Proper hemostasis requires the participation of innumerable biologic elements (Box 31-1). This section groups them into four topics to facilitate understanding: coagulation factors, platelet function, the endothelium, and fibrinolysis. The reader must realize this is for simplicity of learning, and that in biology, the activation creates many reactions and control mechanisms, all interacting simultaneously. The interaction of the platelets, endothelial cells, and proteins to either activate or deactivate coagulation is a highly buffered and controlled process. It is perhaps easiest to think of coagulation as a wave of biologic activity occurring at the site of tissue injury (Figure 31-1).¹ Although there are subcomponents to coagulation itself, the injury/control leading to hemostasis is a four-part event: initiation, acceleration, control, and lysis (recanalization/fibrinolysis). The initiation phase begins with tissue damage, which really is begun with endothelial cell destruction or dysfunction. This initiation phase leads to binding of platelets, as well as protein activations; both happen nearly simultaneously, and each has feedbacks into the other. Platelets adhere, creating an activation or acceleration phase that gathers many cells to the site of injury. From that adhesion a large number of events of cellular/protein messaging cascade. As the activation phase ramps up into an explosive set of reactions, counter-reactions are spun off, leading to control proteins damping the reactions. It is easiest, conceptually, to think of these control mechanisms as analogous to a nuclear reactor. The activation phase would continue to grow and overcome the whole organism unless control rods were inserted (e.g., thrombomodulin, proteins C and S, and tissue plasminogen activator [t-PA]) to stop the spread of the reaction. The surrounding normal endothelium acts quite differently from the disturbed (ischemic) endothelium. Eventually, the control reactions overpower the acceleration reactions and lysis comes into play. The diagram in Figure 31-1 shows lysis as relatively quick, but it can take 24 hours to days to have its full effects. A key concept is that hemostasis is part of a larger body system: inflammation. Most, if not all, of the protein reactions of coagulation control have importance in signaling inflammation leading to other healing mechanisms. Entire books have been written merely to examine these fascinating interactions. It is no wonder that cardiopulmonary bypass (CPB) has such profound inflammatory effects when it is considered that each of the activated coagulation proteins and cell lines then feeds into upregulation of inflammation.

BOX 31-1 Components of Hemostasis

• Coagulation factor activation

• Platelet function

• Vascular endothelium

• Fibrinolysis and modulators of coagulation

Figure 31-1 Coagulation is a sine wave of activity at the site of tissue injury. It goes through four stages: initiation, acceleration, control, and lysis/recanalization. TPA, tissue plasminogen activator; vWF, von Willebrand factor.

(Redrawn from Spiess BD: Coagulation function and monitoring. In Lichtor JL [ed]: Atlas of Clinical Anesthesia. Philadelphia, 1996, Current Medicine.)

During cardiac surgery, the endothelium (locally and systemically) is disturbed.² The coronary arteries are made either partially or fully ischemic for periods with cardioplegia (perfused CPB being relatively rare today). Little known is that high concentrations of potassium are particularly insulting to endothelial cells. Ischemia/reperfusion injury is, therefore, the norm for every cardiac surgical case using CPB.³ Systemic ischemia and reperfusion occur throughout every capillary bed because microair, thrombus, and fat emboli are by-products of CPB⁴ (see Chapters 28 to 30 and 36). It is quite possible that transfusion of red blood cell (RBC) products, through blocking of the microcirculation, actually may contribute to ischemia/reperfusion injury. Transfusion has been shown to evoke endothelial cell hyper-reactivity mediated through cell membrane microparticles.⁴

The coagulation proteins and platelets are both hemodiluted and consumed during CPB. Platelets are activated/inhibited by various means. Coagulation dysfunction in cardiac surgery has been studied for more than 50 years. The complexity of the myriad number of dysfunctions should impress the student of this area. Unfortunately, when a patient is bleeding in the operating room or ICU, there is no universal way to quickly sort out the causative events. Furthermore, the interventions are limited, yet an educated and planned approach (algorithm-driven transfusion/coagulation intervention) does decrease transfusion and influences subsequent bleeding.

