion molecules after stimulation with inflammatory mediators. An inflammatory response is thought to begin in the vessel wall after “invasion” of pathogenic lipoproteins.⁴⁹ The presence of lipoproteins, especially oxidized LDL, results in expression of adhesion molecules such as vascular cell adhesion molecule (VCAM)-1 on the luminal surface of endothelial cells, leading to adherence of monocytes (Fig. 134–3).⁵⁰ Endothelial cell expression of adhesion molecules and accumulation of monocytes can be regarded as endothelial dysfunction because these events may occur in the absence of morphologic changes in the vessel wall. Inflammation may develop without the demonstrable presence of an external microbial pathogen. It is now recognized that diseases with inflammatory component such as psoriasis and systemic lupus erythematosus (SLE) confer an increased risk for atherosclerosis and MI.⁵¹ The complex interactions of inflammation and the endothelium on the initiation and progression of atherosclerosis are reviewed in more detail in “Inflammation and Atherosclerosis” below.

Abnormal Control of Vascular Growth: Smooth Muscle Cells and Extracellular Matrix

Normal endothelium inhibits vascular smooth muscle cell proliferation.⁵² The specific function of vascular smooth muscle cells in atherosclerosis is unclear. However, evidence indicates that, in early atherosclerosis, vascular smooth muscle cells contribute to the development of atheroma through production of proinflammatory mediators such as CC chemokine ligand (CCL)2 (previously termed monocyte chemoattractant protein, or MCP-1) and VCAMs. Although smooth muscle cells primarily play a role in modulating vascular tone, they also are involved in the control of extracellular matrix formation and degradation through matrix modulators such as proteases, protease inhibitors, matrix proteins, and integrins (Fig. 134–4).

The importance of vascular smooth muscle cells in controlling the synthesis of matrix molecules is evident at the clinical level. They provide a thick, fibrous cap that promotes stability and inhibits plaque rupture and ulceration. Factor VII–activating protease, thought to play a role in coagulation and fibrinolysis, is also a potent inhibitor of vascular smooth muscle cell proliferation, and migration in vitro and local application of factor VII–activating protease (but not Marburg I variant) in animal models reduces neointima formation.⁵³ Furthermore, it has been localized to unstable atherosclerotic plaques and may contribute to plaque instability.

Evidence indicates that vascular smooth muscle cells that undergo apoptosis, especially at the shoulder region of the plaque, may create a more unstable cap.⁵⁴ Both intact vascular smooth muscle cells and fibroblasts are thought to stabilize plaques through modulation of extracellular calcification and formation of a fibrocalcific plaque.

Vascular smooth muscle cells arise primarily from the medial layer and are considered monoclonal in origin.⁵⁵ Evidence also indicates that vascular smooth muscle cells may originate from the adventitia.⁵⁶ The rate and timing of smooth muscle cell replication is unclear. It may occur at a constant low rate throughout the development of the atherosclerotic lesion or episodically at a higher rate. Animal studies indicate that new intimal cells may originate from outside the vessel wall from subpopulations of marrow- and non–marrow-derived circulating cells.⁵⁷ Smooth muscle progenitor cells circulating in blood may contribute to the arterial remodeling that occurs after angioplasty and after bypass graft surgery.⁵⁸

Vascular proliferation and inflammation are linked processes. Inflammation-induced impaired NO bioactivity contributes to vascular smooth muscle proliferation.² Overexpression of NO synthase results in reduction of atherosclerotic or restenotic lesion formation in rabbits through both inhibition of vascular smooth muscle cell proliferation and inhibition of adhesion and chemoattractant molecule expression, with subsequent reduction of vascular mononuclear cell infiltration.⁵⁹ Thus, the vascular smooth muscle cell participates in the atherosclerotic process by affecting lipoprotein retention, modulating inflammation, and controlling plaque stability through formation of the fibrous cap. Several vascular disorders involve vascular smooth muscle proliferation as the primary pathophysiologic mechanism, including in-stent restenosis, transplant vasculopathy, and vein bypass graft failure.⁶⁰ The control of smooth muscle proliferation is thought to involve NR4A nuclear receptors that are expressed in atherosclerotic lesion macrophages, smooth muscle cells, and endothelial cells, and are induced by atherogenic stimuli. Inhibition of the transcriptional activity of the NR4A nuclear receptors results in enhanced smooth muscle proliferation.⁶¹ The NR4A nuclear receptors are also expressed in vein segments exposed to arterial pressure and it is postulated that they are responsible for an inhibitory feedback mechanism that occurs in activated vascular cells. Drug-eluting vascular stents that release agents such as sirolimus and paclitaxel interfere with the cell cycle and inhibit restenosis in part via decreased smooth muscle cell proliferation.⁶²

Abnormal Endothelial Control of Blood Fluidity

Endothelial cells normally elaborate a number of antithrombotic substances. Some of these substances are released into blood whereas others are properties of the unactivated endothelial cell surface. These antiplatelet, anticoagulant, and profibrinolytic activities of endothelium, some of which also possess vasodilatory properties (e.g., prostacyclin, NO), act in concert to promote blood fluidity under normal circumstances. Acute activation or chronic dysfunction of endothelial cells alters the hemostatic balance, transforming them from predominantly antithrombotic to prothrombotic cells.⁶³

To this end, endothelial cells modulate the activities of thrombin in health and disease. In the presence of intact and normally functioning endothelium, the prothrombotic actions of thrombin are quenched and the antithrombotic actions of the enzyme predominate. Thrombin binds to thrombomodulin, an integral membrane protein expressed by endothelial cells, and activates protein C (accelerated in the presence of endothelial protein C receptor, another endothelial cell protein) (Chap. 116). Activated protein C, in concert with its cofactor, protein S, has anticoagulant and profibrinolytic actions. It degrades by proteolytic digestion factors Va and VIIIa, and inactivates plasminogen-activator inhibitor (PAI)-1. Simultaneously, by binding to thrombomodulin, enzymatically active procoagulant thrombin is removed from the circulation, thereby limiting its availability to catalyze fibrin formation. Endothelial dysfunction causes loss of thrombomodulin activity from the vascular surface. In fact, increased circulating plasma levels of free (truncated) thrombomodulin represent a marker of endothelial damage. In addition to the role of thrombomodulin in clearance of circulating thrombin, the procoagulant activity of thrombin is normally blocked by endothelial cells through the action of antithrombin, which binds to heparin-like glycosaminoglycans on their luminal surface, thereby catalyzing the inactivation of thrombin by antithrombin. Like thrombomodulin, this thrombin-neutralizing action of endothelial heparan sulfate glycosaminoglycans is lost with endothelial dysfunction.

Endothelial cells do not normally express TF, but they do so upon activation by inflammatory cytokines or exposure to endothelium-activating levels of homocysteine or free thrombin. The procoagulant effects of expression of TF by dysfunctional endothelial cells are potentially compounded by the loss of TF pathway inhibitor (TFPI), which normally is synthesized by endothelial cells. Studies show that monocyte-derived EVs (or microparticles) express TF and platelet-derived EVs express phosphatidylserine and thus support coagulation complex formation.⁶⁴ There is also evidence that P-selectin on platelets binds to P-selection protein ligand-1 on EVs.⁶⁵

Normal endothelium is profibrinolytic. It synthesizes and releases tissue-type plasminogen activator (t-PA); it possesses binding sites for t-PA and plasminogen to provide a surface for the concentrated assembly of the fibrinolytic complex and thereby enhance local plasmin generation; and it fails to produce significant amounts of PAI-1. This profibrinolytic state is converted to an antifibrinolytic state in the presence of endothelial dysfunction. In activated or dysfunctional endothelium, PAI-1 gene expression and PAI-1 secretion are induced; simultaneously, the profibrinolytic properties of normal endothelium are lost (Chap. 135).

The antithrombotic profile of normal endothelium also manifests through the elaboration of several antiplatelet substances. NO is constitutively released into blood by normal endothelial cells and inhibits platelet adhesion and aggregation by stimulating platelet soluble guanylyl cyclase and raising intraplatelet levels of cyclic guanosine monophosphate (see Fig. 134–2).⁶⁶ Physiologic flow and shear forces maintain the activity of endothelial (endothelium-derived) NO synthase (eNOS)⁶⁷ under normal circumstances. Vascular cell-derived carbon monoxide, a product of heme catabolism by heme oxygenase, may have similar antiplatelet activity.^68,69 Prostacyclin (prostaglandin I₂) likewise is released basally by normal endothelial cells and inhibits platelet aggregation by inducing platelet adenylyl cyclase and raising intraplatelet levels of cyclic adenosine monophosphate.⁷⁰

NO, carbon monoxide, and prostaglandin I₂ are labile autacoids, acting only in the immediate vicinity of their release into blood from endothelial cells. An endothelial surface ecto-adenosine diphosphatase (CD39) also blocks platelet activity by metabolizing and disposing of platelet agonist adenosine diphosphate (ADP).⁷¹ In endothelial dysfunction, these various antiplatelet activities are lost, and endothelial release of von Willebrand factor (VWF) is increased, which promotes platelet adhesion. In the case of NO, oxidative stress in the microenvironment of endothelial dysfunction actually “uncouples” eNOS activity^68,72 to preferentially generate superoxide over NO. Oxygen free radicals bind any remaining available NO to produce the toxic product peroxynitrite. Bioactive NO is further reduced in endothelial dysfunction by the presence of asymmetric dimethylarginine, which competes to block eNOS and limit NO production.^67,73

Progenitor Cells and Atherosclerosis

Endothelial progenitor cells (EPCs) are heterogenous in origin and participate in endothelial cell regeneration and neovascularization of ischemic tissue. The mobilization of EPCs from the marrow is stimulated by hypoxia, cytokines such as vascular endothelial growth factor, hormones such as erythropoietin, and statin drugs, whereas mobilization is inhibited in the diabetic state.⁷⁴ The role of EPCs in atherosclerosis is unclear as there are conflicting data.²² A study in apolipoprotein (apo) E–/– mice showed that there is rapid turnover of endothelial cells in atherosclerosis-prone areas and marrow derived EPCs are recruited to sites of atheroprogression.⁷⁵

INFLAMMATION AND ATHEROSCLEROSIS

Innate Immunity and Atherosclerosis

The endothelial response to injury manifests as a chronic inflammatory response that involves both innate and adaptive immunity.⁷⁶ Innate immunity provides the first line of defense for the host and involves several cell types, most importantly macrophages and dendritic cells, which express a limited number of highly conserved sensing molecules such as scavenger receptors and toll-like receptors.^76,77 Microbial infection can be detected by pathogen-associated molecular patterns, which are present in bacteria, viruses, and yeasts, but not in mammalian cells, and are recognized by the toll-like receptors.⁷⁸ Ligation of a pathogen or other substances containing pathogen-associated molecular patterns (such as lipopolysaccharides, aldehyde-derivatized proteins, mannans, teichoic acids) elicits endocytosis or activation of endothelial cells (e.g., through nuclear factor-κB) that results in an inflammatory response (Chaps. 17 and 18).^77,79 Proinflammatory cytokines, such as tumor necrosis factor, nuclear factor-κB and interleukin (IL)-1, magnify the innate inflammatory response.

