Robbins & Cotran Pathologic Basis of Disease (Robbins Pathology) PDF

See TARGETED THERAPY available online at www.studentconsult.com C H A P T E R Hemodynamic Disorders, Thromboembolic Disease, and Shock...

See TARGETED THERAPY available online at www.studentconsult.com C H A P T E R Hemodynamic Disorders, Thromboembolic Disease, and Shock 4 CHAPTER CONTENTS Edema and Effusions 115 Coagulation Cascade 120 Embolism 130 Increased Hydrostatic Pressure 116 Endothelium 123 Pulmonary Embolism (PE) 131 Reduced Plasma Osmotic Pressure 116 Hemorrhagic Disorders 124 Systemic Thromboembolism 131 Sodium and Water Retention 116 Thrombosis 125 Fat Embolism 131 Lymphatic Obstruction 116 Endothelial Injury 126 Air Embolism 132 Hyperemia and Congestion 117 Alterations in Normal Blood Flow 126 Amniotic Fluid Embolism 132 Hemostasis, Hemorrhagic Hypercoagulability 126 Infarction 133 Disorders, and Thrombosis 118 Fate of the Thrombus 129 Shock 134 Normal Hemostasis 118 Disseminated Intravascular Coagulation Pathogenesis of Septic Shock 135 Platelets 119 (DIC) 130 Stages of Shock 137 The health of cells and tissues depends on the circulation Herein, we focus on disorders of hemodynamics (edema, of blood, which delivers oxygen and nutrients and removes effusions, congestion, and shock), provide an overview of wastes generated by cellular metabolism. Under normal disorders of abnormal bleeding and clotting (thrombosis), conditions, as blood passes through capillary beds, proteins and discuss the various forms of embolism. in the plasma are retained within the vasculature, and there is little net movement of water and electrolytes into the tissues. This balance is often disturbed by pathologic condi- EDEMA AND EFFUSIONS tions that alter endothelial function, increase vascular hydrostatic pressure, or decrease plasma protein content, Disorders that perturb cardiovascular, renal, or hepatic all of which promote edema—the accumulation of fluid in function are often marked by the accumulation of fluid tissues resulting from a net movement of water into extra- in tissues (edema) or body cavities (effusions). Under vascular spaces. Depending on its severity and location, normal circumstances, the tendency of vascular hydrostatic edema may have minimal or profound effects. In the lower pressure to push water and salts out of capillaries into the extremities, it may only make one’s shoes feel snugger after interstitial space is nearly balanced by the tendency of plasma a long sedentary day; in the lungs, however, edema fluid colloid osmotic pressure to pull water and salts back into can fill alveoli, causing life-threatening hypoxia. vessels. There is usually a small net movement of fluid into The structural integrity of blood vessels is frequently the interstitium, but this drains into lymphatic vessels and compromised by trauma. Hemostasis is the process of blood ultimately returns to the bloodstream via the thoracic duct, clotting that prevents excessive bleeding after blood-vessel keeping the tissues “dry” (Fig. 4.1). Elevated hydrostatic damage. Inadequate hemostasis may result in hemorrhage, pressure or diminished colloid osmotic pressure disrupts which can compromise regional tissue perfusion and, if this balance and results in increased movement of fluid massive and rapid, may lead to hypotension, shock, and out of vessels. If the net rate of fluid movement exceeds death. Conversely, inappropriate clotting (thrombosis) or the rate of lymphatic drainage, fluid accumulates. Within migration of clots (embolism) can obstruct blood vessels, tissues the result is edema, and if a serosal surface is involved, potentially causing ischemic cell death (infarction). Indeed, fluid may accumulate within the adjacent body cavity as thromboembolism lies at the heart of three major causes of an effusion. morbidity and death in high income countries: myocardial Edema fluids and effusions may be inflammatory or non- infarction, pulmonary embolism (PE), and cerebrovascular inflammatory (Table 4.1). Inflammation-related edema and accident (stroke). effusions are discussed in detail in Chapter 3. These 115 116 CHAPTER 4 Hemodynamic Disorders, Thromboembolic Disease, and Shock LYMPHATICS To thoracic duct and eventually Sodium and Water Retention to left subclavian vein Increased salt retention—with obligate retention of associ- ated water—causes both increased hydrostatic pressure (due to intravascular fluid volume expansion) and dimin- ished vascular colloid osmotic pressure (due to dilution). Salt retention occurs whenever renal function is compro- Increased interstitial Hydrostatic pressure mised, such as in primary kidney disorders and in cardio- fluid pressure vascular disorders that decrease renal perfusion. One of the most important causes of renal hypoperfusion is congestive heart failure, which (like hypoproteinemia) results in the activation of the renin-angiotensin-aldosterone axis. In early heart failure, this response is beneficial, as the retention of sodium and water and other adaptations, including increased Plasma colloid osmotic pressure vascular tone and elevated levels of antidiuretic hormone, improve cardiac output and restore normal renal perfusion. However, as heart failure worsens and cardiac output diminishes, the retained fluid merely increases the hydrostatic Arterial end CAPILLARY BED Venous end pressure, leading to edema and effusions. Figure 4.1 Factors influencing fluid movement across capillary walls. Normally, hydrostatic and osmotic forces are nearly balanced so that there Lymphatic Obstruction is little net movement of fluid out of vessels. Many different pathologic Trauma, fibrosis, invasive tumors, and infectious agents disorders (see Table 4.1) are associated with increases in capillary can all disrupt lymphatic vessels and impair the clearance hydrostatic pressure or decreases in plasma osmotic pressure that lead to of interstitial fluid, resulting in lymphedema in the affected the extravasation of fluid into tissues. Lymphatic vessels remove much of part of the body. A dramatic example is seen in parasitic the excess fluid, but if the capacity for lymphatic drainage is exceeded, tissue edema results. filariasis, in which the organism induces obstructive fibrosis protein-rich exudates accumulate due to increases in vascular Table 4.1 Pathophysiologic Categories of Edema permeability caused by inflammatory mediators. In contrast, Increased Hydrostatic Pressure noninflammatory edema and effusions are protein-poor fluids called transudates. Noninflammatory edema and effusions Impaired Venous Return are common in many disorders, including heart failure, liver Congestive heart failure failure, renal disease, and malnutrition (Fig. 4.2). We will Constrictive pericarditis Ascites (liver cirrhosis) now discuss the various causes of edema. Venous obstruction or compression Thrombosis Increased Hydrostatic Pressure External pressure (e.g., mass) Increases in hydrostatic pressure are mainly caused by Lower extremity inactivity with prolonged dependency disorders that impair venous return. If the impairment is Arteriolar Dilation localized (e.g., a deep venous thrombosis [DVT] in a lower Heat extremity), then the resulting edema is confined to the Neurohumoral dysregulation affected part. Conditions leading to systemic increases in Reduced Plasma Osmotic Pressure (Hypoproteinemia) venous pressure (e.g., congestive heart failure, Chapter 12) are understandably associated with more widespread edema. Protein-losing glomerulopathies (nephrotic syndrome) Liver cirrhosis Reduced Plasma Osmotic Pressure Malnutrition Protein-losing gastroenteropathy Under normal circumstances albumin accounts for almost half of the total plasma protein; it follows that conditions Lymphatic Obstruction leading to inadequate synthesis or increased loss of Inflammatory albumin from the circulation are common causes of reduced Neoplastic Postsurgical plasma oncotic pressure. Reduced albumin synthesis occurs Postirradiation mainly in severe liver diseases (e.g., end-stage cirrhosis, Chapter 18) and protein malnutrition (Chapter 9). An Sodium Retention important cause of albumin loss is the nephrotic syndrome Excessive salt intake with renal insufficiency (Chapter 20), in which albumin leaks into the urine through Increased tubular reabsorption of sodium Renal hypoperfusion abnormally permeable glomerular capillaries. Regardless Increased renin-angiotensin-aldosterone secretion of cause, reduced plasma osmotic pressure leads in a stepwise fashion to edema, reduced intravascular volume, renal Inflammation hypoperfusion, and secondary hyperaldosteronism. Not Acute inflammation only does the ensuing salt and water retention by the kidney Chronic inflammation fail to correct the plasma volume deficit, but it also exacer- Angiogenesis bates the edema, because the primary defect—a low plasma Modified from Leaf A, Cotran RS: Renal Pathophysiology, ed 3, New York, 1985, Oxford University Press, p 146. protein level—persists.  