PHYSIOLOGY OF COAGULATION AND FIBRINOLYSIS

The process of hemostasis is a result of a complex series of interactions among the endothelium, platelets, and the coagulation cascade. In health, hemostasis is limited to the site of vascular injury by natural inhibitors of coagulation and the fibrinolytic system. The endothelium normally is resistant to thrombus formation, and the endothelial surface contains several modulators of coagulation, including a negative surface charge, thrombomodulin, heparan sulfate, and a protein C receptor.⁹ In states of vascular injury, circulating platelets are exposed to subendothelial connective tissue and von Willebrand factor (vWF). Platelets then bind to collagen and vWF via surface glycoprotein receptors and become activated, secreting molecules including thromboxane A₂ and adenosine diphosphate (ADP) that activate locally circulating platelets. The activated platelets that cover the disrupted endothelium adhere via glycoprotein IIb/IIIa receptor binding to fibrinogen, which serves as a bridge to other activated platelets and leads to formation of a platelet aggregate.⁹ Receptors for activated coagulation factors are exposed on the platelet surface in states of activation (Figure 62-2).

Figure 62-2 Endothelial damage exposes vWF, which is a target for platelet binding via the glycoprotein Ib-IX-V receptor complex. Platelet adhesion results in activation and degranulation. Activated platelets aggregate via binding of fibrinogen and vWF to the glycoprotein IIb-IIIa receptor complex. Activation leads to exposure of anionic phospholipid on the platelet membrane, which provides a procoagulant surface to support thrombin generation. Fibrinogen = purple; vWF = green; vascular endothelial cells = brown.

The coagulation cascade was described previously as two separate pathways, intrinsic and extrinsic, both of which activated a common pathway. It is now considered as a series of interdependent enzymatic reactions involving coagulation factors, anticoagulant factors, and fibrinolytic factors (Figure 62-3). Coagulation factors circulate as inactive enzymes (prozymogens).

Figure 62-3 The coagulation cascade. Coagulation is initiated primarily by tissue factor. Tissue factor is inactivated rapidly by TFPI, but the thrombin produced activates factors XIa, VIIIa, and Va, which perpetuate and accelerate coagulation. AT, Antithrombin; TFPI, tissue factor pathway inhibitor. Red bars indicate sites of inhibition by TFPI and AT. Blue = coagulation enzymes; green = TF and enzyme cofactors; red = inhibitors of coagulation.

Tissue factor (TF) is an intrinsic membrane protein present on fibroblasts, neoplastic cells, and most other cell types not normally in contact with blood. Certain stimuli, such as cytokines interleukin-1 (IL-1) and tumor necrosis factor (TNF), can induce monocytes and endothelial cells to synthesize TF.⁹ Factor VII is capable of autoactivation, with activated factor VII (factor VIIa) normally present as 1 per cent of factor VII concentration. Factor VII and factor VIIa bind readily to TF, forming a complex on the cell surface. The TF-VIIa complex is capable of activating additional factor VII bound to TF. The TF-VIIa complex also activates factor IX and factor X, resulting in membrane-bound factor IXa and factor Xa. Factor IX also is activated by factor XIa, a protein in the contact system that is activated by thrombin and by factor XIIa. Factor XII is part of the contact system, which plays a role in fibrinolysis, inflammation, complement activation, and kinin formation but not in the initiation of coagulation.⁹ Activation of factor IX by factor XIa is thought to be less important in vivo than TF-VIIa activation because deficiency of factor XI results in a clinically mild hemostatic disorder in affected individuals.¹⁰ Similarly, a deficiency of factor XII and its cofactor, high-molecular-weight kininogen, does not result in a hemorrhagic tendency. Factor VIII, which circulates in plasma bound to vWF, can be activated by factor Xa or thrombin. Factor VIIIa serves as a cofactor for factor IXa and the resultant “Xase” complex is the most potent activator of factor X. Factor Xa converts prothrombin to thrombin in a slow reaction. The presence of factor Va helps form a prothrombinase complex on the cell surface and accelerates the formation of thrombin markedly. Factor V can be activated by both factor Xa and thrombin. Thrombin cleaves fibrinogen, forming a fibrin monomer. Fibrin monomers polymerize spontaneously and then are cross-linked covalently to form a stable fibrin clot by factor XIIIa, which is activated by thrombin.

Procoagulant activity of the coagulation factors is balanced by inhibitors of coagulation (Figure 62-4). Several physiological mechanisms inhibit coagulation. TF pathway inhibitor regulates the activation of factors IX and X by the TF-VIIa complex. Antithrombin (AT) inhibits thrombin, factor Xa, and factor IXa.¹¹ The activity of AT is enhanced greatly by heparin. Factor Xa also is inhibited by Z protease inhibitor and protein Z.⁹ Thrombin bound to thrombomodulin on the endothelial surface is not capable of activating factors V and VIII but does activate protein C. Activated protein C (APC), along with its cofactor protein S, inactivates factors Va and VIIIa.¹¹ This serves as an important regulatory pathway because the inactivated factors V and VIII are no longer able to participate in the coagulation cascade.