Innate defense involves soluble factors, such as complement, which is involved in atherosclerotic lesion formation. hsCRP has been found to be an important and independent predictor for cardiovascular events.⁸⁰ Natural antibodies that are generated in the absence of known antigen stimulation, mainly immunoglobulin (Ig) M, provide an immediate response against bacteria and viruses but also may be involved in atherosclerosis. For example, innate B lymphocytes, the so-called B1 cells, express a restricted set of germline–encoded antigen receptors that may bind oxidized LDL.

Adaptive Immunity and Atherosclerosis

Compared to innate immunity, adaptive immunity is slower but more precise (Chap. 77).⁷⁶ T cells can be activated by dendritic cells and macrophages, whereas most antigens cannot stimulate B cells without assistance from CD4+ T cells, which recognize the peptide–major histocompatibility complex (MHC) complexes on B cells. By genetic recombination, the number of T-cell and B-cell receptors that can be formed is almost unlimited and far exceed the number of pattern recognition receptors used by the innate immune system. Most CD4+ cells are cytokine-secreting T-helper (Th) cells and express αβ–T-cell receptors, which interact with MHC class II molecules. A smaller number of Th cells express γδ–T-cell receptors, which interact with the nonpolymorphic, nonclassic MHC molecules, CD1, which present certain antigens (particularly lipids and glycolipids). Th cells are classified according to the cytokines they secrete. Th1 cells secrete interferon (IFN)-γ and IL-2 and promote cell-mediated immunity (Chap. 78). Th2 cells secrete IL-4, IL-5, IL-10, and IL-13, and help B cells produce antibodies. CD8+ T cells are primarily cytotoxic killer cells, although they can secrete cytokines, such as tumor necrosis factor-α, IFN-γ, and lymphotoxin. Some thymus-independent antigens can activate these cells without the help of T cells. Oxidized LDL is considered such an antigen because it expresses multiple copies of oxidation-specific epitopes on a single LDL particle.

Adhesion Molecules and Atherosclerosis

Monocyte recruitment to inflammatory foci initially involves the expression of endothelial cell selectins, which mediate monocyte rolling on the endothelium (see Fig. 134–3). The rolling phenomenon is followed by a firmer attachment to endothelial cells mediated by integrins. Perhaps the most important of these is VCAM-1, which is upregulated in cultured endothelial cells in the presence of oxidized LDL. The appearance of this molecule before the development of grossly visible atherosclerotic lesions supports oxidized LDL as an initial recruiter of macrophages. The finding of reduced atherosclerosis in VCAM-1–deficient mice further supports the important role of macrophages and VCAM-1 in the pathogenesis of atherosclerosis.^81,82 Other adhesion molecules, such as P-selectin and intracellular cell adhesion molecule-1, also may be involved in monocyte adhesion at sites of lesion formation.⁸³ The entry of monocytes into the vascular intima leads to their differentiation into resident macrophages. Here they take up cholesterol that has also accumulated in the vascular intima, thereby becoming cholesterol-engorged foam cells.⁸⁴ It has been demonstrated that the accumulation of macrophage foam cells in established atherosclerotic lesions actually originates mainly from the proliferation of macrophages within the lesion rather than from the recruitment of circulating monocytes.⁸⁵ Platelet-derived EVs contain and deliver the chemokine “regulated upon activation, normal T-cell expressed and secreted” (RANTES) to activate endothelium in atherosclerosis to promote attraction of monocytes.⁸⁶

Lipoprotein Phospholipase A₂ and Atherosclerosis

Lipoprotein phospholipase A₂ (Lp-PLA₂) is an inflammatory enzyme belonging to the large family of phospholipases that are capable of hydrolyzing the sn-2 ester bond of phospholipids of cell membranes and lipoproteins.⁸⁷ This enzyme, produced by macrophages, circulates bound to LDL and in the intimal space of the artery can produce oxidized fatty acids and lysophosphatidyl choline. These molecules have a range of potentially atherogenic effects, including chemoattraction of monocytes, increased expression of adhesion molecules, and inhibition of endothelial NO production.⁸⁸ Although originally designated as platelet-activating factor acetylhydrolase because of its ability to degrade platelet-activating factor, the clinical importance of this effect is not thought significant. Numerous epidemiologic studies show that Lp-PLA₂ is a significant biomarker associated with cardiovascular events. The selective inhibition of Lp-PLA₂ with the drug darapladib reduced development of advanced coronary atherosclerosis in diabetic and hypercholesterolemic swine.⁸⁹ A phase 2 clinical study of patients with cardiovascular disease showed that sustained inhibition of plasma Lp-PLA₂ activity with background of intensive atorvastatin therapy resulted in reduction in IL-6 and hsCRP after 12 weeks of darapladib 160 mg, suggesting a possible reduction in inflammatory burden.⁹⁰ However, a prospective double-blind phase III trial of 15,828 patients, darapladib did not reduce the composite end point of cardiovascular death, MI, or stroke. There was a significant reduction in major coronary events and total coronary events.⁹¹

Immune Cells and Atherosclerosis

Macrophages are essential for the clearance of modified lipoproteins and the efflux of lipoprotein-derived cholesterol to HDL receptors for reverse cholesterol transport, the process by which HDL removes cholesterol from cells. Multiple lines of evidence indicate that macrophages promote lesion initiation and progression. For example, hypercholesterolemic mice become markedly resistant to atherosclerosis if they are bred to macrophage-deficient animals.⁹²

The earliest grossly visible sign of atherosclerosis is the fatty streak, which is composed mainly of macrophage foam cells containing relatively large amounts of cholesterol. Foam cells also can derive from smooth muscle cells, as these cells can express scavenger receptors when appropriately activated.^93,94 Formation of the fatty streak is thought to begin with adherence of circulating monocytes to activated endothelial cells at sites in the arterial system prone to atherosclerotic disease, such as at branch points in vessels. Multiple chemoattractant molecules have been identified in these nascent lesions, which recruit monocytes and induce their diapedesis into the subendothelial space where they further differentiate into macrophages. As noted above, however, more recent evidence indicates that microenvironment-supported macrophage proliferation in situ within the atherosclerotic lesion is likewise a key event in atherogenesis.⁸⁴

The chemoattractant CCL2 (MCP-1) facilitates recruitment of monocytes to atherosclerotic lesions, as noted in studies of mouse models of atherosclerosis, such as apoE–/– or LDL receptor (LDLR–/–)–deficient mice fed a Western-style diet. When these mice are crossed to the model lacking CCL2, or its receptor CCR-2, lesion development decreases significantly.^95,96,97

Macrophages and T cells were once thought to be the only inflammatory cells to significantly promote angiogenesis. More recent data showing that neutrophils are found at sites of plaque rupture or erosion and in thrombus from patients with acute coronary artery syndromes indicate that they also have an important role in atherothrombosis.¹⁸ Mast cells have been found in the adventitia of lesions and in areas of plaque hemorrhage; they have been implicated in macrophage apoptosis, increased vascular permeability, degradation of HDL and reduced cholesterol efflux.²² Neutrophils and mast cells are recruited to atherosclerotic lesions in response to CXC-chemokine receptor 2 (CXCR2) signals. The mobilization of neutrophils to atherosclerotic lesions is inhibited by CXC-chemokine receptor 4 (CXCR4) and its ligand CXC-chemokine ligand 12 (CXCRL12; also known as stromal-derived factor (SDF) 1. A subset of CD31+ T cells, so called T_ang cells, was shown to promote endothelial repair and revascularization, and to be inversely correlated with age and cardiovascular disease (CVD) risk in patients undergoing coronary angiography.⁹⁸

Lipid Peroxidation and Atherosclerosis

Macrophages control the amount of cholesterol loading by downregulating the native LDL receptor. Therefore, knowing how cholesterol is taken up into macrophages is important. Cell culture experiments revealed a “foam cell paradox,” in which macrophages engulf only modified lipids. Treatment of native LDL with copper or acetic anhydride (causing acetylation) led to increased LDL uptake through use of the scavenger receptor, leading to the formation of lipid-laden macrophages. These experiments led to the peroxidation theory of atherosclerosis,⁹⁴ whereby LDL modification is an essential step in the development of foam cells. Although the precise mechanisms responsible for LDL oxidation remain unclear, enzymes including myeloperoxidase, inducible NO synthase, and NADPH oxidases are involved in the process.^99,100 Of note, macrophages express each of these enzymes, which normally are used as antimicrobial reactive oxygen species essential for innate immunity.¹⁰¹ Thus, accumulation of cholesterol in the macrophage occurs via scavenger (not LDL) receptors of oxidized (and not native) LDL. Myeloperoxidase is an enzyme thought to cause lipid peroxidation in the intimal space and circulating levels are associated with adverse clinical outcomes in the setting of acute coronary syndromes and predictive of major adverse cardiovascular events.⁹⁹

Scavenger Receptors and Atherosclerosis

Conserved pattern recognition receptors expressed by macrophages include scavenger receptors A and B1 and CD36, all of which internalize oxidized LDL.^102,103 Macrophages express various genes in response to oxidized LDL, including peroxisome proliferator-activated receptor-γ and adenosine triphosphate–binding cassette transporter A1, which profoundly influence macrophage-mediated inflammation and atherosclerotic activity.

Cell culture studies indicate that scavenger receptor A recognizes acetylated LDL but, unlike the LDL receptor, is not downregulated in response to increased cholesterol content and thus likely accounts for foam cell formation.¹⁰⁴ However, no evidence indicates that acetyl LDL is generated in vivo, indicating other modifications of LDL, such as oxidation, may be required for foam cell formation.^105,106 Another scavenger receptor presumed to be involved in the atherosclerotic process is CD36, a receptor that avidly binds oxidized LDL.