Hyperemia and congestion 117 Clinical Features HEART FAILURE MALNUTRITION, HEPATIC SYNTHESIS, The consequences of edema range from merely annoying NEPHROTIC SYNDROME to rapidly fatal. Subcutaneous edema is important primarily because it signals potential underlying cardiac or renal Capillary Renal blood flow disease; however, when significant, it can also impair wound hydrostatic Plasma albumin healing and the clearance of infections. Pulmonary edema pressure Activation of the is a common clinical problem that is most frequently seen renin-angiotensin in the setting of left ventricular failure; it can also occur system with renal failure, acute respiratory distress syndrome (Chapter 15), and pulmonary inflammation or infection. Retention of RENAL Edema in the pulmonary interstitium and the alveolar spaces Na+ and H2O FAILURE impedes gas exchange (leading to hypoxemia) and also creates a favorable environment for bacterial infection. Blood volume Plasma osmotic Pulmonary edema is often exacerbated by pleural effusions, pressure which may further compromise gas exchange by compressing the underlying pulmonary parenchyma. Peritoneal effusions EDEMA (ascites) resulting most commonly from portal hypertension are prone to seeding by bacteria, leading to serious and Figure 4.2 Mechanisms of systemic edema in heart failure, renal failure, sometimes fatal infections. Brain edema is life threatening; malnutrition, hepatic failure, and nephrotic syndrome. if severe, brain substance can herniate (extrude) through the foramen magnum, or the brain stem vascular supply can be compressed. Either condition can injure the medullary of lymphatic channels and lymph nodes. This may result centers and cause death (Chapter 28). in edema of the external genitalia and lower limbs that is so massive as to earn the appellation elephantiasis. Severe edema of the upper extremity may also complicate surgical KEY CONCEPTS removal and/or irradiation of the breast and associated axillary lymph nodes in patients with breast cancer. EDEMA Edema is the result of the movement of fluid from the vasculature into the interstitial spaces; the fluid may be protein-poor (tran- sudate) or protein-rich (exudate). MORPHOLOGY Edema may be caused by: Edema is easily recognized grossly; microscopically, it is appreciated Increased hydrostatic pressure (e.g., heart failure) as clearing and separation of the extracellular matrix (ECM) and Decreased colloid osmotic pressure caused by reduced plasma subtle cell swelling. Edema is most commonly seen in subcutaneous albumin, either due to decreased synthesis (e.g., liver disease, tissues, the lungs, and the brain. Subcutaneous edema can be protein malnutrition) or to increased loss (e.g., nephrotic diffuse or more conspicuous in regions with high hydrostatic syndrome) pressures. Its distribution is often influenced by gravity (e.g., it Increased vascular permeability (e.g., inflammation) appears in the legs when standing and the sacrum when recumbent), Lymphatic obstruction (e.g., infection or neoplasia) a feature termed dependent edema. Finger pressure over Sodium and water retention (e.g., renal failure) markedly edematous subcutaneous tissue displaces the interstitial fluid and leaves a depression, a sign called pitting edema. Edema resulting from renal dysfunction often appears initially in parts of the body containing loose connective tissue, such as HYPEREMIA AND CONGESTION the eyelids; periorbital edema is thus a characteristic finding Hyperemia and congestion both stem from increased blood in severe renal disease. With pulmonary edema, the lungs are volumes within tissues, but have different underlying often two to three times their normal weight, and sectioning mechanisms and consequences. Hyperemia is an active yields frothy, blood-tinged fluid—a mixture of air, edema, and process in which arteriolar dilation (e.g., at sites of inflam- extravasated red cells. Brain edema can be localized or general- mation or in skeletal muscle during exercise) leads to ized depending on the nature and extent of the pathologic process increased blood flow. Affected tissues turn red (erythema) or injury. The swollen brain exhibits narrowed sulci and distended because of increased delivery of oxygenated blood. Conges- gyri, which are compressed by the unyielding skull (Chapter 28). tion is a passive process resulting from reduced venous Effusions involving the pleural cavity (hydrothorax), the outflow of blood from a tissue. It can be systemic, as in pericardial cavity (hydropericardium), or the peritoneal cavity cardiac failure, or localized, as in isolated venous obstruction. (hydroperitoneum or ascites) are common in a wide range Congested tissues have an abnormal blue-red color (cyanosis) of clinical settings.Transudative effusions are typically protein-poor, that stems from the accumulation of deoxygenated hemo- translucent, and straw colored; an exception are peritoneal effusions globin in the affected area. In long-standing chronic passive caused by lymphatic blockage (chylous effusion), which may be congestion, the associated chronic hypoxia may result in milky due to the presence of lipids absorbed from the gut. In ischemic tissue injury and scarring. In chronically congested contrast, exudative effusions are protein-rich and often cloudy tissues, capillary rupture can also produce small hemorrhagic due to the presence of white cells. foci; subsequent catabolism of extravasated red cells can 118 CHAPTER 4 Hemodynamic Disorders, Thromboembolic Disease, and Shock leave residual telltale clusters of hemosiderin-laden mac- within intact blood vessels or within the chambers of the rophages. As a result of increased hydrostatic pressures, heart. As is discussed in Chapters 11 and 12, thrombosis congestion commonly leads to edema. has a central role in the most common and clinically impor- tant forms of cardiovascular disease. Although useful, it must be recognized that this division MORPHOLOGY between bleeding and thrombotic disorders sometimes breaks down, in that generalized activation of clotting sometimes Congested tissues take on a dusky reddish-blue color (cyanosis) paradoxically produces bleeding due to the consumption due to red cell stasis and the presence of deoxygenated hemoglobin. of coagulation factors, as in disseminated intravascular coagula- Microscopically, acute pulmonary congestion is marked by tion (DIC). To provide context for understanding disorders engorged alveolar capillaries, alveolar septal edema, and focal of bleeding and clotting, this discussion begins with normal intra-alveolar hemorrhage. In chronic pulmonary congestion, hemostasis, focusing on the contribution of platelets, coagula- which is often caused by congestive heart failure, the septa are tion factors, and endothelium. thickened and fibrotic, and the alveoli often contain numerous macrophages laden with hemosiderin (heart failure cells) derived from phagocytosed red cells. In acute hepatic congestion, the Normal Hemostasis central vein and sinusoids are distended. Because the centrilobular Hemostasis is a precisely orchestrated process involving area is at the distal end of the hepatic blood supply, centrilobular platelets, clotting factors, and endothelium that occurs at hepatocytes may undergo ischemic necrosis, and the periportal the site of vascular injury and culminates in the formation hepatocytes—better oxygenated because of proximity to hepatic