Protein Coagulation Activations

Coagulation Pathways

The coagulation factors participate in a series of activating and feedback inhibition reactions, ending with the formation of an insoluble clot.⁵ A clot is the sum total of platelet-to-platelet interactions, leading to the formation of a platelet plug (initial stoppage of bleeding). The cross-linking of platelets to each other by way of the final insoluble fibrin leads to a stable clot. Clot is not simply the activation of proteins leading to more protein deposition. Clinicians have been shaped in their thinking about coagulation by the historic way that coagulation proteins were discovered and the resulting coagulation tests. It is that teaching of the coagulation cascade, with resultant monitoring technology, that has led to some transfusion behaviors. The way coagulation has been classically taught (protein cascades) is not the way coagulation proceeds biologically.

With few exceptions, the coagulation factors are glycoproteins (GPs) synthesized in the liver, which circulate as inactive molecules termed zymogens. Factor activation proceeds sequentially, each factor serving as substrate in an enzymatic reaction catalyzed by the previous factor in the sequence. Hence this classically has been termed a cascade or waterfall sequence. Cleavage of a polypeptide fragment changes an inactive zymogen to an active enzyme. The active form is termed a serine protease because the active site for its protein-splitting activity is a serine amino acid residue. Many reactions require the presence of calcium ion (Ca²⁺) and a phospholipid surface (platelet phosphatidylserine). The phospholipids occur most often either on the surface of an activated platelet or endothelial cell and occasionally on the surface of white cells. So anchored, their proximity to one another permits reaction rates profoundly accelerated (up to 300,000-fold) from those measured when the enzymes remain in solution. The factors form four interrelated arbitrary groups (Figure 31-2): the contact activation, intrinsic, extrinsic, and common pathways. They were so labeled historically by the human need for order. In biology, they are all highly interactive, occur simultaneously on the surface of cells, and have feedback loops with cross-reactions.

Figure 31-2 Depiction of coagulation protein activation sequence.

Asterisks denote participation of calcium ion. HMWK, high-molecular-weight kininogen; PK, prekallikrein.

Contact Activation

Factor XII, high-molecular-weight kininogen (HMWK), prekallikrein (PK), and factor XI form the contact or surface activation group. The in vivo events that activate factor XII remain unconfirmed. Clinicians do know that ex vivo contact with ionically charged surfaces will activate factor XII. Because factor XII autoactivates by undergoing a shape change in the presence of a negative charge, in vitro coagulation tests use glass, silica, kaolin, and other compounds with negative surface charge (see Chapter 17). The glycocalyx of endothelial cells has a repelling charge for coagulation proteins. One potential in vivo mechanism for factor XII activation is disruption of the endothelial cell layer, which exposes the underlying negatively charged collagen matrix. Activated platelets also provide negative charges on their membrane surfaces. HMWK anchors the other surface activation molecules, PK and factor XI, to damaged endothelium or activated platelets. Factor XIIa cleaves both factor XI, to form factor XIa, and PK, to form kallikrein. Figure 31-3 depicts the events of surface activation.

Figure 31-3 A–C, Activation of the contact factors XII, XI, and prekallikrein (PK). The cofactor, high-molecular-weight kininogen (HMWK), binds factor XI and PK to the endothelial surface. Kallikrein (K) amplifies factor XII activation.

(From Colman RW, Marder VJ, Salzman EW, Hirsh J: Overview of hemostasis. In Colman RW, Hirsh J, Marder VJ, Salzman EW [eds]: Hemostasis and Thrombosis, 3rd ed. Philadelphia: JB Lippincott, 1994, p 3.)

This system also interlinks to the complement cascade and fibrinolytic process as follows. Kallikrein converts HMWK to bradykinin, which activates the complement proteins. Kallikrein also may convert plasminogen to plasmin (see later). This latter function is quite weak, however, and of unknown significance in vivo.

It was rather tempting historically to attribute all coagulation abnormalities to activation of the contact system. This seemed an obvious explanation for the coagulopathy of CPB. The circuits are most often made of polyvinylchloride with a negatively charged surface. Today, it is known that the effects of contact activation in the entire scheme of CPB coagulation dysfunction are actually quite small. Of note, patients with completely absent factor XII do just fine and do not exhibit excess bleeding, nor are they particularly dry after CPB. Therefore, the presence or absence of factor XII in the evolution of humankind seems to not be critical for survival. It also should reveal that the surface activation mechanism is not the driving force behind the bleeding/consumptive coagulopathy of CPB.