Circulating IgG and IgM antibodies against products of lipid peroxidation are present in the plasma of animals and humans.¹⁰⁷ These antibodies closely correlate with measures of lipid peroxidation and with atherosclerotic progression and regression in murine models.¹⁰⁸ Immunization of hypercholesterolemic rabbits and mice with products of oxidized LDL, such as malonyldialdehyde LDL or copper-oxidized LDL, inhibits the progression of atherosclerotic lesion formation.^{109,110,111,112} These experiments have been interpreted to indicate that an immunologic response to oxidized LDL components can alter the atherosclerotic process.

Leukocyte-derived 5-lipoxygenase also contributes to atherosclerosis susceptibility in mice.¹¹³ Animal studies indicate the importance of lipoxygenases in atherosclerosis as disruption of the 12,15-lipoxygenase gene diminishes atherosclerosis in apoE-deficient mice, and overexpression of 15-lipoxygenase in vascular endothelium accelerates early atherosclerosis in LDL receptor-deficient mice.^114,115 This enzyme is under study as a potential target to inhibit the atherosclerotic process.¹¹⁶

Gut Microbiome There are newer data indicating that intestinal microbes are involved in cardiometabolic diseases.¹¹⁷ Systemic inflammation is activated is the setting of chronic bacterial translocation (secondary to increased intestinal permeability) leading to macrophage influx into adipose tissue resulting in insulin resistance and nonalcoholic fatty liver disease. The increased inflammation may also be secondary to trimethylamine-N-oxide via influx of macrophages and cholesterol accumulation via upregulation of macrophage scavenger receptors and reduction in reverse cholesterol transport. Thus, gut microbiota may accelerate atherosclerosis risk.

Accumulation of Low-Density Lipoprotein in the Vascular Wall

Three potential factors lead to accumulation of LDL in the vascular wall: increased permeability of the endothelium, prolonged retention of lipoproteins in the intima, and slow removal of lipoproteins from the vessel wall.¹¹⁸ Rabbits fed a high-cholesterol diet develop aortic wall lesions at specific lesion-susceptible sites; however, endothelial permeability is not increased at those sites, indicating that LDL is selectively retained in these regions.^119,120 Retention of LDL molecules likely results from their adherence to proteoglycans in the vessel wall.¹²¹ LDL genetically engineered to not bind to proteoglycans is hypothesized to be less atherogenic than native LDL.²⁰

Oxidized LDL and its products, oxidized phospholipids and oxysterols, have other properties that make them potentially proatherogenic.¹²² These properties include proinflammatory characteristics, such as chemotactic signaling for monocytes, smooth muscle cells, and T lymphocytes (but not for B lymphocytes or neutrophils, neither of which is found in lesions) and increased expression of VCAM-1 on, and stimulation of CCL2 release from, endothelial cells.¹²³ Oxidized LDL also may contribute to instability of the atherosclerotic plaque via induction of type 1 metalloproteinase expression and increase in TF activity.¹²⁴ For oxidized LDL to be a ligand for the scavenger receptor, extensive degradation of the polyunsaturated fatty acid in the sn-2 position of phospholipids by oxidation is essential.

To test the oxidized LDL hypothesis, several clinical studies have been conducted using antioxidant vitamins, most commonly vitamin E; however, most of the completed studies gave negative results.^125,126 At the present time, treatment with vitamin E does not appear to be beneficial in preventing cardiovascular events.

High-Density Lipoprotein and Atherosclerosis

A low level of HDL cholesterol is a strong predictor of adverse cardiovascular events, presumably because the low level is associated with insufficient reverse cholesterol transport. Animal studies using liver-directed gene transfer of human apoA–apoI resulted in significant promotion of reverse cholesterol transport and regression of preexisting atherosclerotic lesions in LDL receptor-deficient mice.^127,128 The HDL level may not be as important as amount of reverse cholesterol transport. For example, the capacity of HDL to accept cholesterol from macrophages is predictive of atherosclerotic burden.¹²⁹ However, HDL has additional antiatherogenic properties that may confer protection against atherosclerosis.¹³⁰ For example, HDL is protective against oxidation of LDL, at least in part because of paraoxonase, an enzyme physically associated with HDL that degrades organophosphates.¹³¹ Paraoxonase polymorphisms are associated with increased risk of CVD, also indicating that oxidized LDL is an important factor in atherosclerotic development.¹³²

Research studies currently are evaluating novel ways to increase HDL levels or to use apoA–apoI variants and mimetics that hopefully will cause regression of atherosclerosis. So far, initial clinical studies were not successful. Cholesteryl ester transfer protein promotes the transfer of cholesteryl esters from antiatherogenic HDLs to proatherogenic apoB-containing lipoproteins, including very-low-density lipoproteins (VLDLs), VLDL remnants, intermediate-density lipoproteins, and LDLs. A deficiency of this molecule results in increased HDL levels and decreased LDL levels, a lipid profile that is antiatherogenic. A large clinical study in humans showed that inhibition of the transfer protein with torcetrapib increased HDL levels but was associated with increased mortality and hypertension.¹³³ It is supposed that the increase in mortality was a result of an off-target effect of the drug increasing blood pressure and not because of cholesterol ester transfer protein inhibition. Clinical trials evaluating the effect of other inhibitors of cholesteryl ester transfer protein on atherosclerosis and cardiovascular events have not shown clinical efficacy thus far. Along similar lines, delivery of a mutant form of apoA1 (apoA1 Milano) resulted in regression of plaque size as measured by intravascular ultrasound in a small phase II clinical trial.¹³⁴ However, studies evaluating the effect of apoA1 mimetics on atherosclerosis also are have not shown compelling data regarding reducing plaque size or cardiovascular events.¹³⁵

CD40, CD40 Ligand, and Atherosclerosis

Studies indicate that human atherosclerotic lesions express the immune mediator CD40 and its soluble ligand, sCD40L. Increasing evidence indicates that the CD40–sCD40L signaling pathway plays a central role in several inflammatory processes, including atherosclerosis and graft rejection following transplantation.¹³⁶ Interruption of CD40 signaling in hyperlipidemic mice reduces the size of aortic atherosclerotic lesions and their lipid, macrophage, and T-lymphocyte content.¹³⁷ Atorvastatin, lovastatin, pravastatin, and simvastatin reduce IFN-γ–induced CD40 expression in a dose-dependent manner. Activation of atheroma-associated cells with human recombinant sCD40L is reduced when cells are treated with statins. In addition, retrospective ex vivo immunostaining of human carotid atherosclerotic lesions of patients treated with simvastatin for more than 3 months revealed less CD40 expression and atheroma-associated cells compared with patients who were not treated with the drug. A reduction in sCD40L is associated with pravastatin or cerivastatin therapy.¹³⁸ These findings support the notion that statins are antiinflammatory, in addition to their cholesterol-lowering effects.

Transforming Growth Factor-β and Atherosclerosis

Transforming growth factor (TGF)-β is a cytokine secreted by macrophages, smooth muscle cells, and the Th3 subset of Th cells that has multiple regulatory functions. TGF-β is speculated to contribute to plaque stabilization because it stimulates collagen synthesis and is fibrogenic. One study found that inhibition of TGF-β signaling by neutralizing antibodies led to a larger plaque size with an unstable phenotype.¹³⁹ Further studies are needed to clarify the role of TGF-β in atherosclerotic plaque initiation and growth.

ABO Blood Type and Cardiovascular Risk The ABO blood group is determined from presence of A and B antigens on the surface of the red blood cells and is thought to confer cardiovascular risk.¹⁴⁰ These antigens are also expressed on the surface of platelets and the endothelium and consist of terminal carbohydrate molecules which are synthesized by the sequential action of the ABO glycosyltransferases. Genetic studies showed that carriers of single nucleotide polymorphisms (SNPs) that mark non-O blood group types have higher levels of plasma VWF when compared to O individuals. Epidemiologic and genetic studies show that the non-O blood group is associated with adverse cardiovascular events. One study demonstrated that the SNP rs514659 was associated with coronary artery diseases (CADs) when complicated by MI but not with CAD without MI, suggesting that the primary relationship of ABO to clinical CAD is through modulation of coronary thrombosis or plaque rupture in patients with established coronary atherosclerosis rather than through primary promotion of atherosclerosis per se.

Infection and Atherosclerosis

Several infectious agents have been implicated as pathogens in atherosclerosis.¹⁴¹ A well-studied infectious pathogen is C. pneumoniae. Animals infected with this agent develop atherosclerosis, and patients with CVD have higher titers of antibodies against this pathogen. Viruses, such as herpes simplex and cytomegalovirus, also are implicated in human atherosclerotic lesion formation. Gingivitis as a consequence of poor dental hygiene or smoking may lead to cellular immune activation and provoke atherosclerosis by cytokines or antibodies.¹⁴² Endogenous proteins, such as heat shock proteins, also are implicated in atherosclerosis. One study showed that progression of carotid disease correlated with antibodies against heat shock proteins 65 and 60.¹⁴³

Splenectomy and Atherosclerosis

The relationship between the immune system and atherosclerosis is complex, as evident from an animal study that showed that splenectomy of cholesterol-fed apoE–/– mice led to significantly increased atherosclerosis.¹⁴⁴ This proatherogenic effect was rescued by transfer of either purified B cells or T cells from the spleens of atherosclerotic apoE–/– donors. A long-term study of soldiers who underwent splenectomy after trauma found the soldiers had a twofold increased incidence of CAD, providing evidence that the spleen has antiatherogenic activity.¹⁴⁵ Further studies are needed to determine if splenectomy significantly impacts the atherosclerotic process, and if so, by what mechanism.

Genetics and Myocardial Infarction

Atherosclerotic disease is a complex human trait involving multiple genes and environmental factors. Through the study of linkage analysis of families and sibling pairs as well as candidate genes and genome-wide association studies, the genetic predisposition to MI is starting to be understood.¹⁴⁶ The clinical importance of this knowledge is the potential identification of markers of disease for risk prediction and potential intervention to lower the risk of atherosclerotic-based cardiovascular events.

The use of genome-wide linkage analyses of families or sib-pairs has identified chromosomal loci linked to or genetic variations in the arachidonic 5-lipoxygenase-activating protein gene (ALOX5AP)¹⁴⁷ and leukotriene A₄ hydrolase gene (LTA₄H).¹⁴⁸ The genes are both involved in inflammation-related pathway of leukotriene B₄ production. Interestingly, a small molecule inhibitor of ALOX5AP was shown to reduce leukotriene production and plasma levels of C-reactive protein.