of a blood clot, which serves to prevent or limit the extent arterioles—may only develop fatty change. In chronic passive of bleeding. The general sequence of events leading to hepatic congestion, the centrilobular regions are grossly red- hemostasis at a site of vascular injury is shown in Fig. 4.4. brown and slightly depressed (because of cell death) and are Arteriolar vasoconstriction occurs immediately and mark- accentuated against the surrounding zones of uncongested tan edly reduces blood flow to the injured area (see Fig. liver (nutmeg liver) (Fig. 4.3A). Microscopically, there is centri- 4.4A). It is mediated by reflex neurogenic mechanisms lobular congestion and hemorrhage, hemosiderin-laden macro- and may be augmented by the local secretion of factors phages, and variable degrees of hepatocyte dropout and necrosis such as endothelin, a potent endothelium-derived vaso- (see Fig. 4.3B). constrictor. This effect is transient, however, and bleeding would resume if not for activation of platelets and coagulation factors. Primary hemostasis: the formation of the platelet plug. Disrup- HEMOSTASIS, HEMORRHAGIC tion of the endothelium exposes subendothelial von DISORDERS, AND THROMBOSIS Willebrand factor (vWF) and collagen, which promote platelet adherence and activation. Activation of platelets Hemostasis can be defined simply as the process by which results in a dramatic shape change (from small rounded blood clots form at sites of vascular injury. Hemostasis is discs to flat plates with spiky protrusions that markedly essential for life and is deranged to varying degrees in a increase surface area), as well as the release of secretory broad range of disorders, which can be divided into two granules. Within a few minutes, the secreted products groups. In hemorrhagic disorders, characterized by excessive recruit additional platelets that undergo aggregation to bleeding, hemostatic mechanisms are either blunted or form a primary hemostatic plug (see Fig. 4.4B). insufficient to prevent blood loss. By contrast, in thrombotic Secondary hemostasis: deposition of fibrin. Vascular injury disorders blood clots (often referred to as thrombi) form exposes tissue factor at the site of injury. Tissue factor is A B Figure 4.3 Liver with chronic passive congestion and hemorrhagic necrosis. (A) Central areas are red and slightly depressed compared with the surrounding tan viable parenchyma, forming a “nutmeg liver” pattern (so-called because it resembles the cut surface of a nutmeg). (B) Centrilobular necrosis with degenerating hepatocytes and hemorrhage. (Courtesy Dr. James Crawford, Department of Pathology, University of Florida, Gainesville, Fla.)  Hemostasis, hemorrhagic disorders, and thrombosis 119 Figure 4.4 Normal hemostasis. (A) After vascular injury, neurohumoral A. VASOCONSTRICTION factors induce transient vasoconstriction. (B) Platelets bind via glycoprotein Ib (GpIb) receptors to von Willebrand factor (vWF) on Endothelium Basement membrane Arteriole smooth muscle exposed extracellular matrix (ECM) and are activated, undergoing a shape change and granule release. Released adenosine diphosphate (ADP) and thromboxane A2 (TxA2) induce additional platelet aggregation through platelet GpIIb-IIIa receptor binding to fibrinogen, and form the primary hemostatic plug. (C) Local activation of the coagulation cascade (involving Site of injury tissue factor and platelet phospholipids) results in fibrin polymerization, “cementing” the platelets into a definitive secondary hemostatic plug. (D) Counterregulatory mechanisms, mediated by tissue plasminogen activator (t-PA, a fibrinolytic product) and thrombomodulin, confine the hemostatic process to the site of injury. Endothelin release Reflex ECM (collagen) augments vasoconstriction vasoconstriction a membrane-bound procoagulant glycoprotein that is normally expressed by subendothelial cells in the vessel wall, such as smooth muscle cells and fibroblasts. Tissue B. PRIMARY HEMOSTASIS factor binds and activates factor VII (see later), setting in motion a cascade of reactions that culminates in thrombin generation. Thrombin cleaves circulating fibrinogen into insoluble fibrin, creating a fibrin meshwork, 2 Shape change 4 Recruitment and also is a potent activator of platelets, leading to 3 Granule release additional platelet aggregation at the site of injury. This 1 Platelet adhesion (ADP, TxA2) Aggregation sequence, referred to as secondary hemostasis, consolidates (hemostatic vWF the initial platelet plug (see Fig. 4.4C). 5 plug) Clot stabilization and resorption. Polymerized fibrin and platelet aggregates undergo contraction to form a solid, permanent plug that prevents further hemorrhage. At Endothelium Basement this stage, counterregulatory mechanisms (e.g., tissue membrane Collagen plasminogen activator [t-PA] made by endothelial cells) are set into motion that limit clotting to the site of injury (see Fig. 4.4D) and eventually lead to clot resorption and C. SECONDARY HEMOSTASIS tissue repair. It should be emphasized that endothelial cells are central regulators of hemostasis; the balance between the 2 Phospholipid –coagulation factor antithrombic and prothrombotic activities of endothelium complexes 3 Thrombin activation determines whether thrombus formation, propagation, 4 Fibrin polymerization or dissolution occur. Normal endothelial cells express a 1 Tissue factor Tissue factor variety of anticoagulant factors that inhibit platelet aggrega- 1 tion and coagulation and promote fibrinolysis; after injury or activation, however, this balance shifts, and endothelial cells acquire numerous procoagulant activities (activation of platelets and clotting factor, described earlier; see also Fibrin Fig. 4.10). Besides trauma, endothelium can be activated by microbial pathogens, hemodynamic forces, and pro- inflammatory mediators. D. THROMBUS AND ANTITHROMBOTIC EVENTS We now describe roles of platelets, coagulation factors, and endothelium in hemostasis in greater detail, following the scheme illustrated in Fig. 4.4. Trapped neutrophil Release of: t-PA (fibrinolysis) Trapped red Platelets thrombomodulin cells Platelets play a critical role in hemostasis by forming the (blocks coagulation primary plug that initially seals vascular defects and by cascade) Polymerized fibrin providing a surface that binds and concentrates activated coagulation factors. Platelets are disc-shaped anucleate cell fragments that are shed from megakaryocytes in the bone marrow into the bloodstream. Their function depends on several glycoprotein receptors, a contractile cytoskeleton, and two types of cytoplasmic granules. α-Granules have the adhesion molecule P-selectin on their membranes (Chapter 3) 120 CHAPTER 4 Hemodynamic Disorders, Thromboembolic Disease, and Shock and contain proteins involved in coagulation, such as fibrino- through a special G-protein–coupled receptor referred gen, coagulation factor V, and vWF, as well as protein factors to as protease-activated receptor-1 (PAR-1), which is switched that may be involved in wound healing, such as fibronectin, on by a proteolytic cleavage carried out by thrombin. platelet factor 4 (PF4, a heparin-binding chemokine), platelet- ADP is a component of dense-body granules; thus, platelet derived growth factor (PDGF), and transforming growth activation and ADP release begets additional rounds of factor-β. Dense (or δ) granules contain adenosine diphosphate platelet activation, a phenomenon referred to as recruit- (ADP), adenosine triphosphate, ionized calcium, serotonin, ment. ADP acts by binding two G-protein–coupled and epinephrine. receptors. P2Y1 and P2Y12. Activated platelets also produce After a traumatic vascular injury, platelets encounter the prostaglandin thromboxane A2 (TxA2), a potent inducer constituents of the subendothelial connective tissue, such of platelet aggregation. An understanding of the biochemi- as vWF and collagen. On contact with these proteins, platelets cal pathways involved in platelet activation has led to undergo a sequence of reactions that culminate in the forma- the development of drugs with antiplatelet activities. tion of a platelet plug (see Fig. 4.4B). Aspirin, the oldest of all, inhibits platelet aggregation