Figure 62-4 Physiological mechanisms that downregulate coagulation. Tissue factor pathway inhibitor limits activation of coagulation by tissue factor, AT directly inhibits factors Xa and thrombin, while the protein C pathway inactivates factors Va and VIIIa, which are cofactors in the activation of factors Xa and thrombin. AT, Antithrombin; EPCR, endothelial protein C receptor. Red = inhibitory factors; beige = vascular endothelium.

The fibrinolytic system solubilizes fibrin clots, maintaining the balance between fibrin clot formation and dissolution. Thrombin stimulates endothelial cells to release tissue-type plasminogen activator (tPA).¹¹ Plasminogen is activated to plasmin by tPA when both are bound to fibrin. Plasmin splits fibrinogen and fibrin to produce fibrin degradation products (FDPs). These products are themselves anticoagulants, inhibiting thrombin and fibrin polymerization.¹¹ Cross-linked fibrin is relatively more resistant to plasmin degradation. Urokinase plasminogen activator acts similarly to tPA and is formed by activation of prourokinase. Prourokinase bound to its receptor on activated endothelial cells is activated by plasmin, by autoactivation, and by kallikrein, a component of the contact system.¹¹ Urokinase then cleaves plasminogen to form plasmin. The fibrinolytic system is regulated by α-2 antiplasmin, plasminogen activator inhibitor-1 (PAI-1), and thrombin-activated fibrinolysis inhibitor.

DISRUPTION OF NORMAL PROCOAGULANT, ANTICOAGULANT, AND FIBRINOLYTIC BALANCE

DIC results from excess systemic generation of thrombin, which leads to a disruption of balance between coagulation and fibrinolysis. Major causes of marked activation of coagulation sufficient to lead to DIC include endothelial injury, tissue injury, and monocyte activation. TF is the key event in most cases of initiation of DIC, and studies in animal models of Escherichia coli endotoxemia showed that inhibition of the TF pathway eliminated thrombin production.¹² Systemic generation of thrombin has been demonstrated to be mediated by the extrinsic pathway of coagulation via TF and factor VIIa.¹² TF is expressed on activated mononuclear cells, endothelial cells, and neoplastic cells.² Endotoxin, IL-1, and TNF induce TF synthesis and expression on endothelial cells.¹³ TNFα transcription is stimulated by the TF-VIIa complex, leading to increased TF synthesis and resulting in a positive feedback loop.¹³ TNFα stimulates the release of other cytokines, including IL-1 and IL-6.¹³ IL-6 contributes to systemic thrombin generation by causing TF expression on mononuclear cells.

PLATELET DYSFUNCTION AND CONSUMPTION

FDPs D and E have a tendency to bind to platelet membranes, leading to platelet dysfunction. Platelet consumption and incorporation into thrombi lead to thrombocytopenia, which may range from mild to severe in patients with DIC. Persistent activation of the coagulation cascade may result in consumption of coagulation factors, prolongation of coagulation times, and hemorrhage.

ABNORMALITIES OF THE FIBRINOLYTIC SYSTEM

As a result of widespread thrombin production and subsequent release of tPA, plasminogen is converted to plasmin and increased levels of plasmin circulate systemically during DIC, acting on fibrin and fibrinogen to produce FDPs X, Y, D, and E.⁴ Plasmin also splits cross-linked fibrin, creating the D-dimer fragment. Other enzymes degraded by plasmin include factors V, VIII, IX, and XI, which results in decreased concentrations of these clotting factors.¹ The FDPs in circulation interfere with fibrin polymerization, leading to hemorrhage. FDPs sometimes are referred to as fibrin split products (FSPs) or fibrin-related antigens.¹⁴ FDPs activate macrophages and monocytes to release IL-1, IL-6, and PAI-1, a potent inhibitor of fibrinolysis.¹ During peak activation of coagulation, endotoxin, IL-1, and TNF increase PAI-1 synthesis and reduce thrombomodulin expression, thus inhibiting fibrinolysis and favoring coagulation.²

INHIBITOR CONSUMPTION

AT and protein C concentrations decrease in patients with DIC. AT levels are reduced in DIC because of consumption, reduced synthesis, and degradation by neutrophil elastase.² Protein C activity is compromised as a result of reduced synthesis, reduced thrombomodulin expression on endothelial cells, and binding of cofactor protein S to C4b-binding protein.² TF pathway inhibitor function also is insufficient in states of DIC.²

END-ORGAN DAMAGE

Systemic circulation of thrombin results in the splitting of fibrinogen to form fibrinopeptides A and B and the fibrin monomer.¹ Fibrin monomers polymerize spontaneously and factor XIII, activated by thrombin, stabilizes the cross-linked fibrin, forming a clot that contributes to tissue ischemia. IL-1 and IL-6 cause vascular endothelial disruption and thus contribute to end-organ damage.¹⁵ Thrombin-induced release of endothelin from endothelial cells leads to vasoconstriction and vasospasm, predisposing to thrombus formation and vascular occlusion.¹