Several association studies of unrelated individuals have identified genetic variations that confer susceptibility to atherosclerotic disease and cardiovascular events. Studies using genome-wide linkage analysis identified four SNPs on chromosome 9p21.3 that were associated with MI in white cohorts.^146,149 Other genetic polymorphisms also contribute to increased risk of CVD through a variety of mechanisms.¹⁵⁰

ATHEROSCLEROTIC PLAQUE

Plaque Classification

The American Heart Association classification of atherosclerotic plaques into types I through VIII is based on lesion composition and structure (Fig. 134–5).²³ Types I through III atherosclerotic plaques have foam cells organized in a fatty streak, ranging from those not visible on close examination (type I) to those that are apparent on examination (type III). Types I through III lesions are small and clinically silent, whereas types IV through VI lesions may obstruct the lumen and produce a clinical event. Type IV lesions contain a confluent pool of lipid and in most patients do not cause anginal symptoms because of the ability of the artery to remodel outward. Type V lesions contain a fibromuscular cap resulting from replacement of tissue disrupted by accumulated lipid and hematoma or organized thrombotic deposits. Type VI lesions involve thrombosis that may be either mural or obstructive. Of note, a type IV lesion may develop type VI changes without ever passing through a type V change and accumulating significant fibrous tissue. Plaques that are complex and primarily composed of calcium are type VII lesions or, if fibrous tissue predominates, are type VIII lesions.

Vulnerable Plaque and the Vulnerable Patient

The pathologic mechanisms responsible for converting chronic coronary atherosclerosis to an acute coronary event result, in part, from plaque disruption, a term that was synonymously used with plaque rupture.^151,152 The term vulnerable plaque was used by Muller and colleagues^153,154 to describe rupture-prone plaques as the underlying cause of most clinical coronary events. The current definition for “vulnerable plaque” includes all thrombosis-prone plaques and those with a high probability of undergoing rapid progression, thus becoming culprit plaques (Fig. 134–6).¹⁵⁵ Criteria for development of the vulnerable plaque have been proposed based on histopathologic study of culprit plaques (Table 134–2).¹⁵⁵ The major criteria involve the presence of active inflammation, a thin cap with large lipid core, endothelial denudation with superficial platelet aggregation, a fissured plaque, and stenosis greater than 90 percent. The minor criteria for a vulnerable plaque include superficial calcified nodule, glistening yellow plaque, intraplaque hemorrhage, endothelial dysfunction, and outward (positive) remodeling. Some studies indicate plaques that are heavily calcified and without a significant lipid core are more stable.^25,48

An important concept concerning plaque remodeling is that atherosclerotic plaques commonly grow outward (positive remodeling) before a luminal stenosis occurs. Therefore, a contrast dye coronary angiogram may underestimate the plaque burden in the vessel. Arterial thrombosis may result from plaque hemorrhage (majority of events) or occur in an area of endothelial denudation (30 to 40 percent) without breach of the intimal space.¹⁵⁶ Thrombosis has also been reported in plaques that have a superficial calcified nodule protruding into the lumen.¹⁵⁶ Most atherosclerotic plaques that underlie a fatal or nonfatal MI are, as shown by angiography, less than 70 percent stenosed.¹⁵⁷ Some patients have more than one vulnerable plaque, which underscores the importance of medical therapy in addition to coronary revascularization.¹⁵⁸ Several technologies are currently being tested to identify the location of the vulnerable plaque.¹⁵⁹ Hopefully these developing technologies will shed more light on the natural history of the vulnerable plaque and afford the ability to conduct studies using local or regional antiatherosclerotic therapy.

Because of the dynamic interaction of atherosclerotic plaque with circulating blood, the term cardiovascular vulnerable patient has been proposed to define subjects susceptible to an acute coronary syndrome (ACS) or sudden cardiac death based on atherosclerotic plaque or blood or myocardial vulnerability.¹⁶⁰ The vulnerable (thrombogenic) blood includes serum markers of atherosclerosis and inflammation, such as hsCRP, inflammatory cytokines (e.g., IL-6, sCD40L), EVs, and hypercoagulable factors. The blood markers of vulnerability that reflect the hypercoagulable state include those of the fibrinolytic system and platelets (Table 134–3).¹⁶¹ Patients may have an MI because of a nonfatal or fatal arrhythmia as a result of coronary atherosclerosis or other nonatherosclerotic disease, such as hypertrophic cardiomyopathy or right ventricular dysplasia. Thus, a vulnerable patient should be considered from the standpoint of the combined presence of a vulnerable atherosclerotic plaque, vulnerable blood (prone to thrombosis), and/or vulnerable myocardium (prone to life-threatening arrhythmia).

Arterial Thrombosis

Atherothrombosis refers to the occurrence of thrombosis upon atherosclerotic lesions,¹⁶¹ the typical setting for arterial thrombosis. It represents the acute event that converts chronic atherosclerosis—a silent, asymptomatic, progressive disease—into symptomatic, life-threatening clinical complications, including acute MI, stroke, and critical limb ischemia. The previous section described in detail the current concepts of the consecutive stages of atherosclerotic lesion development.

However, thrombosis is not simply the final occlusive event. It also contributes to atherosclerosis lesion development. Intraplaque hemorrhage and in situ thrombosis localizes thrombin activity within plaques. Thus, atheroma evolution is not only a proliferative process but also involves thrombosis.¹⁶²

Pathobiology of Arterial Thrombi

Fundamental pathologic and pathophysiologic distinctions exist between arterial and venous thrombi (Table 134–4). Arterial thrombi usually are occlusive in smaller arteries and arterioles. Nonocclusive mural thrombi often occur in the lumina of the heart chambers and large arteries, such as the aorta and the iliac and common carotid arteries. In any arterial vessel, however, thrombi develop almost invariably upon preexisting abnormal intimal surfaces, which typically are atherosclerotic lesions. Less commonly, arterial thrombosis is superimposed on other forms of vascular disease, such as vasculitis or traumatic injury.¹⁶³ Thus, in the high-flow and high-pressure arterial system, thrombi form in response to increased local shear forces and exposure of thrombogenic substances on damaged vascular surfaces. Arterial thrombi, referred to as white thrombi, are composed mainly of platelets and relatively little fibrin or red cells. Leukocytes are likewise actively recruited into growing, platelet-rich arterial thrombi.¹⁶⁴

Thus, at sites of atherosclerotic plaque rupture, circulating platelets are activated not only by thrombogenic substances exposed to them by a disrupted plaque but also directly by the locally increased shear forces the platelets encounter.⁸⁵ At any given point in the circulation, shear rates are maximal adjacent to the vessel wall (measured as “wall shear rates”), and they are minimal in the center of the vessel lumen where velocity of flowing blood is the greatest. Normally, wall shear rates are in the range of 300 to 800 s^–1 in large arteries, and they increase to about 500 to 1600 s^–1 in arterioles of the microcirculation. However, in pathologically stenotic vessels the wall shear rates can reach 10,000 s^–1 or even higher. Increased shear stress in the microenvironment of an atherosclerotic plaque is usually compounded by turbulent blood flow. These locally abnormal hemodynamic forces can directly activate platelets as they pass through the region. Disturbed flow can simultaneously cause localized endothelial dysfunction.¹⁶⁵

High shear stresses, especially in the presence of marked shear gradients around stenotic sites, are sufficient to cause the release of VWF from endothelial cells, and promote the unfolding and binding of VWF to its receptors on platelet surface glycoprotein (GP) Ib-V-IX. This interaction, which does not occur in the normal circulation, mediates the adhesion of platelets to the intimal surface and triggers GPIb-V-IX–dependent platelet thrombus formation. The mechanisms of arterial thrombogenesis are further elaborated below in the section on “Platelet Activation.”

In contrast, wall shear rates are much lower in the venous circulation where the hemodynamic forces are insufficient to directly activate platelets.¹⁶⁶ Venous thrombi are almost always occlusive and may form virtual casts of the vessel in which they arise. Unlike the setting for arterial thrombi, gross vascular damage generally is not found at sites of venous thrombosis. Any ultrastructural abnormalities of adjacent endothelium likely are the consequences rather than the causes of thrombus formation. Therefore, in the low-flow and low-pressure venous system, reduced blood flow (stasis) and systemic activation of the coagulation cascade play the primary pathophysiologic roles. Venous thrombi are composed predominantly of red cells enmeshed in fibrin and contain relatively few platelets; hence, they have been described pathologically as red thrombi.

The generalizations described are consistent with the following clinical observations: (1) hereditary hypercoagulable states (also called “thrombophilias”), characterized by chronic hyperactivity of the coagulation system, are primarily associated with venous rather than arterial thrombosis; and (2) anticoagulants that prevent fibrin formation (e.g., heparin, warfarin) are generally used to prevent venous thrombosis, whereas antiplatelet agents (e.g., aspirin) are more effective in preventing arterial thrombosis. The differences between arterial and venous thrombosis are not, however, absolute because both types of thrombi are composed of different amounts of platelets, fibrin, and leukocytes. In addition, all thrombi continually undergo propagation, organization, embolization, lysis, and rethrombosis, and this dynamic remodeling results in their constantly changing compositions.

Site-Specific Arterial Thrombosis

The model of atherothrombosis described is best characterized in coronary arteries. This pathophysiology may not be entirely applicable to arterial thrombosis at other sites. It cannot be assumed that the local determinants of thrombosis that are operative in the coronary arteries are identical to those encountered in the cerebrovascular and peripheral arterial circulations. Basic regional differences may involve (1) distribution and composition of atherosclerotic lesions, (2) variable local rheology, and (3) underlying vascular cell heterogeneity.

Atherosclerosis is highly localized within the systemic vasculature. Lesion formation particularly affects the carotid artery bifurcation, coronary arteries (especially the left coronary artery bifurcation), abdominal aorta (especially its posterior wall downstream of the renal arteries, but with little disease usually present in the upstream thoracic aorta), and profunda femoral arteries. These lesion-prone sites in the arterial circulation correspond to regions where wall shear stress is very low and may even oscillate between positive and negative directions (i.e., reversal of flow) during the cardiac cycle. A strong correlation exists between local hemodynamic conditions of low shear stress and the development of atherosclerotic plaque formation and intimal thickening.^167,168,169 However, as arteries become progressively diseased and as stenoses develop at these sites, local hemodynamics change. Stenotic flows are characterized by sharp increases in shear rate that achieve their peak just upstream of the stenosis throat, with development of intensive turbulence downstream of the stenosis. The mechanisms of platelet activation and accumulation that initiate arterial thrombosis at these high-shear sites are further described in “Platelet Activation” below.