Platelet adhesion is mediated largely via interactions and produces a mild bleeding defect by inhibiting between the platelet surface receptor glycoprotein Ib cyclooxygenase, a platelet enzyme that is required for (GpIb) and vWF in the subendothelial matrix (Fig. 4.5). TxA2 synthesis. More recently, drugs that inhibit platelet Platelets also adhere to exposed collagen via the platelet function by antagonizing PAR-1 or P2Y12 have been collagen receptor Gp1a/IIa. Notably, genetic deficiencies developed. All of these antiplatelet drugs are used in the of vWF (von Willebrand disease, Chapter 14) or GpIb treatment of coronary artery disease. (Bernard-Soulier syndrome) result in bleeding disorders, Platelet aggregation follows their activation. The confor- attesting to the importance of these factors. mational change in glycoprotein IIb/IIIa that occurs with Platelets rapidly change shape following adhesion, being platelet activation allows binding of fibrinogen, a large converted from smooth discs to spiky “sea urchins” with bivalent plasma polypeptide that forms bridges between greatly increased surface area. This change is accompanied adjacent platelets, leading to their aggregation. Predict- by conformational changes in cell surface glycoprotein ably, inherited deficiency of GpIIb-IIIa results in a bleeding IIb/IIIa that increase its affinity for fibrinogen and by the disorder called Glanzmann thrombasthenia. The initial wave translocation of negatively charged phospholipids (particu- of aggregation is reversible, but concurrent activation of larly phosphatidylserine) to the platelet surface. These thrombin stabilizes the platelet plug by causing further phospholipids bind calcium and serve as nucleation sites platelet activation and aggregation, and by promoting for the assembly of coagulation factor complexes. irreversible platelet contraction. Platelet contraction is Secretion (release reaction) of granule contents occurs along dependent on the cytoskeleton and consolidates the with changes in shape; these two events are often referred aggregated platelets. In parallel, thrombin also converts to together as platelet activation. Platelet activation is fibrinogen into insoluble fibrin, cementing the platelets triggered by a number of factors, including the coagulation in place and creating the definitive secondary hemostatic factor thrombin and ADP. Thrombin activates platelets plug. Entrapped red cells and leukocytes are also found in hemostatic plugs, in part due to adherence of leukocytes to P-selectin expressed on activated platelets. Deficiency: Bernard-Soulier syndrome KEY CONCEPTS PLATELET ADHESION, ACTIVATION, Deficiency: Glanzmann GpIb AND AGGREGATION thrombasthenia Platelet Endothelial injury exposes the underlying basement membrane ECM; platelets adhere to the ECM primarily through the binding GpIIb-IIIa Fibrinogen of platelet GpIb receptors to VWF. complex Adhesion leads to platelet activation, an event associated with secretion of platelet granule contents, including calcium (a GpIb Endothelium cofactor for several coagulation proteins) and ADP (a mediator ADP induces of further platelet activation); dramatic changes in shape and conformational membrane composition; and activation of GpIIb/IIIa receptors. change The GpIIb/IIIa receptors on activated platelets form bridging von Willebrand factor cross-links with fibrinogen, leading to platelet aggregation. Concomitant activation of thrombin promotes fibrin deposition, Deficiency: von Willebrand cementing the platelet plug in place. Subendothelium disease Figure 4.5 Platelet adhesion and aggregation. Von Willebrand factor Coagulation Cascade functions as an adhesion bridge between subendothelial collagen and the glycoprotein Ib (GpIb) platelet receptor. Aggregation is accomplished by The coagulation cascade is a series of amplifying enzymatic fibrinogen bridging GpIIb-IIIa receptors on different platelets. Congenital reactions that lead to the deposition of an insoluble fibrin deficiencies in the various receptors or bridging molecules lead to the clot. As discussed later, the dependency of clot formation diseases indicated in the colored boxes. ADP, Adenosine diphosphate. on various factors differs in the laboratory test tube and in  Hemostasis, hemorrhagic disorders, and thrombosis 121 A B Figure 4.6 The coagulation cascade in the laboratory and in vivo. (A) Clotting is initiated in the laboratory by adding phospholipids, calcium, and either a negatively charged substance such as glass beads (intrinsic pathway) or a source of tissue factor (extrinsic pathway). (B) In vivo, tissue factor is the major initiator of coagulation, which is amplified by feedback loops involving thrombin (dotted lines). The red polypeptides are inactive factors, the dark green polypeptides are active factors, and the light green polypeptides correspond to cofactors (reaction accelerators). Factors marked with an asterisk (*) are vitamin K dependent as are protein C and S (not depicted). Warfarin acts as an anticoagulant by inhibiting the γ-carboxylation of the vitamin K–dependent coagulation factors. Vitamin K is an essential cofactor for the synthesis of all of these vitamin K–dependent clotting factors. blood vessels in vivo (Fig. 4.6). However, clotting in vitro an enzyme (an activated coagulation factor), a substrate and in vivo both follow the same general principles, as follows. (an inactive proenzyme form of a coagulation factor), and The cascade of reactions in the pathway can be likened a cofactor (a reaction accelerator). These components are to a “dance” in which coagulation factors are passed from assembled on a negatively charged phospholipid surface, one partner to the next (Fig. 4.7). Each reaction step involves which is provided by activated platelets. Assembly of Active coagulation factor (enzyme) Phospholipid Inactive coagulation surface IXa factor (substrate) X Xa Ca2+ Xa II IIa Cofactor VIIIa Activated factor X (Xa) Cofactor Va Figure 4.7 Schematic illustration of the conversion of factor X to factor Xa, which in turn converts factor II (prothrombin) to factor IIa (thrombin) (see Fig. 4.6B). The initial reaction complex consists of a proteolytic enzyme (factor IXa), a substrate (factor X), and a reaction accelerator (factor VIIIa), all assembled on a platelet phospholipid surface. Calcium ions hold the assembled components together and are essential for the reaction. Activated factor Xa becomes the protease for the second adjacent complex in the coagulation cascade, converting prothrombin substrate (II) to thrombin (IIa) using factor Va as the reaction accelerator. 122 CHAPTER 4 Hemodynamic Disorders, Thromboembolic Disease, and Shock reaction complexes also depends on calcium, which binds to γ-carboxylated glutamic acid residues that are present in factors II, VII, IX, and X. The enzymatic reactions that Platelet produce γ-carboxylated glutamic acid use vitamin K as a aggregation Endothelium activation cofactor and are antagonized by drugs such as coumadin, ECM used as an anticoagulant. Lymphocyte Based on assays performed in clinical laboratories, the TxA2 activation coagulation cascade can be divided into the extrinsic and Fibrin intrinsic pathways (see Fig. 4.6A). Thrombin The prothrombin time (PT) assay assesses the function of the proteins in the extrinsic pathway (factors VII, X, V, Neutrophil Monocyte II [prothrombin], and fibrinogen). In brief, tissue factor, adhesion activation PDGF phospholipids, and calcium are added to plasma, and the time for a fibrin clot to form is recorded. The partial thromboplastin time (PTT) assay screens the PDGF function of the proteins in the intrinsic pathway (factors XII, XI, IX, VIII, X, V, II, and fibrinogen). In this assay, clotting of plasma is initiated by the addition of negatively Smooth charged particles (e.g., ground glass) that activate factor muscle cell XII (Hageman factor) together with phospholipids and calcium, and the time to fibrin clot formation is recorded. Figure 4.8 Role of thrombin in hemostasis and cellular activation. Thrombin plays a critical role in generating cross-linked fibrin (by cleaving Although the PT and PTT assays are of great utility in fibrinogen to fibrin and by