Hemodynamic Disorders, Thromboembolic Disease, and Shock

Increased Hydrostatic Pressure

Increases in hydrostatic pressure are mainly caused by disorders that impair venous return. If the impairment is localized (e.g., a deep venous thrombosis [DVT] in a lower extremity), then the resulting edema is confined to the affected part. Conditions leading to systemic increases in venous pressure (e.g., congestive heart failure, Chapter 12 ) are understandably associated with more widespread edema.

Reduced Plasma Osmotic Pressure

Under normal circumstances albumin accounts for almost half of the total plasma protein; it follows that conditions leading to inadequate synthesis or increased loss of albumin from the circulation are common causes of reduced plasma oncotic pressure. Reduced albumin synthesis occurs mainly in severe liver diseases (e.g., end-stage cirrhosis, Chapter 18 ) and protein malnutrition ( Chapter 9 ). An important cause of albumin loss is the nephrotic syndrome ( Chapter 20 ), in which albumin leaks into the urine through abnormally permeable glomerular capillaries. Regardless of cause, reduced plasma osmotic pressure leads in a stepwise fashion to edema, reduced intravascular volume, renal hypoperfusion, and secondary hyperaldosteronism. Not only does the ensuing salt and water retention by the kidney fail to correct the plasma volume deficit, but it also exacerbates the edema, because the primary defect—a low plasma protein level—persists.

Sodium and Water Retention

Increased salt retention—with obligate retention of associated water—causes both increased hydrostatic pressure (due to intravascular fluid volume expansion) and diminished vascular colloid osmotic pressure (due to dilution). Salt retention occurs whenever renal function is compromised, such as in primary kidney disorders and in cardiovascular 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 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 pressure, leading to edema and effusions.

Lymphatic Obstruction

Trauma, fibrosis, invasive tumors, and infectious agents can all disrupt lymphatic vessels and impair the clearance of interstitial fluid, resulting in lymphedema in the affected part of the body. A dramatic example is seen in parasitic filariasis, in which the organism induces obstructive fibrosis of lymphatic channels and lymph nodes. This may result 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 removal and/or irradiation of the breast and associated axillary lymph nodes in patients with breast cancer.

Morphology

Edema is easily recognized grossly; microscopically, it is appreciated as clearing and separation of the extracellular matrix (ECM) and subtle cell swelling. Edema is most commonly seen in subcutaneous tissues, the lungs, and the brain. Subcutaneous edema can be diffuse or more conspicuous in regions with high hydrostatic pressures. Its distribution is often influenced by gravity (e.g., it appears in the legs when standing and the sacrum when recumbent), a feature termed dependent edema. Finger pressure over 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 the eyelids; periorbital edema is thus a characteristic finding in severe renal disease. With pulmonary edema, the lungs are often two to three times their normal weight, and sectioning yields frothy, blood-tinged fluid—a mixture of air, edema, and extravasated red cells. Brain edema can be localized or generalized depending on the nature and extent of the pathologic process or injury. The swollen brain exhibits narrowed sulci and distended gyri, which are compressed by the unyielding skull ( Chapter 28 ).

Effusions involving the pleural cavity (hydrothorax), the pericardial cavity (hydropericardium), or the peritoneal cavity ( hydroperitoneum or ascites ) are common in a wide range of clinical settings. Transudative effusions are typically protein-poor, translucent, and straw colored; an exception are peritoneal effusions caused by lymphatic blockage (chylous effusion), which may be milky due to the presence of lipids absorbed from the gut. In contrast, exudative effusions are protein-rich and often cloudy due to the presence of white cells.

Clinical Features

The consequences of edema range from merely annoying to rapidly fatal. Subcutaneous edema is important primarily because it signals potential underlying cardiac or renal disease; however, when significant, it can also impair wound healing and the clearance of infections. Pulmonary edema is a common clinical problem that is most frequently seen in the setting of left ventricular failure; it can also occur with renal failure, acute respiratory distress syndrome ( Chapter 15 ), and pulmonary inflammation or infection. Edema in the pulmonary interstitium and the alveolar spaces impedes gas exchange (leading to hypoxemia) and also creates a favorable environment for bacterial infection. Pulmonary edema is often exacerbated by pleural effusions, which may further compromise gas exchange by compressing the underlying pulmonary parenchyma. Peritoneal effusions (ascites) resulting most commonly from portal hypertension are prone to seeding by bacteria, leading to serious and sometimes fatal infections. Brain edema is life threatening; 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 centers and cause death ( Chapter 28 ).