Striking heterogeneity is seen in the composition of atherothrombotic plaques, even within the same individual. In addition to plaque composition, the basic structural differences between specific arteries contribute to differences in thrombogenic substrates that are exposed upon arterial injury. For example, carotid and iliac arteries contain relatively more elastic fibers and proportionately fewer smooth muscle cells than coronary arteries.¹⁷⁰ Furthermore, ACSs typically result from disruption of only modestly stenotic, lipid-rich plaques, whereas disruption-prone, high-risk plaques in the carotid arteries usually are severely stenotic. Thus, a proposed more appropriate term is high-risk plaque rather than vulnerable plaque (which connotes its composition) to define a disruption-prone or thrombosis-prone plaque in different parts of the circulation.¹⁷¹

The pathophysiology of arterial thrombosis at different sites in the circulation may also be determined in part by vascular bed-specific heterogeneity of endothelial and smooth muscle cells. Endothelial cell-derived anticoagulant and procoagulant activities are differentially expressed throughout the vascular tree. Endothelial cell heterogeneity throughout the circulation is a function of varying organ- and tissue-specific microenvironments, hemodynamic forces, and site-specific changes in epigenetic footprinting. The heterogeneity of endothelial cells and the vascular bed-specific signaling pathways that control endothelial gene expression have been considered to play an important role in the localization of arterial thrombosis.¹⁷² Heterogeneity of vascular smooth muscle cells likewise exists throughout the arterial tree. They vary in embryonic origin, sources of progenitors, and lineage. With subsequent development, they acquire various phenotypes that can be traced to preferential sites within vessel walls.¹⁷³

Less is known about the pathophysiology of cerebrovascular thrombosis, and even less about peripheral arterial thrombosis, than about coronary artery thrombosis. Future research in these areas should permit the development of more rational antithrombotic strategies in noncoronary artery thrombosis.

Overview of Arterial Thrombotic Process

Arterial thrombosis typically occurs in the presence of underlying atherosclerosis (hence the term “atherothrombosis”). Less frequently, however, it may also occur in nonatherosclerotic arteries, such as in the setting of vasculitis.

Atherothrombosis Disruption of an atherosclerotic plaque triggers an explosive cascade of events that results in the formation of a platelet-rich thrombus at the site of arterial injury.⁵⁷ Focal loss of the antithrombotic and the vasodilatory properties of endothelium is compounded by plaque rupture or erosion. These events induce the local activation of platelets and the coagulation system by exposure of blood to previously encrypted thrombogenic substances (e.g., subendothelial cells, such as smooth muscle cells and fibroblasts; subendothelial structures, such as collagen; and subendothelial prothrombotic substances, such as TF, from all of which flowing blood is normally insulated by the barrier of a healthy endothelial monolayer). The local milieu for thrombus formation is aggravated by focal vasoconstriction, rapidly increased shear forces, and platelet-mediated recruitment of leukocytes. Platelet and coagulation activation are inseparable, reciprocally self-amplifying processes. Activation of platelets generates procoagulant properties on their cell surfaces. Combined with non–platelet-dependent local activators of the coagulation cascade, platelet activation culminates in the formation of thrombin, which itself is a potent stimulus for further platelet activation. Superimposed on these local determinants of arterial thrombosis, the thrombotic process may be modulated by systemic, circulating factors. The factors include the systemic state of activation of platelets and coagulation, which may be governed by acquired or genetic factors and by hormonal influences (e.g., adrenergic state).

Arterial thrombi generally are localized to the site of acute vascular injury. They are prevented from extending beyond this site by the restoration of hemostatic balance that promotes blood fluidity along healthy endothelial surfaces immediately adjacent to the site of injury. Thrombus propagation may occur, however, through a bloodborne pool of thrombogenic substances that originate at the site of vascular injury and thrombosis. These substances can be in the form of platelets, leukocytes, red cells, sloughed endothelial cells, other cellular microparticles, and circulating active TF derived from leukocytes activated within the thrombus.^174,175 In fact, cellular microparticles¹⁷⁶ constitute the main reservoir of bloodborne TF, the principal initiator of coagulation.

Thrombus persistence within an artery depends on the local balance between prothrombotic, antithrombotic, and fibrinolytic factors. Ulcerated and thrombotic atherosclerotic plaques, particularly in the aorta, tend to persist or recur.¹⁷⁷ Atherosclerotic plaques of the aortic arch have been detected in almost one-third of patients with cryptogenic stroke. Although aortic arch atheroma are more frequent and more severe causes of cryptogenic stroke in individuals older than 55 years of age, patent foramen ovale (and presumably paradoxical embolism) are more strongly associated with cryptogenic stroke in those younger than 55 years of age.¹⁷⁸

Advances in the development of coronary stents have created a new form of arterial thrombosis that usually can only be prevented by administration of two different platelet inhibitors (e.g., aspirin in combination with a thienopyridine derivative, such as clopidogrel or prasugrel). Although drug-eluting stents that deliver sirolimus or paclitaxel have been successful in reducing the problem of restenosis that is caused by smooth muscle cell proliferation and intimal hyperplasia following the coronary intervention, they have actually increased the occurrence of “late stent thrombosis” compared to bare metal stents. This form of arterial thrombosis, which typically occurs after discontinuation of (dual) antiplatelet therapy, is probably caused by eluting drugs interfering with endothelialization of the stent surface.¹⁷⁹

Thrombosis in Nonatherosclerotic Arteries Thrombosis may occur in arteries that are affected by vasculitis.¹⁸⁰ As both SLE and atherosclerosis are immune-driven processes, it is to be expected that some patients with active SLE are more susceptible to accelerated atherosclerosis (and related atherothrombosis) resulting from autoantibody-mediated proatherogenic mechanisms.^181,182 However, even in the absence of underlying atherosclerosis, various types of arterial thrombosis can complicate active vasculitis. For example, patients with SLE may have MI with angiographically normal coronary arteries. Giant cell arteritis, which characteristically targets the extracranial carotid and vertebral arteries, leads to inflammation and necrosis of the arterial wall and subsequent arterial occlusions in a distribution that is quite different from that of atherosclerosis. Takayasu arteritis has an unusual predilection for the aortic arch and its branches, leading to panarteritis, medial layer enlargement with luminal narrowing, and sometimes thrombotic occlusion. Other types of vasculitis and autoimmune processes that may cause arterial thrombosis include polyarteritis nodosa, Behçet disease, and antiphospholipid antibody syndrome.

Arterial thrombosis in the absence of atherosclerosis is also seen with immune- and nonimmune disorders of platelets and/or the vascular endothelium, such as heparin-induced thrombocytopenia (with arterial thrombosis most commonly occurring in the aorta, iliofemoral arteries, as well as in cerebral and coronary arteries),¹⁸³ and in the myeloproliferative neoplasms (e.g., essential thrombocythemia, polycythemia vera; Chaps. 84,85,86).^184,185,186

Platelet Activation

Disruption of an advanced atherosclerotic plaque results in abrupt exposure of highly thrombogenic material to flowing blood. This process leads locally to both thrombin generation and platelet activation, which operate simultaneously in a mutually self-amplifying process. As noted above in the section on “Pathobiology of Arterial Thrombi,” plaque rupture and the development of new intimal surface irregularities also suddenly alter local rheologic characteristics, increasing local shear rates. Increased shear stress resulting from sudden changes in degree of stenosis following rupture is compounded by increased focal vasoconstriction induced by thrombin, thromboxane A₂, and other vasoactive substances released in the milieu of acute injury.

At high shear rates (>1000 s^–1), platelets must be initially tethered to the vascular surface through a shear-activated interaction between the platelet membrane GPIbα (of the GPIb–IX–V complex) and its adhesive ligand, VWF.^187,188,189 Platelet adhesion also involves collagen binding to platelet collagen receptors (integrin α₂β₁ and GPVI). Other matrix constituents that become exposed to platelets and serve as adhesive ligands include fibronectin, laminin, fibrinogen, and fibrin. These initial adhesive interactions induce intracellular signaling pathways that activate platelets. High shear stress also activates platelets both directly,¹⁹⁰ as noted previously, and by lowering the threshold of platelet activation by chemical agonists to which platelets are simultaneously exposed in the microenvironment of the arterial thrombus.¹⁹¹ Thus, following adhesion, platelets are explosively activated by several interacting pathways: (1) intracellular signaling initiated by the adhesion event itself, (2) direct action of locally increased shear stress, and (3) agonists released (e.g., ADP, thromboxane A₂) and generated (e.g., thrombin) at the site of vascular injury.

Finally, the occlusive arterial platelet thrombus is created by the aggregation of platelets. This process is mediated by several alternative ligands (VWF, fibrinogen, fibronectin) that bind to their activated receptors in the platelet integrin α_IIbβ₃ complex. Stability of the platelet aggregate is conferred by additional ligand–receptor interactions, including CD40L binding to integrin α_IIbβ₃.¹⁹² Platelet thrombus stabilization is designed to counteract shear forces that promote not only the formation of arterial thrombi but also their embolization.

The importance of the inflammatory component of arterial thrombosis,¹⁶⁴ which is characterized by complex interactions among leukocytes, endothelial cells, and platelets, is increasingly being recognized. Activated platelets recruit leukocytes to the site of vascular damage, promoting their adhesion to endothelium and their activation on endothelium-bound chemokines. In fact, the presence of leukocytosis in myeloproliferative neoplasms is a better predictor of pathologic thrombosis than the platelet count.