activating factor XIII), as well as activating several other coagulation factors (see Fig. 4.6B). Through protease- evaluating coagulation factor function in patients, they activated receptors (PARs, see text), thrombin also modulates several do not recapitulate the events that lead to coagulation in cellular activities. It directly induces platelet aggregation and TxA2 vivo. This point is most clearly made by considering the production, and activates endothelial cells, which respond by expressing clinical effects of deficiencies of various coagulation factors. adhesion molecules and cytokine mediators (e.g., PDGF). Thrombin also Deficiencies of factors V, VII, VIII, IX, and X are associated directly activates leukocytes. ECM, Extracellular matrix; PDGF, platelet- with moderate to severe bleeding disorders, and prothrombin derived growth factor. See Fig. 4.10 for additional anticoagulant activities mediated by thrombin. (Courtesy Shaun Coughlin, MD, PhD, deficiency is likely incompatible with life. In contrast, factor Cardiovascular Research Institute, University of California at San Francisco; XI deficiency is only associated with mild bleeding, and modified with permission.) individuals with factor XII deficiency do not bleed and in fact may be susceptible to thrombosis. By contrast, there is evidence from experimental models suggesting that in some circumstances factor XII may contribute to thrombosis. These Pro-inflammatory effects. PARs also are expressed on paradoxical findings may stem from involvement of factor inflammatory cells, endothelium, and other cell types XII in several pathways, including the pro-inflammatory (Fig. 4.8), and activation of these receptors by thrombin bradykinin pathway as well as fibrinolysis (discussed later). is believed to mediate pro-inflammatory effects that Based on the effects of various factor deficiencies in contribute to tissue repair and angiogenesis. humans, it is believed that, in vivo, factor VIIa/tissue factor Anticoagulant effects. Remarkably, through mechanisms complex is the most important activator of factor IX and described later, on encountering normal endothelium, that factor IXa/factor VIIIa complex is the most important thrombin changes from a procoagulant to an anticoagu- activator of factor X (see Fig. 4.6B). The mild bleeding lant; this reversal in function prevents clots from extending tendency seen in patients with factor XI deficiency is likely beyond the site of the vascular injury. explained by the ability of thrombin to activate factor XI (as well as factors V and VIII), a feedback mechanism that Factors That Limit Coagulation amplifies the coagulation cascade. Once initiated, coagulation must be restricted to the site Among the coagulation factors, thrombin is the most of vascular injury to prevent deleterious consequences. important, because its various enzymatic activities control One limiting factor is simple dilution; blood flowing past diverse aspects of hemostasis and link clotting to inflam- the site of injury washes out activated coagulation factors, mation and repair. Among thrombin’s most important which are rapidly removed by the liver. A second is the activities are the following: requirement for negatively charged phospholipids, which, Conversion of fibrinogen into cross-linked fibrin. Thrombin as mentioned, are mainly provided by platelets that have directly converts soluble fibrinogen into fibrin monomers been activated by contact with subendothelial matrix at sites that polymerize into an insoluble fibril, and also amplifies of vascular injury. However, the most important counter- the coagulation process, not only by activating factor XI, regulatory mechanisms involve factors that are expressed but also by activating two critical cofactors: factors V and by intact endothelium adjacent to the site of injury (described VIII. It also stabilizes the secondary hemostatic plug by later). activating factor XIII, which covalently cross-links fibrin. Activation of the coagulation cascade also sets into motion Platelet activation. Thrombin is a potent inducer of platelet a fibrinolytic cascade that limits the size of the clot and activation and aggregation through its ability to activate contributes to its later dissolution (Fig. 4.9). Fibrinolysis is PAR-1, thereby linking platelet function to coagulation. largely accomplished through the enzymatic activity of  Hemostasis, hemorrhagic disorders, and thrombosis 123 α2-antiplasmin α2-antiplasmin/plasmin complexes Plasminogen activator Free plasmin inhibitors (PAI) Tissue plasminogen Thrombin activator (t-PA) Fibrin clot and urokinase Fibrin degradation products Plasminogen Platelets Plasmin Endothelium Figure 4.9 The fibrinolytic system, illustrating various plasminogen activators and inhibitors (see text). plasmin, which breaks down fibrin and interferes with its Endothelium polymerization. An elevated level of breakdown products The balance between the anticoagulant and procoagulant of fibrinogen (often called fibrin split products), most notably activities of endothelium often determines whether clot fibrin-derived D-dimers, are a useful clinical marker of several formation, propagation, or dissolution occurs (Fig. 4.10). thrombotic states (described later). Plasmin is generated by Normal endothelial cells express a multitude of factors that enzymatic catabolism of the inactive circulating precursor inhibit the procoagulant activities of platelets and coagulation plasminogen, either by a factor XII–dependent pathway factors and that augment fibrinolysis. These factors act in (possibly explaining the association of factor XII deficiency concert to prevent thrombosis and to limit clotting to sites and thrombosis) or by plasminogen activators. The most of vascular damage. However, if injured or exposed to pro- important plasminogen activator is t-PA; it is synthesized inflammatory factors, endothelial cells lose many of their principally by endothelium and is most active when bound antithrombotic properties. Here, we complete the discussion to fibrin. This characteristic makes t-PA a useful therapeutic of hemostasis by focusing on the antithrombotic activities agent, since its fibrinolytic activity is largely confined to of normal endothelium; we return to the “dark side” of sites of recent thrombosis. Once activated, plasmin is in endothelial cells later when discussing thrombosis. turn tightly controlled by counterregulatory factors such The antithrombotic properties of endothelium can be as α2-plasmin inhibitor, a plasma protein that binds and divided into activities directed at platelets, coagulation rapidly inhibits free plasmin. factors, and fibrinolysis. Platelet inhibitory effects. An obvious effect of intact endothelium is to serve as a barrier that shields platelets from subendothelial vWF and collagen. However, normal KEY CONCEPTS endothelium also releases a number of factors that inhibit COAGULATION FACTORS platelet activation and aggregation. Among the most important are prostacyclin (PGI2), nitric oxide (NO), and Coagulation occurs via the sequential enzymatic conversion of adenosine diphosphatase; the latter degrades ADP, already a cascade of circulating and locally synthesized proteins. discussed as a potent activator of platelet aggregation. Tissue factor elaborated at sites of injury is the most important The major regulator of NO and PGI2 production appears initiator of the coagulation cascade in vivo. to be flow; precisely what senses flow is uncertain, though At the final stage of coagulation, thrombin converts fibrinogen changes in cell shape and cytoskeleton are correlated. into insoluble fibrin that contributes to formation of the definitive PGI2 is produced by COX-1, which is expressed consti- hemostatic plug. tutively by “healthy” endothelium under normal flow Coagulation normally is restricted to sites of vascular injury conditions. NO is the product of endothelial nitric oxide by: synthase eNOS. Limiting enzymatic activation to phospholipid surfaces provided Anticoagulant effects. Normal endothelium shields coagula- by activated platelets or endothelium tion factors from tissue factor in vessel walls and expresses Circulating inhibitors of coagulation