Intrinsic System

Intrinsic activation forms factor XIa from the products of surface activation. Factor XIa splits factor IX to form factor IXa, with Ca²⁺ required for this process. Then factor IXa activates factor X with help from Ca²⁺, a phospholipid surface (platelet-phosphatidylserine), and a GP cofactor, factor VIIIa. Figure 31-4 displays a stylized version of factor X activation. The phospholipids and GP cofactors are on the surface of platelets.

Figure 31-4 Factor VIII facilitates the activation of factor X by factor IXa. Calcium tethers the molecules to the phospholipid surface.

(From Horrow JC: Desmopressin and antifibrinolytics. Int Anesthesiol Clin 28:230, 1990.)

Extrinsic System

Activation of factor X can proceed independently of factor XII by substances classically thought to be extrinsic to the vasculature. This is of historic interest because today it is known that the expression of tissue factor is actually a highly regulated event in endothelial cells. Any number of endothelial cell insults can lead to the production of tissue factor by the endothelial cell.³^–¹⁰ At rest, the endothelial cell is quite antithrombotic. However, with ischemia, reperfusion, sepsis, or cytokines (particularly interleukin-6), the endothelial cell will stimulate its production of intracellular nuclear factor-κB and send messages for the production of messenger RNA for tissue factor production.⁶ This can happen quickly and the resting endothelial cell can turn out large amounts of tissue factor. It is widely held today that the activation of tissue factor is what drives many of the abnormalities of coagulation after cardiac surgery, rather than contact activation.^7,⁸ In some tissues, cells outside the vasculature contain large amounts of tissue factor. They are released when cells are damaged/ruptured. Thromboplastin, also known as tissue factor, released from tissues into the vasculature, acts as a cofactor for initial activation of factor X by factor VII. Factors VII and X then activate one another with the help of platelet phospholipid and Ca²⁺, thus rapidly generating factor Xa. (Factor VIIa also activates factor IX, thus linking the extrinsic and intrinsic paths.)

Thromboplastin straddles the extravascular cell membrane, with its extracellular portion available to bind factor VIIa. Cytokines (particularly tumor necrosis factor-α and interleukin-6) and endotoxins can stimulate its expression on endothelium.^9,¹⁰ It anchors factor VIIa to the cell surface, thus facilitating activation of factor X. The amount of available factor Va also seems to be quite important for the adequate functioning of the normal coagulation cascades.

Common Pathway

Using membrane phospholipids (phosphatidylserine) as a catalyst site, Ca²⁺ as a ligand, and factor Va as cofactor, factor Xa splits prothrombin (factor II) to thrombin (factor IIa). The combination of factors Xa, Va, and Ca²⁺ is termed the prothrombinase complex. Factor Xa anchors to the membrane surface (of platelets) via Ca²⁺. Factor Va, assembling next to it, initiates a rearrangement of the complex, vastly accelerating binding of the substrate, prothrombin. Most likely, the factor Xa formed from the previous reaction is channeled along the membrane to this next reaction step without detaching from the membrane.

Figure 31-5 depicts the steps involved in formation of thrombin from its precursor, prothrombin. The by-product, fragment F1.2, serves as a plasma marker of prothrombin activation. An alternative scheme generates a different species, meizothrombin, involved more specifically in activation of coagulation inhibitors.¹¹

Figure 31-5 Activation of prothrombin by factor Xa proceeds in a multistep fashion.

A, On the phospholipid surface, the prothrombinase complex consists of prothrombin (factor II), factor Xa, and factor Va. B, The first activation step in which the prothrombin fragment F1.2 is split from prothrombin to form prethrombin (II*). C, Molecular rearrangement of prethrombin yields thrombin.

Thrombin cleaves the fibrinogen molecule to form soluble fibrin monomer and polypeptide fragments termed fibrinopeptides A and B. Fibrin monomers associate to form a soluble fibrin matrix. Factor XIII, activated by thrombin, cross-links these fibrin strands to form an insoluble clot. Patients with lower levels of factor XIII have been found to have more bleeding after cardiac surgery.^11,¹²

Vitamin K

Those factors that require calcium (II, VII, IX, X) depend on vitamin K to add between 9 and 12 γ-carboxyl groups to glutamic acid residues near their amino termini. Calcium tethers the negatively charged carboxyl groups to the phospholipid surface (platelets), thus facilitating molecular interactions. Some inhibitory proteins also depend on vitamin K (proteins C and S).