Tissue Factor and Phospholipids

Tissue factor is a cell-surface–bound transmembrane protein that normally is not exposed to circulating blood. When expressed, TF initiates coagulation by binding to factor VIIa and activates factors IX and X, thereby triggering the common pathway of coagulation and the formation of thrombin. Strong evidence indicates that TF is the principal thrombogenic factor in the lipid-rich core of atherosclerotic plaques. While much of this TF is associated with monocytes/macrophages and vascular smooth muscle cells, more recent studies suggest that TF-positive microparticles are the most abundant sources of TF in atherosclerotic plaques. The main inhibitor of TF-mediated coagulation is TFPI. In atherosclerotic plaques TFPI colocalizes with TF and therefore may play an atheroprotective role.^162,193

Upon rupture of the atherosclerotic plaque, exposure of vascular TF to flowing blood initiates the coagulation cascade. Coagulation reactions are accelerated on the surfaces of activated platelets, microparticles, and on other activated cells in the microenvironment of vascular injury. The surfaces of these activated cells express anionic phospholipids, particularly phosphatidylserine. Apoptotic cells, with which advanced lesions are enriched, likewise translocate phospholipids from the inner to the outer leaflet of the cell membrane.¹⁹⁴ Plasma lipoproteins can provide a phospholipid surface for the assembly of enzymatic complexes of the coagulation cascade; in particular, oxidized LDL, LDL, and VLDL have procoagulant effects.^195,196 In contrast, HDL has multiple antithrombotic actions, including suppression of the coagulation cascade, stimulation of fibrinolysis, and stimulation of endothelial cell release of prostacyclin and NO, which are inhibitors of platelet activation.¹⁹⁷

Arterial thrombosis is triggered by the acute exposure of circulating blood to TF and anionic phospholipids, leading to explosive thrombin formation. Thrombin, a potent platelet agonist, further fuels the platelet activation process described in the previous section. These reactions create a self-amplifying process that is tightly localized to the site of vascular injury. The arterial thrombus is further contained to this site by the restoration of normal, antithrombotic endothelium in adjacent areas of the vessel wall.

Systemic Factors

As described above in “Overview of Arterial Thrombotic Process,” the pathophysiology of arterial thrombosis is primarily determined by local, “solid-state” factors that operate in concert in the immediate microenvironment of acute vascular injury, typically disruption of an atherosclerotic plaque. However, interindividual differences in systemic, circulating factors can modify susceptibility to the focal formation of an arterial thrombus.¹⁹⁸ Systemic determinants of blood thrombogenicity (i.e., hypercoagulability) can enhance the local risk of arterial thrombosis. There is increasing evidence for an association between venous and arterial thrombosis, with several studies now showing that patients with venous thromboembolism (deep vein thrombosis and/or pulmonary embolism) are at increased risk of having coexisting asymptomatic atherosclerosis or subsequent symptomatic atherothrombotic events. Conversely, patients with clinically overt atherosclerotic CVD are at increased risk of venous thromboembolism.^199,200,201 In addition to certain thrombophilic abnormalities, such as antiphospholipid antibody syndrome, hyperhomocysteinemia, and the myeloproliferative neoplasms, which are known to predispose individuals to both venous and arterial thromboembolism, some traditional cardiovascular risk factors (e.g., advanced age, obesity, metabolic syndrome, abnormal lipid profiles, immobility, estrogens) also appear to be independent risk factors for venous thromboembolism.^{39,202,203,204}

Genetic determinants of the coagulation system may exert modifying effects on susceptibility to arterial thrombosis. The known hypercoagulable states that predispose to venous thrombosis (e.g., factor V Leiden, prothrombin gene mutation, antithrombin deficiency, protein C and protein S deficiencies) generally are weakly²⁰⁵ or not at all associated with increased risk of arterial thrombosis. However, decreased mortality from ischemic heart disease has been noted in patients with hemophilia A or B and even in carriers of hemophilia.²⁰⁶ This finding most likely results from reduced arterial thrombotic tendency in these individuals because early atherogenesis itself does not appear to be significantly affected by the coexistence of hemophilia.²⁰⁷ Conversely, some epidemiologic studies have correlated elevated levels of fibrinogen and some other coagulation factors with both subclinical atherosclerosis and clinical cardiovascular events,^208,209 although cause-and-effect relationships between elevated levels of hemostatic factors and cardiovascular risk have not been established.

Several lines of evidence suggest that genetic determinants of increased platelet reactivity likewise enhance focal determinants of arterial thrombosis. Animal models of atherosclerosis in pigs and mice with von Willebrand disease suggest that an extremely low or absent VWF level exerts a protective effect on the development and distribution of atherosclerotic lesions,^210,211 although these observations are inconclusive. Whether or not von Willebrand disease protects against development of human atherosclerosis remains in dispute.

Platelet membrane glycoproteins are highly polymorphic and can be recognized as alloantigens or autoantigens. Polymorphisms in platelet membrane glycoprotein receptors have been considered to increase platelet reactivity, thereby potentially contributing to susceptibility to arterial thrombosis.^212,213 The first such genetic variation reported involves the HPA-1a/HPA-1b polymorphism, which results in a Leu33Pro substitution in the β₃ subunit of the platelet integrin α_IIbβ₃ complex. The 33Pro (HPA-1b) allele was found to be associated with risk of MI in young individuals.²¹⁴ Most, but not all, subsequent studies have agreed that the HPA-1b allele represents an inherited risk factor for ACS.²¹³ Other platelet receptor polymorphisms that have been inconclusively linked to risk of CVD include three different polymorphisms of the integrin α_IIb (HPA-3), GPIb gene, and a polymorphism of the collagen receptor integrin α₂β₁. However, as is the case for the soluble hemostatic factors, lack of a clear relationship among genotype, phenotype, and clinical manifestations has failed to establish convincing cause-and-effect relationships for any of these genetic variations.

Although none of these individual hemostatic proteins or platelet polymorphisms plays a clear, dominant role in the pathophysiology of arterial thrombosis, future application of platelet proteomics²¹⁵ and genomics are likely to reveal new disorders of platelet activation associated with arterial thrombosis.

High blood levels of catecholamines likely contribute systemically to localized arterial thrombus formation. Catecholamines may be increased by physical or emotional stress or by cigarette smoking, thereby triggering acute cardiovascular events in these settings. In addition to their vasoactive actions, catecholamines are direct platelet agonists and enhance shear stress-induced platelet activation.^191,216

Changes in lipid metabolism may exert systemic prothrombotic actions. The thrombogenicity of lipoprotein(a) has been attributed to its structural similarity to plasminogen, leading to reduced plasmin formation and impaired thrombolysis.¹⁶³ Elevated LDL cholesterol can contribute to blood hypercoagulability.²¹⁷ The prothrombotic state of diabetes involves multiple mechanisms, including platelet hyperreactivity and increased leukocyte procoagulant activity.¹⁷⁷

MYOCARDIAL INFARCTION

MI is a term that reflects necrosis of cardiac myocytes caused by prolonged ischemia. In the past, MI was defined by the combination of two of three characteristics: typical symptoms (i.e., chest discomfort), a rise in serum enzymatic markers derived from myocardial cells, and a typical electrocardiographic pattern involving the development of Q waves. The advent of sensitive and specific serologic biomarkers and precise imaging techniques has led to the development of revised criteria for MI.²¹⁸ For example, patients can be diagnosed with a “ST-segment elevation MI”²¹⁹ or “non–Q-wave or non–ST-segment elevation” MI (NSTEMI)²²⁰ if certain criteria are met. The criteria agreed upon by the American College of Cardiology for acute, evolving, or recent MI²¹⁸ are as follows:

1. Typical rise and gradual fall (troponin) or more rapid rise and fall (creatinine kinase-MB isoform) or biochemical markers of myocardial necrosis with at least one of the following: (A) ischemic symptoms; (B) development of pathologic Q waves on the electrocardiogram (ECG); (C) electrocardiographic changes indicative of ischemia (ST segment elevation or depression); or (D) coronary artery intervention (e.g., coronary angioplasty).

2. Pathologic findings of an acute MI.

The criteria for established MI²¹⁸ (i.e., event that occurred in the past) is any one of the following:

1. Development of new pathologic Q waves on serial ECGs. The patient may or may not remember previous symptoms. Biochemical markers of myocardial necrosis may have normalized, depending on the length of time since the infarct developed.

2. Pathologic findings of a healed or healing MI.

Clinical Features of Acute Coronary Syndromes

Stable angina pectoris is ischemic discomfort symptomatology caused by a narrowed coronary artery that does not allow sufficient oxygen delivery to meet the metabolic demands of the myocardium. Unstable angina is defined clinically as a change in the pattern of stable angina to more frequent or more severe symptoms, uninterrupted angina symptoms for 20 minutes or more, or the development of angina at rest. The term acute coronary syndrome has evolved as a useful description of the spectrum of patients presenting with angina pectoris caused by unstable angina through MI.²²⁰ The underlying pathologic mechanism for the development of ACS is usually a vulnerable atherosclerotic plaque with either plaque rupture or plaque ulceration leading to thrombosis. Unstable angina and NSTEMI are differentiated by pathologic elevation in the levels of cardiac biomarkers that confirm MI.

Angina pectoris can be associated with other symptoms, such as diaphoresis, dizziness, nausea, clamminess, and fatigue. Some patients with ACS present with atypical symptoms rather than chest pain. The presentation may be dyspnea alone, nausea and/or vomiting, palpitations/syncope, or cardiac arrest. Rarely, patients with diabetes mellitus and other patients have a “silent MI” diagnosed incidentally on ECG or cardiac imaging study.

The initial ECG is often not diagnostic in patients with ACS. In one clinical study, the ECG was not diagnostic in approximately 45 percent and was normal in 20 percent of patients who subsequently were shown to have experienced an acute MI.²²¹ ST-segment elevation and Q waves are consistent with ST-segment elevation myocardial infarction (STEMI), but other conditions, such as acute pericarditis with early repolarization variant and hypertrophic cardiomyopathy with Q waves, may mimic the ECG manifestations of STEMI.

Laboratory Features of Acute Myocardial Infarction

A variety of serum biomarkers are used to evaluate patients with suspected acute MI. The three most commonly used tests are (1) troponin I and troponin T, (2) creatine kinase (CK) and its isoform CK-myocardial band (MB), and (3) myoglobin. An elevated serum concentration of one or more of the three biomarkers is seen in almost all patients with acute MI. The preferred biomarkers are the troponins because the troponin assays are more specific than the other tests.