factors, such as anti- multiple factors that actively oppose coagulation, most thrombin III, whose activity is augmented by heparin-like notably thrombomodulin, endothelial protein C receptor, molecules expressed on endothelial cells heparin-like molecules, and tissue factor pathway inhibi- Expression of thrombomodulin on normal endothelial cells, tor. Thrombomodulin and endothelial protein C receptor which binds thrombin and converts it to an anticoagulant bind thrombin and protein C, respectively, in a complex Activation of fibrinolytic pathways (e.g., by association of t-PA on the endothelial cell surface. When bound in this with fibrin) complex, thrombin loses its ability to activate coagulation 124 CHAPTER 4 Hemodynamic Disorders, Thromboembolic Disease, and Shock INHIBIT Heparin-like Tissue factor Thrombomodulin THROMBOSIS molecule pathway inhibitor Endothelial effects t-PA PGI2, NO, and Anti-thrombin adenosine III Thrombin diphosphatase Inactivates tissue factor-VIIa complexes Thrombin Inhibits platelet Protein C Active protein C aggregation Inactivates thrombin (also factors IXa and Xa) (requires protein S) Activates Inactivates factors Va and VIIIa fibrinolysis FAVOR THROMBOSIS Extrinsic coagulation sequence Platelet adhesion (held together by fibrinogen) vWF Exposure of membrane-bound tissue factor Collagen Figure 4.10 Anticoagulant activities of normal endothelium (top) and procoagulant properties of injured or activated endothelium (bottom). NO, Nitric oxide; PGI2, prostacyclin; t-PA, tissue plasminogen activator; vWF, von Willebrand factor. The thrombin receptor is one of the protease-activated receptors (PAR). factors and platelets, and instead cleaves and activates properly to ensure hemostasis. The presentation of abnormal protein C, a vitamin K–dependent protease that requires bleeding varies widely. At one end of the spectrum are a cofactor, protein S. Activated protein C/protein S massive bleeds associated with ruptures of large vessels complex is a potent inhibitor of coagulation cofactors Va such as the aorta or of the heart; these catastrophic events and VIIIa. Heparin-like molecules on the surface of endo- simply overwhelm hemostatic mechanisms and are often thelium bind and activate antithrombin III, which then fatal. Diseases associated with sudden, massive hemorrhage inhibits thrombin and factors IXa, Xa, XIa, and XIIa. The include aortic dissection and aortic abdominal aneurysm clinical utility of heparin and related drugs is based on (Chapter 11) and myocardial infarction (Chapter 12) com- their ability to stimulate antithrombin III activity. Tissue plicated by rupture of the aorta or the heart. At the other factor pathway inhibitor (TFPI), like protein C, requires end of the spectrum are subtle defects in clotting that only protein S as a cofactor and, as the name implies, binds become evident under conditions of hemostatic stress, such and inhibits tissue factor/factor VIIa complexes. as surgery, childbirth, dental procedures, menstruation, or Fibrinolytic effects. Normal endothelial cells synthesize trauma. Among the most common causes of mild bleeding t-PA, as already discussed, a key component of the tendencies are inherited defects in vWF (Chapter 14), aspirin fibrinolytic pathway. consumption, and uremia (renal failure); the latter alters platelet function through uncertain mechanisms. Between Hemorrhagic Disorders these extremes lie deficiencies of coagulation factors (the hemophilias, Chapter 14), which are usually inherited and Disorders associated with abnormal bleeding inevitably lead to severe bleeding disorders if untreated. stem from primary or secondary defects in vessel walls, Additional specific examples of disorders associated with platelets, or coagulation factors, all of which must function abnormal bleeding are discussed throughout the book. The  Hemostasis, hemorrhagic disorders, and thrombosis 125 A B Figure 4.11 (A) Punctate petechial hemorrhages of the colonic mucosa, a consequence of thrombocytopenia. (B) Fatal intracerebral bleed. following are general principles related to abnormal bleeding supply or causes herniation of the brain stem (Chapter 28). and its consequences. Finally, chronic or recurrent external blood loss (e.g., peptic Defects of primary hemostasis (platelet defects or von Wil- ulcer or menstrual bleeding) causes iron loss and can lead lebrand disease) often present with small bleeds in skin to iron deficiency anemia. In contrast, when red cells are or mucosal membranes. These bleeds typically take the retained (e.g., hemorrhage into body cavities or tissues), form of petechiae, minute 1- to 2-mm hemorrhages (Fig. iron is recovered and recycled for use in the synthesis of 4.11A), or purpura, which are slightly larger (≥3 mm) than hemoglobin. petechiae. It is believed that the capillaries of the mucosa and skin are particularly prone to rupture following minor Thrombosis trauma and that under normal circumstances platelets seal these defects virtually immediately. Mucosal bleeding The primary abnormalities that lead to thrombosis are (1) associated with defects in primary hemostasis may also take endothelial injury, (2) stasis or turbulent blood flow, and the form of epistaxis (nosebleeds), gastrointestinal bleed- (3) hypercoagulability of the blood (the so-called Virchow ing, or excessive menstruation (menorrhagia). A feared triad) (Fig. 4.12). Thrombosis is one of the scourges of modern complication of very low platelet counts (thrombocytopenia) man, because it underlies the most serious and common is intracerebral hemorrhage, which may be fatal. forms of cardiovascular disease. Here, the focus is on its Defects of secondary hemostasis (coagulation factor defects) causes and consequences; its role in cardiovascular disorders often present with bleeds into soft tissues (e.g., muscle) is discussed in detail in Chapters 11 and 12. or joints. Bleeding into joints (hemarthrosis) following minor trauma is particularly characteristic of hemophilia (Chapter 14). It is unknown why severe defects in second- Hypercholesterolemia Inflammation ary hemostasis present with this peculiar pattern of bleeding; as with severe platelet defects, intracranial ENDOTHELIAL INJURY hemorrhage, sometimes fatal, may also occur. Generalized defects involving small vessels often present with “palpable purpura” and ecchymoses. Ecchymoses (some- times simply called bruises) are hemorrhages of 1 to 2 cm in size. In both purpura and ecchymoses, the volume of extravasated blood may be large enough to create a THROMBOSIS palpable mass of blood known as a hematoma. Purpura and ecchymoses are particularly characteristic of systemic disorders that disrupt small blood vessels (e.g., vasculitis, Chapter 11) or that lead to blood vessel fragility (e.g., ABNORMAL amyloidosis, Chapter 6; scurvy, Chapter 9). HYPERCOAGULABILITY BLOOD FLOW Inherited The clinical significance of hemorrhage depends on the Stasis (e.g., factor V Leiden) (e.g., atrial fibrilation, bed rest) volume of the bleed, the rate at which it occurs, and its Turbulence Acquired location. Rapid loss of up to 20% of the blood volume may (e.g., disseminated cancer) (e.g., atherosclerotic have little impact in healthy adults; greater losses, however, vessel narrowing) can cause hemorrhagic (hypovolemic) shock (discussed later). Figure 4.12 The Virchow triad in thrombosis. Endothelial integrity is the Bleeding that is relatively trivial in the subcutaneous tissues most important factor. Injury to endothelial cells can alter local blood flow can cause death if located in the brain (see Fig. 4.11B); because and affect coagulability. Abnormal blood flow (stasis or turbulence), in turn, the skull is unyielding, intracranial hemorrhage may increase can cause endothelial injury. These factors may promote thrombosis intracranial pressure to a level that compromises the blood independently or in combination. 