Key Concepts

Edema

Edema is the result of the movement of fluid from the vasculature into the interstitial spaces; the fluid may be protein-poor (transudate) or protein-rich (exudate).

Edema may be caused by:

• Increased hydrostatic pressure (e.g., heart failure)

• Decreased colloid osmotic pressure caused by reduced plasma albumin, either due to decreased synthesis (e.g., liver disease, protein malnutrition) or to increased loss (e.g., nephrotic syndrome)

• Increased vascular permeability (e.g., inflammation)

• Lymphatic obstruction (e.g., infection or neoplasia)

• Sodium and water retention (e.g., renal failure)

Platelets

Platelets play a critical role in hemostasis by forming the primary plug that initially seals vascular defects and by 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 ) and contain proteins involved in coagulation, such as fibrinogen, coagulation factor V, and vWF, as well as protein factors that may be involved in wound healing, such as fibronectin, platelet factor 4 (PF4, a heparin-binding chemokine), platelet-derived growth factor (PDGF), and transforming growth factor-β. Dense (or δ) granules contain adenosine diphosphate (ADP), adenosine triphosphate, ionized calcium, serotonin, and epinephrine.

After a traumatic vascular injury, platelets encounter constituents of the subendothelial connective tissue, such as vWF and collagen. On contact with these proteins, platelets undergo a sequence of reactions that culminate in the formation of a platelet plug (see Fig. 4.4B ).

• Platelet adhesion is mediated largely via interactions between the platelet surface receptor glycoprotein Ib (GpIb) and vWF in the subendothelial matrix ( Fig. 4.5 ). Platelets also adhere to exposed collagen via the platelet collagen receptor Gp1a/IIa. Notably, genetic deficiencies of vWF (von Willebrand disease, Chapter 14 ) or GpIb (Bernard-Soulier syndrome) result in bleeding disorders, attesting to the importance of these factors.

  

• Platelets rapidly change shape following adhesion, being converted from smooth discs to spiky “sea urchins” with greatly increased surface area. This change is accompanied by conformational changes in cell surface glycoprotein IIb/IIIa that increase its affinity for fibrinogen and by the translocation of negatively charged phospholipids (particularly phosphatidylserine) to the platelet surface. These phospholipids bind calcium and serve as nucleation sites for the assembly of coagulation factor complexes.

• Secretion (release reaction) of granule contents occurs along with changes in shape; these two events are often referred to together as platelet activation. Platelet activation is triggered by a number of factors, including the coagulation factor thrombin and ADP. Thrombin activates platelets through a special G-protein–coupled receptor referred to as protease-activated receptor-1 (PAR-1), which is switched on by a proteolytic cleavage carried out by thrombin. ADP is a component of dense-body granules; thus, platelet activation and ADP release begets additional rounds of platelet activation, a phenomenon referred to as recruitment . ADP acts by binding two G-protein–coupled receptors. P2Y ₁ and P2Y ₁₂ . Activated platelets also produce the prostaglandin thromboxane A ₂ (TxA ₂ ), a potent inducer of platelet aggregation. An understanding of the biochemical pathways involved in platelet activation has led to the development of drugs with antiplatelet activities. Aspirin , the oldest of all, inhibits platelet aggregation and produces a mild bleeding defect by inhibiting cyclooxygenase, a platelet enzyme that is required for TxA ₂ synthesis. More recently, drugs that inhibit platelet function by antagonizing PAR-1 or P2Y ₁₂ have been developed. All of these antiplatelet drugs are used in the treatment of coronary artery disease.

• Platelet aggregation follows their activation. The conformational change in glycoprotein IIb/IIIa that occurs with platelet activation allows binding of fibrinogen, a large bivalent plasma polypeptide that forms bridges between adjacent platelets, leading to their aggregation. Predictably, inherited deficiency of GpIIb-IIIa results in a bleeding disorder called Glanzmann thrombasthenia . The initial wave of aggregation is reversible, but concurrent activation of thrombin stabilizes the platelet plug by causing further platelet activation and aggregation, and by promoting irreversible platelet contraction. Platelet contraction is dependent on the cytoskeleton and consolidates the aggregated platelets. In parallel, thrombin also converts fibrinogen into insoluble fibrin, cementing the platelets in place and creating the definitive secondary hemostatic 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.

Key Concepts

Platelet Adhesion, Activation, and Aggregation

• Endothelial injury exposes the underlying basement membrane ECM; platelets adhere to the ECM primarily through the binding of platelet GpIb receptors to VWF.