Therapy for Acute Coronary Syndromes

Therapy for Acute Myocardial Infarction The initial management of patients with STEMI depends upon prompt recognition and therapy to reduce morbidity and mortality. A carefully coordinated plan of care is essential for optimal results in patients with STEMI, given that multiple therapies usually are initiated simultaneously. The goals of therapy are to reduce ischemic pain, stabilize hemodynamic status, and quickly establish myocardial reperfusion. The American College of Cardiology (ACC)/American Heart Association (AHA) guidelines for management of patients with acute MI are available at the ACC website.²²²

Antiplatelet Agents Unless contraindicated, all patients with acute MI should be given antiplatelet therapy. The Antiplatelet Trialists’ Collaboration indicated a 30 percent reduction in vascular events with an absolute benefit of 38 vascular events prevented per 1000 patients at 1 month with antiplatelet therapy.²²⁰ Aspirin 325 mg/day or a P2Y₁₂-receptor antagonist such as clopidogrel is commonly used in the setting of MI. Vorapaxar, a protease-activated receptor-1 (PAR-1) antagonist, is indicated for the reduction of thrombotic events in patients with a history of MI or with peripheral artery disease (PAD).²²³ Contraindications to antiplatelet therapy include active bleeding, coagulopathy, and severe, untreated hypertension (a relative contraindication). The combination of dipyridamole and aspirin has not been proven to provide incremental clinical benefit over aspirin alone.

β-Adrenergic Blockade The control of heart rate with β-adrenergic blocker agents has been efficacious in the setting of acute MI or unstable angina.²²² According to guidelines, oral β-blocker should be initiated during the first 24 hours of care of STEMI. Intravenous administration of β-blockers should be given only to selected, hemodynamically stable patients according to guidelines.

Management of Chest Pain A cornerstone of ischemic pain management has been intravenous nitroglycerin (beginning at 5 to 10 mcg/min) in combination with morphine sulfate if necessary. Nitroglycerin also may improve hypertension and symptoms of heart failure, if present. Intravenous nitroglycerin therapy has not been proven to improve mortality and usually is discontinued within 24 to 48 hours of presentation.²²² Patients who have taken drugs (e.g., sildenafil) for erectile dysfunction within the preceding 24 hours are at increased risk for vasodilation and hypotension, so caution is advised in these patients when intravenous nitroglycerin is given.

Reperfusion Therapy The overriding goal of treatment of STEMI is restoration of myocardial blood flow and salvage of myocardial tissue. A decision should be made immediately whether the patient will undergo a primary (direct) percutaneous coronary intervention (PCI) or receive a fibrinolytic agent. The currently preferred approach is PCI, but the relative advantages and limitations of each therapy should be considered. The most important factor to consider is whether PCI is immediately available. Several randomized trials indicate enhanced survival with PCI compared to fibrinolysis, with a lower rate of intracranial hemorrhage and recurrent MI.^224,225 Transfer to a center that can provide PCI, if necessary, should be accomplished in less than 2 hours.²²⁶

Fibrinolytic therapy should be given immediately if PCI cannot be performed promptly.²¹⁹ Prior to fibrinolysis, the patient should be initially assessed for possible contraindications, which include active bleeding, history of cerebrovascular disease, intracranial neoplasm, drug allergy, and trauma. A systolic blood pressure greater than 175 torr is a relative contraindication but should not prohibit therapy, especially if the pressure can be rapidly controlled. Many different fibrinolytic regimens with different dosing schemes are available. Streptokinase was the first thrombolytic agent tested but has proved less effective than alteplase.²²⁷ In addition, streptokinase is antigenic and can cause an allergic reaction, particularly with repeat administration. Other thrombolytic agents, such as tenecteplase and reteplase, have reportedly similar results compared to alteplase.²²⁸ Tenecteplase is popular on hospital formularies because of its relatively easy single-bolus administration and reported lower rate of noncerebral bleeding.²²⁹

Anticoagulation Heparin, both unfractionated and low molecular weight, is commonly used in patients with STEMI.²¹⁹ The exact role of heparin therapy with different fibrinolytic agents is evolving. Patients who undergo primary PCI usually are given unfractionated heparin 7500 U subcutaneously twice daily or low-molecular-weight heparin, for example, enoxaparin, 1 mg/kg twice daily unless contraindications are evident. For patients receiving intravenous unfractionated heparin, the recommended dose is an initial 60 to 70 U/kg bolus (maximum: 5000 U) followed by 12 to 15 U/kg per hour (maximum: 1000 U/h) as continuous infusion with monitoring of the activated partial thromboplastin time (aPTT) measured at 6 hours. The heparin dose is adjusted to maintain an aPTT between 50 and 75 seconds.

Current guidelines recommend maintaining the aPTT at 50 to 75 seconds for short-term use. Heparin should be continued beyond this period only in the case of high risk of systemic or venous thromboembolism. Patients can be switched to a subcutaneously administered heparin or converted to oral warfarin during the high-risk period. The anticoagulant drugs unfractionated heparin, enoxaparin, fondaparinux, and bivalirudin are all excreted by the kidneys; consequently, although the first dose is usually safe, longer-term therapy should be guided by assessment of creatinine clearance.²³⁰ The Coumadin-Aspirin Reinfarction Study (CARS) did not show a significant benefit with the combination of low-dose warfarin (1 or 3 mg) and aspirin 80 mg daily compared to aspirin 160 mg daily monotherapy on cardiovascular morbidity in patients who had an MI.²³¹

Statins All patients with MI should be started on a 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor (statin) unless the MI was caused by a nonatherosclerotic process such as coronary vasospasm, vasculitis, or embolus. Numerous studies indicate that statins reduce the risk of subsequent MI by approximately 30 to 50 percent.²³² Current evidence suggests that a serum LDL level less than 80 mg/dL with statin treatment is more efficacious in retarding atherosclerotic disease progression than a serum LDL level of 100 mg/dL or above.²³³ Other nonstatin drugs, such as ezetimibe, PCSK9 inhibitors, and microsomal triglyceride transfer protein inhibitors, also reduce cholesterol levels but relative reduction in cardiovascular events compared to statins is not yet clear.

Therapy for Unstable Angina Pectoris and Non–ST-Elevation Myocardial Infarction The distinction between unstable angina and NSTEMI initially may be difficult because levels of troponins and/or CK-MB may not be elevated until hours after presentation. Similar to STEMI, the initial treatment of unstable angina and NSTEMI includes supplemental oxygen, pain control, and bed rest.²³⁰ Nitrates, given either intravenously or subcutaneously, are the treatment of choice for angina pectoris. Oral β-blockers also are routinely given to patients with unstable angina to relieve symptoms of angina and to reduce the risk of progression to MI.

Treatment of unstable angina and NSTEMI involves administration of an antiplatelet agent and anticoagulation.²³⁰ Fibrinolytic therapy is not beneficial in patients with unstable angina, and its use is associated with unacceptably high bleeding risk. Antiplatelet treatment, most commonly aspirin at a dose of 325 mg daily, was shown in the Antithrombotic Trialists’ Collaboration to reduce the combined end point of subsequent nonfatal MI, nonfatal stroke, or vascular death (8.0 percent vs. 13.3 percent) in patients with non–ST-segment elevation ACS.²³⁴ Clinical trials involving patients with non–ST-segment elevation ACS have demonstrated significantly reduced cardiovascular events and mortality with aspirin administration, mostly at a lower dose of 80 to 100 mg orally once per day.^235,236,237 Some patients do not benefit from aspirin, and this finding has generated an interest as to whether these patients are “aspirin resistant.” Nonrandomized studies indicate that aspirin resistance may occur, but because of the limitations of these studies, the definition and prognostic significance of this phenomenon are uncertain.²³⁸

The thienopyridine clopidogrel (75 mg/day) is effective in reducing the risk of MI and mortality in patients with unstable angina.²²⁰ The combination of aspirin and clopidogrel has been tested in patients with NSTEMI and unstable angina. The combination of these antiplatelet agents resulted in improved survival and decreased progression to MI.²³⁹ The patients with non–ST-segment elevation ACS who underwent PCI benefited the most from the combination of aspirin and clopidogrel.²⁴⁰ However, the combination was associated with an increase in major bleeding and reoperation for bleeding in patients who underwent coronary artery bypass grafting (CABG). Therefore, a 5-day, but preferably a 7-day, period off clopidogrel is recommended before CABG.²⁴¹

A meta-analysis of randomized clinical trials found that intravenous platelet integrin IIb/IIIa inhibitors substantially benefited patients with non–ST-segment elevation ACS undergoing coronary intervention.²⁴² The integrin α_IIbβ_IIIa receptor antagonist abciximab (ReoPro) is a monoclonal antibody fragment that reduces short-term and long-term clinical events in patients with ACS undergoing angioplasty with or without stent placement. Other platelet integrin α_IIbβ_IIIa antagonists, such as tirofiban and Integrilin, also are effective and safe in treating unstable angina when combined with heparin anticoagulation.²⁴³ Guidelines from an ACC/AHA task force recommend administration of an integrin α_IIbβ_IIIa inhibitor, in addition to aspirin and heparin, for patients with unstable angina/NSTEMI undergoing planned PCI.²²⁰

Unfractionated heparin reduces the rate of MI and death, and relieves anginal pain, when used in combination with an antiplatelet agent.²²⁰ Intravenous heparin usually is given as a 5000-U bolus followed by continuous infusion. Low-molecular-weight heparins can be substituted for unfractionated heparin. Some studies have shown superior efficacy of low-molecular-weight heparins, but other studies have not indicated a significant difference. Direct thrombin inhibitors, such as hirudin and bivalirudin, have been shown to reduce the rate of death, nonfatal MI, and refractory angina compared to heparin.^244,245 The American College of Chest Physicians (ACCP) recommends lepirudin (recombinant hirudin), argatroban, bivalirudin, or danaparoid in patients with a history of heparin-associated thrombocytopenia,²⁴⁶ although some of these agents are no longer available in the United States.

Therapy for Stable Angina Pectoris Patients with stable angina pectoris can be treated with either medical management or revascularization.²⁴⁷ Limited clinical trial data comparing revascularization, either percutaneous or surgical, to medical therapy are available. The older trials evaluating percutaneous and surgical revascularization were limited by several factors: antiplatelet treatment, angiotensin-converting enzyme inhibitors, and aggressive lipid lowering with statins were not given as background medical therapy of angina. Given these limitations, determining whether revascularization is better than medical management for long-term care of patients with stable angina in modern practice is difficult.