126 CHAPTER 4 Hemodynamic Disorders, Thromboembolic Disease, and Shock Endothelial Injury Altered blood flow contributes to thrombosis in several Endothelial injury leading to platelet activation almost clinical settings. Ulcerated atherosclerotic plaques not only inevitably underlies thrombus formation in the heart and expose subendothelial vWF and tissue factor but also cause the arterial circulation, where the high rates of blood flow turbulence. Aortic and arterial dilations called aneurysms impede clot formation. Notably, cardiac and arterial clots result in local stasis and are therefore fertile sites for throm- are typically rich in platelets, and it is believed that platelet bosis (Chapter 11). Acute myocardial infarction results in adherence and activation is a necessary prerequisite for areas of noncontractile myocardium and sometimes in cardiac thrombus formation under high shear stress, such as exists aneurysm; both are associated with stasis and flow abnor- in arteries. This insight provides part of the reasoning behind malities that promote the formation of cardiac mural thrombi the use of aspirin and other platelet inhibitors in coronary (Chapter 12). Rheumatic mitral valve stenosis results in left artery disease and acute myocardial infarction. atrial dilation; in conjunction with atrial fibrillation, a dilated Obviously, severe endothelial injury may trigger atrium is a site of profound stasis and a prime location for thrombosis by exposing vWF and tissue factor. However, thrombosis (Chapter 12). Hyperviscosity (such as is seen inflammation and other noxious stimuli also promote with polycythemia vera; Chapter 13) increases resistance thrombosis by shifting the pattern of gene expression in to flow and causes small vessel stasis, and the deformed endothelium to one that is “prothrombotic.” This change red cells in sickle cell anemia (Chapter 14) impede blood flow is sometimes referred to as endothelial activation or dysfunc- through small vessels, with the resulting stasis also predispos- tion and can be produced by diverse exposures, including ing to thrombosis. physical injury, infectious agents, abnormal blood flow, inflammatory mediators, metabolic abnormalities, such Hypercoagulability as hypercholesterolemia or homocystinemia, and toxins Hypercoagulability refers to an abnormally high tendency absorbed from cigarette smoke. Endothelial activation is of the blood to clot, and is typically caused by alterations believed to have an important role in triggering arterial in coagulation factors. Hypercoagulability has a particularly thrombotic events. important role in venous thrombosis and can be divided The role of endothelial cell activation and dysfunction into primary (genetic) and secondary (acquired) disorders in arterial thrombosis is discussed in detail in Chapters 11 (Table 4.2). Of the inherited causes of hypercoagulability, and 12. Here it suffices to mention several of the major prothrombotic alterations: Table 4.2 Hypercoagulable States Procoagulant changes. Endothelial cells activated by inflammatory cytokines downregulate the expression of Primary (Genetic) thrombomodulin, already described as a key modulator of Common thrombin activity, enhancing the procoagulant and pro- Factor V mutation: factor V Leiden (Arg to Gln substitution in amino inflammatory actions of thrombin. In addition, inflamed acid residue 506 leading to resistance to activated protein C) endothelium also downregulates the expression of other Prothrombin mutation (G20210A noncoding sequence variant leading anticoagulants, such as protein C and tissue factor protein to increased prothrombin levels) inhibitor, changes that further promote a procoagulant Increased levels of factors VIII, IX, XI, or fibrinogen (genetics unknown) state. Rare Antifibrinolytic effects. Activated endothelial cells secrete Antithrombin III deficiency plasminogen activator inhibitors (PAIs), which limit fibrino- Protein C deficiency lysis, and downregulate the expression of t-PA, alterations Protein S deficiency that also favor the development of thrombi. Very Rare Fibrinolysis defects Alterations in Normal Blood Flow Homozygous homocystinuria (deficiency of cystathione β-synthetase) Turbulence contributes to arterial and cardiac thrombosis Secondary (Acquired) by causing endothelial injury or dysfunction as well as by forming countercurrents that contribute to local pockets of Strong Risk Factors for Thrombosis stasis, whereas stasis is a major contributor to the develop- Prolonged bed rest or immobilization ment of venous thrombi Normal blood flow is laminar such Myocardial infarction that the platelets (and other blood cellular elements) flow Atrial fibrillation Tissue injury (surgery, fracture, burn) centrally in the vessel lumen, separated from the endothelium Cancer by a slower moving layer of plasma. Turbulence and stasis Prosthetic cardiac valves therefore: Disseminated intravascular coagulation Promote endothelial activation, enhancing procoagulant Heparin-induced thrombocytopenia activity and leukocyte adhesion, in part through flow- Antiphospholipid antibody syndrome induced changes in the expression of adhesion molecules Other Risk Factors for Thrombosis and pro-inflammatory factors Cardiomyopathy Disrupt laminar flow and bring platelets into contact with Nephrotic syndrome the endothelium Hyperestrogenic states (pregnancy and postpartum) Prevent washout and dilution of activated clotting factors by Oral contraceptive use fresh flowing blood and the inflow of clotting factor Sickle cell anemia Smoking inhibitors  Hemostasis, hemorrhagic disorders, and thrombosis 127 point mutations in the factor V gene and prothrombin gene In disseminated cancers, release of various procoagulants are the most common. These are listed below: from tumors predisposes to thrombosis. The hypercoagulabil- Factor V Leiden. Approximately 2% to 15% of Caucasians ity seen with advancing age may be due to reduced carry a single-nucleotide mutation in factor V that is endothelial PGI2 production. Smoking and obesity promote called factor V Leiden, after the city in the Netherlands hypercoagulability by unknown mechanisms. where it was discovered. Among individuals with recur- Among the acquired thrombophilic states, heparin- rent DVT, the frequency of this mutation is considerably induced thrombocytopenia and antiphospholipid antibody higher, approaching 60%. This mutation renders factor syndrome are particularly important clinical problems that V resistant to cleavage and inactivation by protein C. As deserve special mention. a result, an important antithrombotic counterregulatory pathway is lost (see Fig. 4.10). The inheritance pattern Heparin-Induced Thrombocytopenia (HIT) Syndrome for factor V Leiden is autosomal dominant. Heterozygotes HIT syndrome is a serious, potentially life-threatening have a fivefold increased relative risk of venous throm- disorder that occurs following the administration of bosis, and homozygotes have a 50-fold increase. unfractionated heparin. It results from the formation of Prothrombin gene mutation. A single nucleotide change antibodies that recognize complexes of heparin and PF4 on (G20210A) in the 3′-untranslated region of the prothrom- the surface of platelets (Chapter 14), as well as complexes bin gene is another common mutation (1% to 2% of the of heparin-like molecules and PF4-like proteins on endothelial population) associated with hypercoagulability. It leads cells. PF4 protein is normally found in platelet alpha granules to elevated prothrombin levels and an almost threefold and is released on activation of platelets. Released PF4 binds increased risk of venous thrombosis. to heparin and undergoes a conformational change that Other inherited causes. Rare inherited causes of primary results in the formation of a neoantigen against which IgG hypercoagulability include deficiencies of anticoagulants antibodies are formed. PF4-IgG immune complex (Fig. 4.13) such as antithrombin III, protein C, or protein S; affected attaches to and cross-links the Fc receptors on the platelet individuals typically present with venous thrombosis surface, which leads to platelet activation and aggregation. and recurrent thromboembolism beginning in adolescence Platelet activation results in the release of more PF4, creating or early adulthood. more target antigen for HIT antibodies. The prothrombotic Homocysteinemia. Elevated levels of homocysteine may state may also be augmented by activation of endothelium be inherited or acquired. Marked