• Adhesion leads to platelet activation, an event associated with secretion of platelet granule contents, including calcium (a cofactor for several coagulation proteins) and ADP (a mediator of further platelet activation); dramatic changes in shape and membrane composition; and activation of GpIIb/IIIa receptors.

• The GpIIb/IIIa receptors on activated platelets form bridging cross-links with fibrinogen, leading to platelet aggregation.

• Concomitant activation of thrombin promotes fibrin deposition, cementing the platelet plug in place.

Coagulation Cascade

The coagulation cascade is a series of amplifying enzymatic reactions that lead to the deposition of an insoluble fibrin clot. As discussed later, the dependency of clot formation on various factors differs in the laboratory test tube and in blood vessels in vivo ( Fig. 4.6 ). However, clotting in vitro and in vivo both follow the same general principles, as follows.

The cascade of reactions in the pathway can be likened to a “dance” in which coagulation factors are passed from one partner to the next ( Fig. 4.7 ). Each reaction step involves an enzyme (an activated coagulation factor), a substrate (an inactive proenzyme form of a coagulation factor), and a cofactor (a reaction accelerator). These components are assembled on a negatively charged phospholipid surface, which is provided by activated platelets. Assembly of 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 produce γ-carboxylated glutamic acid use vitamin K as a cofactor and are antagonized by drugs such as coumadin, used as an anticoagulant.

Based on assays performed in clinical laboratories, the coagulation cascade can be divided into the extrinsic and intrinsic pathways (see Fig. 4.6A ).

• The prothrombin time (PT) assay assesses the function of the proteins in the extrinsic pathway (factors VII, X, V, II [prothrombin], and fibrinogen). In brief, tissue factor, 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 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 charged particles (e.g., ground glass) that activate factor XII (Hageman factor) together with phospholipids and calcium, and the time to fibrin clot formation is recorded.

Although the PT and PTT assays are of great utility in evaluating coagulation factor function in patients, they do not recapitulate the events that lead to coagulation in vivo. This point is most clearly made by considering the clinical effects of deficiencies of various coagulation factors. Deficiencies of factors V, VII, VIII, IX, and X are associated with moderate to severe bleeding disorders, and prothrombin deficiency is likely incompatible with life. In contrast, factor XI deficiency is only associated with mild bleeding, and 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 paradoxical findings may stem from involvement of factor XII in several pathways, including the pro-inflammatory bradykinin pathway as well as fibrinolysis (discussed later).

Based on the effects of various factor deficiencies in humans, it is believed that, in vivo, factor VIIa/tissue factor complex is the most important activator of factor IX and that factor IXa/factor VIIIa complex is the most important activator of factor X (see Fig. 4.6B ). The mild bleeding tendency seen in patients with factor XI deficiency is likely explained by the ability of thrombin to activate factor XI (as well as factors V and VIII), a feedback mechanism that amplifies the coagulation cascade.

Among the coagulation factors, thrombin is the most important, because its various enzymatic activities control diverse aspects of hemostasis and link clotting to inflammation and repair. Among thrombin's most important activities are the following:

• Conversion of fibrinogen into cross-linked fibrin . Thrombin directly converts soluble fibrinogen into fibrin monomers that polymerize into an insoluble fibril, and also amplifies the coagulation process, not only by activating factor XI, but also by activating two critical cofactors: factors V and VIII. It also stabilizes the secondary hemostatic plug by activating factor XIII, which covalently cross-links fibrin.

• Platelet activation . Thrombin is a potent inducer of platelet activation and aggregation through its ability to activate PAR-1, thereby linking platelet function to coagulation.

• Pro-inflammatory effects . PARs also are expressed on inflammatory cells, endothelium, and other cell types ( Fig. 4.8 ), and activation of these receptors by thrombin is believed to mediate pro-inflammatory effects that contribute to tissue repair and angiogenesis.

  

• Anticoagulant effects. Remarkably, through mechanisms described later, on encountering normal endothelium, thrombin changes from a procoagulant to an anticoagulant; this reversal in function prevents clots from extending beyond the site of the vascular injury.

Factors That Limit Coagulation

Once initiated, coagulation must be restricted to the site of vascular injury to prevent deleterious consequences. One limiting factor is simple dilution; blood flowing past the site of injury washes out activated coagulation factors, which are rapidly removed by the liver. A second is the requirement for negatively charged phospholipids, which, as mentioned, are mainly provided by platelets that have been activated by contact with subendothelial matrix at sites of vascular injury. However, the most important counterregulatory mechanisms involve factors that are expressed by intact endothelium adjacent to the site of injury (described later).