Both PCI and coronary bypass surgery significantly reduce angina. The Coronary Artery Surgery Study (CASS) showed more patients remained symptom-free after CABG compared to medical therapy 5 years after the procedure.²⁴⁸ At 10 years, however, no significant difference in symptoms was observed. Clinical trials showed significant improvement in angina with PCI compared to medical therapy; however, patients who underwent the former had similar rates of death and MI as those undergoing medical therapy and were less likely to have angina and more likely to have undergone a coronary bypass graft.²⁴⁹

Restenosis is a complex process involving inflammation, cellular proliferation, thrombosis, and matrix deposition. Restenosis occurring after PCI may result in flow-limiting luminal narrowing in 20 to 30 percent of therapeutically dilated vessels.²⁵⁰ Numerous pharmacologic agents, including heparin,²⁵¹ have been given in an attempt to reduce the restenosis rate but have met with limited or no success. Intraarterial radiation (brachytherapy) reduces the restenosis rate but is cumbersome to perform because of radiation safety issues and has fallen out of favor. Drug-eluting arterial stents, including the immunosuppressive macrocyclic lactone rapamycin (sirolimus)²⁵² and the chemotherapeutic agent paclitaxel (Taxol),²⁵³ significantly reduce the rate of restenosis. Because drug-eluting stents were not available at the time of the previous clinical trials, extrapolating the benefits of PCI versus CABG or over medical therapy is difficult. The medical management of patients with stable angina pectoris should include antiplatelet therapy, statin drug treatment, a β-blocker, an angiotensin-converting enzyme inhibitor, and a long-acting nitrate.

PERIPHERAL ARTERY DISEASE

PAD is a term that encompasses any arterial disease of the lower extremities, upper extremities, and iliac vessels. It most commonly results from atherosclerosis. Patients who have atherosclerotic disease that compromises blood flow to the extremities may present with exertional pain in a muscle group, called claudication (derived from the Latin claudicare meaning “to limp”). Claudication is an intermittent but reproducible discomfort of a defined group of muscles that is induced by exercise and relieved with rest.²⁵⁴ Acute limb ischemia is a relatively rare problem in patients with PAD. In general, it is caused by in situ thrombosis or an embolic event from arrhythmias, such as atrial fibrillation, or after manipulation of an artery or aorta with a catheter. Approximately 4 percent of patients with claudication progress to critical limb ischemia, which is defined as rest pain and/or foot ulceration that heralds impending tissue loss.

The 5-year mortality rate is estimated to be 30 percent in patients with lower-extremity PAD.²⁵⁵ Approximately 75 percent of mortality results from a cardiovascular event, such as MI or stroke.²⁵⁵ The ankle-brachial index is a noninvasive measure of limb vascular pressure in the lower extremities and has been noted in several studies to be predictive of cardiovascular events.²⁵⁴ However, a decreased index is not just a predictor; it also is a physical finding that indicates significant atherosclerotic plaque burden is present. Other noninvasive imaging studies for PAD include the combination of segmental pressures and pulse volume recordings, duplex Doppler ultrasound, computed tomographic angiography and magnetic resonance imaging.^256,257

Medical therapy for patients with PAD includes risk-factor modification, antiplatelet therapy, and treatment of claudication symptoms with exercise rehabilitation and possible pharmacologic agents. The risk factors for development of peripheral atherosclerosis include cigarette smoking, diabetes mellitus, hypertension, and dyslipidemia.²⁵⁸ Aggressive management of risk factors for PAD is recommended to prevent disease progression.²⁵⁹ Treatment with antiplatelet agents reduces the risk of cardiovascular events, such as MI and stroke, in patients with PAD.²⁵⁵ The Antithrombotic Trialists’ Collaboration evaluated 9214 patients with PAD enrolled in 42 trials and found that use of antiplatelet drugs, such as aspirin 75 to 325 mg/day, resulted in a proportional reduction of 23 percent in serious vascular events.²³⁴ Evaluation of patients with PAD in the Physicians’ Health Study found that aspirin 325 mg every other day decreased the need for peripheral artery surgery.²⁶⁰ However, no difference between the aspirin and placebo groups with regard to development of claudication was observed. Several studies have evaluated the ADP receptor blockers ticlopidine and clopidogrel. Clopidogrel was evaluated in 19,185 patients in the Clopidogrel Versus Aspirin in Patients at Risk of Ischaemic Events (CAPRIE) study.²⁶¹ A dose of clopidogrel 75 mg/day had a modest but significant advantage over aspirin 325 mg/day in preventing stroke, MI, and peripheral vascular disease. Subgroup analysis revealed that the patients with PAD benefited the most with clopidogrel treatment. One study evaluating the effect of aspirin, 100 mg, compared to placebo in asymptomatic patients with diabetes mellitus and PAD found no benefit in reducing cardiovascular events.²⁶² The PAR-1 antagonist, vorapaxar, is approved to prevent cardiovascular events in patients with PAD.²²³ The clinical consensus is that antiplatelet therapy should be offered to all patients with PAD unless contraindicated by allergy or comorbidities.²⁶³

The options for treating claudication symptoms include exercise rehabilitation, pharmacologic agents, and a revascularization procedure. Several studies indicate exercise rehabilitation improves the symptoms of claudication, and a supervised program is better than an unstructured program, and comparable to percutaneous revascularization.^264,265 Two drugs are approved by the FDA for treatment of claudication symptoms: pentoxifylline, a methylxanthine derivative that may improve abnormal red cell deformability and reduce blood viscosity, and cilostazol, a type III phosphodiesterase inhibitor with antiplatelet and vasodilating properties. Cilostazol is generally considered more effective than pentoxifylline for improving walking distance in patients with claudication.²⁶⁶ The addition of cilostazol to either aspirin or clopidogrel does not increase the bleeding risk.²⁶⁷ A revascularization procedure in patients with stable, intermittent claudication generally is reserved for those with severe lifestyle-limiting symptoms or manifestation of critical limb ischemia.

CEREBROVASCULAR DISEASE

The etiology of ischemic stroke is multifactorial and can be categorized into embolic, small-vessel disease, large-vessel disease, and cryptogenic. Carotid artery disease accounts for approximately 30 percent of strokes. Major risk factors for developing carotid artery atherosclerosis are hypertension, diabetes, smoking, and dyslipidemia.²⁶⁸ Emerging risk factors for stroke include hyperhomocysteinemia and an elevated plasma level of lipoprotein(a). An elevated hsCRP level is a risk factor associated with ischemic stroke in both men and women. However, at this time hsCRP is not routinely measured as an additional marker for increased risk of stroke. Similar to CAD and PAD, control of atherosclerotic risk factors is essential in the primary prevention of stroke in patients with evidence of carotid atherosclerosis and for those who have undergone carotid endarterectomy.^269,270

Carotid endarterectomy is indicated for patients with symptoms and a greater than 50 percent stenosis²⁷¹ or for patients who are asymptomatic with a greater than 60 to 99 percent stenosis of the common carotid or internal carotid arteries.²⁷² Carotid stent with embolic protection is a therapy used for treatment of carotid atherosclerosis in selective patients.²⁷³

Two antiplatelet drug regimens are approved for prevention of stroke: clopidogrel (Plavix) and the combination of aspirin 25 mg and dipyridamole 200 mg daily. Approval of clopidogrel is based on the CAPRIE study, which showed a reduction in the combined end point of stroke, MI, and death in patients treated with clopidogrel 75 mg/day compared to those treated with aspirin 325 mg/day.²⁶¹ The FDA indication for dipyridamole/aspirin is primarily based on the European Stroke Protection Study 2, which noted a reduction in stroke with the combination of dipyridamole 200 mg and aspirin 25 mg given together (Aggrenox) twice per day.²⁷⁴ The Prevention Regimen for Effectively Avoiding Second Strokes (PRoFESS) trial was a secondary stroke–prevention trial comparing the combination of aspirin and extended-release dipyridamole (Aggrenox) versus clopidogrel (Plavix) in preventing stroke recurrence after a first event. The difference between the agents was not statistically significant for the primary outcome of recurrent stroke.²⁷⁵ Fish oil (omega-3 fatty acids) lowers triglycerides and VLDLs and may reduce serum viscosity by lowering fibrinogen. Some studies suggest that fish oil consumption lowers the risk of ischemic stroke. The effect of fish oils on carotid atherosclerosis is unknown.²⁷⁶

Atheromatous embolism refers to the dislodgment into the bloodstream of arterial plaque material, including cholesterol crystals (“cholesterol embolism”) from ulcerated vascular plaques. The cholesterol embolization syndrome involves systemic microembolism to the end arteries of almost any circulatory bed. Atheroembolism most characteristically originates from lesions in the abdominal aorta and ileofemoral arteries. Cholesterol emboli that lodge in an arteriole incite an acute inflammatory response, followed by a foreign-body reaction, intravascular thrombus formation, endothelial proliferation, and eventually fibrosis. These processes generally result in ischemia that sometimes leads to infarction and necrosis.²⁷⁷ Mortality rate of clinically diagnosed atheroembolism can be as high as 80 percent, depending on the anatomic location and size of the vascular beds involved.²⁷⁸

Patients with atheroembolism, including the cholesterol embolization syndrome, generally have advanced atherosclerosis, often complicated by a history of hypertension, diabetes mellitus, renal failure, or aortic aneurysms. Atherosclerotic plaques can disrupt and embolize spontaneously; however, the clinical syndrome typically is triggered by vascular intervention, including vascular surgery, catheterization, angioplasty, endarterectomy, or angiography. Anticoagulation or thrombolytic therapy may be risk factors with atheroembolism.²⁷⁸ Clinical presentation depends on the sites of embolization. When these sites involve the distal extremity microcirculation, the “blue toe syndrome” may develop. The syndrome presents with the acute appearance of painful and tender discoloration or mottled blue and patchy appearance of one or more toes that may progress to ulceration and gangrene. Other common cutaneous manifestations are livedo reticularis involving the legs, buttocks, or abdomen, painful nodules, and purpura. Cerebrovascular embolism can cause transient neurologic abnormalities. Cholesterol emboli lodged in retinal arterial bifurcations can be visualized by ophthalmoscopy as bright, refractile, yellow rectangular crystals. Visceral organs most commonly affected by atheroembolism include the kidneys, sometimes causing renal failure, and the gastrointestinal tract, where abdominal pain, ischemic colitis, and bleeding may ensue.

Diagnosis is based on clinical presentation associated with imaging evidence of atherosclerosis of the arterial supply of affected organs.²⁷⁸ Transient eosinophilia occurs in most cases.²⁷⁹ Treatment of atheroembolism should include surgical removal or bypass of the source of emboli. No medical treatment modalities have been established to be effective. Anticoagulation or fibrinolytic therapy may increase the risk of further atheroembolism.