elevations of homo- by binding of HIT antibodies to PF4-like proteins on their cysteine may be caused by an inherited deficiency of surface. Binding of HIT antibodies to platelets results in cystathione β-synthetase. Acquired causes include their removal by macrophages (hence the thrombocytopenia deficiency of vitamin B6, B12, and folic acid. Prothrombotic in the syndrome name). Although thrombocytopenia is the effects of homocysteine may be due to thioester linkages most common manifestation, thrombosis is the most serious formed between homocysteine metabolites and a variety complication. It occurs in approximately 50% of cases and of proteins, including fibrinogen. affects both veins and arteries. Necrosis of the skin, gangrene of the limbs, stroke, and myocardial infarction are some of The most common thrombophilic genotypes found in the sequelae. Diagnosis requires the demonstration of various populations (heterozygosity for factor V Leiden and anti–PF4-heparin antibodies. Low-molecular-weight heparin heterozygosity for the prothrombin G20210A variant) impart preparations induce HIT less frequently, and other classes only a moderately increased risk of thrombosis; most of anticoagulants such as direct inhibitors of factor X and individuals with these genotypes, when otherwise healthy, thrombin may also obviate the risk. are free from thrombotic complications. However, factor V and prothrombin mutations are frequent enough that Antiphospholipid Antibody Syndrome (APS) homozygosity and compound heterozygosity are not rare, APS is an autoimmune disorder characterized by: and such genotypes are associated with greater risk. More- Presence of one or more antiphospholipid (aPL) autoantibodies over, individuals with such mutations have a significantly Venous or arterial thromboses, or pregnancy complications increased frequency of venous thrombosis in the setting of such as recurrent miscarriages, unexplained fetal death, and other acquired risk factors (e.g., pregnancy or prolonged premature birth. bed rest). Thus, factor V Leiden heterozygosity may trigger deep vein thrombosis (DVT) when combined with enforced APS may be primary or secondary. Individuals with a inactivity, such as during prolonged airplane travel. Con- well-defined autoimmune disease, such as systemic lupus sequently, inherited causes of hypercoagulability must be erythematosus (Chapter 6), are designated as having second- considered in patients younger than 50 years of age who ary antiphospholipid syndrome (hence the earlier term lupus present with thrombosis—even when acquired risk factors anticoagulant syndrome). In primary antiphospholipid syn- are present. drome, patients exhibit only the manifestations of a hyper- Unlike hereditary disorders, the pathogenesis of acquired coagulable state and lack evidence of other well-defined thrombophilia is frequently multifactorial (see Table 4.2). In autoimmune disorders. Approximately 50% of the patients some cases (e.g., cardiac failure or trauma), stasis or vascular with APS have the primary form, and the rest occur in injury may be most important. Hypercoagulability due to association with well-defined autoimmune disease, most oral contraceptive use or the hyperestrogenic state of commonly SLE. Our focus here is on the primary form. pregnancy is probably caused by increased hepatic synthesis The clinical manifestations of APS are varied; they include of coagulation factors and reduced anticoagulant synthesis. recurrent thromboses, repeated miscarriages, cardiac valve 128 CHAPTER 4 Hemodynamic Disorders, Thromboembolic Disease, and Shock α-granule However, unlike most other clinical features, fetal loss does Fc receptor not appear to be caused by thrombosis, but rather seems to Platelet PF-4 stem from antibody-mediated interference with the growth and differentiation of trophoblasts, leading to a failure of placentation. Activation Although antiphospholipid antibodies are clearly associ- ated with thrombotic diatheses, they have also been identified in 5% to 15% of apparently normal individuals, implying that they are not sufficient to cause the full-blown syndrome. It is postulated that a “second hit” is required that may be provided by infection, smoking, or pregnancy, among others. Although these antibodies induce a hypercoagulable Heparin state in vivo, they interfere with phospholipids and thus inhibit coagulation in vitro, thereby prolonging the PTT. The Heparin + PF-4 antibodies also frequently result in a false-positive serologic test for syphilis because the antigen in the standard assay is embedded in cardiolipin, which cross reacts with phos- B B cell pholipids of Treponema pallidum. Diagnosis of APS is based on clinical features and demonstration of aPL antibodies in the serum. Therapy of APS involves various forms of anticoagulation. Anti heparin + PF-4 IgG MORPHOLOGY IgG binding to Thrombi can develop anywhere in the cardiovascular system and platelet Fc vary in size and shape depending on the involved site and the receptor underlying cause. Arterial or cardiac thrombi usually begin at sites of turbulence or endothelial injury, whereas venous thrombi characteristically occur at sites of stasis. Thrombi are focally Platelet activation attached to the underlying vascular surface, particularly at the point of initiation. From here, arterial thrombi tend to grow Platelet aggregation Platelet removal by retrograde, and venous thrombi extend in the direction of blood splenic macrophages flow; thus both propagate toward the heart. The propagating portion of a thrombus is often poorly attached and therefore THROMBOSIS THROMBOCYTOPENIA prone to fragmentation and embolization. Thrombi often have grossly and microscopically apparent Figure 4.13 Mechanism of heparin-induced thrombocytopenia. laminations called lines of Zahn, which are pale platelet and fibrin deposits alternating with darker red cell–rich layers. Such laminations signify that a thrombus has formed in flowing blood; their presence can therefore distinguish antemortem clots from vegetations, and thrombocytopenia. Depending on the the bland nonlaminated clots that occur postmortem (see later). vascular bed involved, the clinical presentations can include Thrombi occurring in heart chambers or in the aortic lumen pulmonary embolism (following lower extremity venous are designated mural thrombi. Abnormal myocardial contraction thrombosis), pulmonary hypertension (from recurrent (arrhythmias, dilated cardiomyopathy, or myocardial infarction) subclinical pulmonary emboli), valvular heart disease, stroke, or endomyocardial injury (myocarditis or catheter trauma) bowel infarction, or renovascular hypertension. promotes cardiac mural thrombi (Fig. 4.14A), and ulcerated The pathogenesis of antiphospholipid syndrome is atherosclerotic plaque and aneurysmal dilation underlie aortic complex and not fully understood. The aPL antibodies are thrombi (see Fig. 4.14B). directed against anionic membrane phospholipids or proteins Arterial thrombi are frequently occlusive; the most common associated with phospholipids. Proteins that are recognized sites in decreasing order of frequency are the coronary, cerebral, by these antibodies include cardiolipin and β2-glycoprotein I. and femoral arteries. They typically consist of a friable meshwork This glycoprotein is found in plasma, but it has strong avidity of platelets, fibrin, red cells, and degenerating leukocytes. Although for phospholipids expressed on the surfaces of endothelial these are usually superimposed on a ruptured atherosclerotic cells, monocytes, platelets, thrombin, and trophoblasts. plaque, other vascular injuries (vasculitis, trauma) may be the Anti–β2-glycoprotein antibodies are suspected to have a major underlying cause. role in APS by activating endothelial cells, monocytes, and Venous thrombosis (phlebothrombosis) is almost invari- platelets. Their pathogenicity is supported by the observation ably occlusive, with the thrombus forming a long luminal cast. that transfer of these antibodies into rodents can induce Because these thrombi form in the sluggish venous circulation, thrombosis. Patients with APS also show evidence of comple- they tend to contain more enmeshed red cell