Activation of the coagulation cascade also sets into motion a fibrinolytic cascade that limits the size of the clot and contributes to its later dissolution ( Fig. 4.9 ). Fibrinolysis is largely accomplished through the enzymatic activity of plasmin, which breaks down fibrin and interferes with its polymerization. An elevated level of breakdown products of fibrinogen (often called fibrin split products), most notably fibrin-derived D-dimers, are a useful clinical marker of several thrombotic states (described later). Plasmin is generated by enzymatic catabolism of the inactive circulating precursor plasminogen , either by a factor XII–dependent pathway (possibly explaining the association of factor XII deficiency and thrombosis) or by plasminogen activators. The most important plasminogen activator is t-PA; it is synthesized principally by endothelium and is most active when bound to fibrin. This characteristic makes t-PA a useful therapeutic agent, since its fibrinolytic activity is largely confined to sites of recent thrombosis. Once activated, plasmin is in turn tightly controlled by counterregulatory factors such as α ₂ -plasmin inhibitor, a plasma protein that binds and rapidly inhibits free plasmin.

Key Concepts

Coagulation Factors

• Coagulation occurs via the sequential enzymatic conversion of a cascade of circulating and locally synthesized proteins.

• Tissue factor elaborated at sites of injury is the most important initiator of the coagulation cascade in vivo.

• At the final stage of coagulation, thrombin converts fibrinogen into insoluble fibrin that contributes to formation of the definitive hemostatic plug.

• Coagulation normally is restricted to sites of vascular injury by:

  • Limiting enzymatic activation to phospholipid surfaces provided by activated platelets or endothelium

  • Circulating inhibitors of coagulation factors, such as antithrombin III, whose activity is augmented by heparin-like molecules expressed on endothelial cells

  • Expression of thrombomodulin on normal endothelial cells, which binds thrombin and converts it to an anticoagulant

  • Activation of fibrinolytic pathways (e.g., by association of t-PA with fibrin)

Endothelium

The balance between the anticoagulant and procoagulant activities of endothelium often determines whether clot formation, propagation, or dissolution occurs ( Fig. 4.10 ). Normal endothelial cells express a multitude of factors that inhibit the procoagulant activities of platelets and coagulation factors and that augment fibrinolysis. These factors act in concert to prevent thrombosis and to limit clotting to sites of vascular damage. However, if injured or exposed to pro-inflammatory factors, endothelial cells lose many of their antithrombotic properties. Here, we complete the discussion of hemostasis by focusing on the antithrombotic activities of normal endothelium; we return to the “dark side” of endothelial cells later when discussing thrombosis.

The antithrombotic properties of endothelium can be divided into activities directed at platelets, coagulation 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 endothelium also releases a number of factors that inhibit platelet activation and aggregation. Among the most important are prostacyclin (PGI ₂ ), nitric oxide (NO), and adenosine diphosphatase ; the latter degrades ADP, already discussed as a potent activator of platelet aggregation. The major regulator of NO and PGI ₂ production appears to be flow; precisely what senses flow is uncertain, though changes in cell shape and cytoskeleton are correlated. PGI ₂ is produced by COX-1, which is expressed constitutively by “healthy” endothelium under normal flow conditions. NO is the product of endothelial nitric oxide synthase eNOS.

• Anticoagulant effects. Normal endothelium shields coagulation factors from tissue factor in vessel walls and expresses multiple factors that actively oppose coagulation, most notably thrombomodulin, endothelial protein C receptor, heparin-like molecules, and tissue factor pathway inhibitor. Thrombomodulin and endothelial protein C receptor bind thrombin and protein C, respectively, in a complex on the endothelial cell surface. When bound in this complex, thrombin loses its ability to activate coagulation factors and platelets, and instead cleaves and activates protein C, a vitamin K–dependent protease that requires a cofactor, protein S. Activated protein C/protein S complex is a potent inhibitor of coagulation cofactors Va and VIIIa. Heparin-like molecules on the surface of endothelium bind and activate antithrombin III, which then inhibits thrombin and factors IXa, Xa, XIa, and XIIa. The clinical utility of heparin and related drugs is based on their ability to stimulate antithrombin III activity. Tissue factor pathway inhibitor (TFPI), like protein C, requires protein S as a cofactor and, as the name implies, binds and inhibits tissue factor/factor VIIa complexes.

• Fibrinolytic effects. Normal endothelial cells synthesize t-PA, as already discussed, a key component of the fibrinolytic pathway.