Modulators of the Coagulation Pathway

Thrombin, the most important coagulation modulator, exerts a pervasive influence throughout the coagulation factor pathways. It activates factors V, VIII, and XIII; cleaves fibrinogen to fibrin; stimulates platelet recruitment, chemotaxis of leukocytes and monocytes; releases t-PA, prostacyclin, and nitric oxide from endothelial cells; releases interleukin-1 from macrophages; and with thrombomodulin, activates protein C, a substance that then inactivates factors Va and VIIIa.¹² Note the negative feedback aspect of this last action (Figure 31-6). Coagulation function truly centers around the effects of thrombin. The platelets, tissue factor, and contact activation all are interactive and are activated by a rent in the surface of the endothelium or through the loss of endothelial coagulation control. Platelets adhere to a site of injury and, in turn, are activated, leading to sequestration of other platelets. It is the interaction of all of those factors together that eventually creates a critical mass. Once enough platelets are interacting together, with their attached surface concomitant serine protease reactions, then a thrombin burst is created. Only when enough thrombin activation has been encountered in a critical time point is a threshold exceeded, and the reactions become massive and much larger than the sum of the whole. It is thought that the concentration and ability of platelets to react fully affect the ability to have a critical thrombin burst. CPB may affect the ability to get that full thrombin burst because of its effects on platelet number, platelet-to-platelet interactions, and the decreased amounts of protein substrates.

Figure 31-6 Modulating effects of protein C on coagulation.

Thrombomodulin from endothelial cells accelerates thrombin activation of protein C (Pr C). In the presence of protein S (Pr S), activated protein C [(act Pr C)a] inactivates factors V and VIII. Protein C and protein S are vitamin K dependent.

The many serine proteases that compose the coagulation pathways are balanced by serine protease inhibitors, termed serpins.¹³ This biologic yin and yang leads to an excellent buffering capacity. It is only when the platelet-driven thrombin burst so overwhelms the body’s localized anticoagulation or inhibitors that clot proceeds forward. Serpins include α₁-antitrypsin, α₂-macroglobulin, heparin cofactor II, α₂-antiplasmin, antithrombin (AT; also termed antithrombin III [AT III]), and others.

AT III constitutes the most potent and widely distributed inhibitor of blood coagulation. It binds to the active site (serine) of thrombin, thus inhibiting action of thrombin. It also inhibits, to a much lesser extent, the activity of factors XIIa, XIa, IXa, and Xa; kallikrein; and the fibrinolytic molecule, plasmin. Thrombin bound to fibrin is protected from the action of AT, thus explaining the poor efficacy of heparin in treating established thrombosis. AT III is a relatively inactive zymogen. To be most effective, AT must bind to a unique pentasaccharide sequence contained on the wall of endothelial cells in the glycosaminoglycan surface known as heparan; the same active sequence is present in the drug heparin.

An important note is that activated AT III is active only against free thrombin (fibrin-bound thrombin cannot be seen by AT III).¹⁴ Prothrombin circulates in the plasma but is not affected by heparin-AT III complexes; it is only thrombin, and thrombin does not circulate freely. Most thrombin in its active form is either bound to GP binding sites of platelets or in fibrin matrices. When blood is put into a test tube and clot begins to form (such as in an activated coagulation time [ACT]), 96% of thrombin production is yet to come. Most thrombin generation is on the surface of platelets and on clot-held fibrinogen. Platelets, through their GP binding sites and phospholipid folds, protect activated thrombin from attack by AT III. Therefore, the biologic role of AT III is to create an anticoagulant surface on endothelial cells. It is not present biologically to sit and wait for a dose of heparin before CPB.

CPB dilutes AT III substantially, and the further consumption of AT III during CPB (thrombin generation) leads, in some patients, to profoundly low levels of this important inhibitor.¹⁵ Research work adding AT III back to the CPB circuit has shown promise in that by doing so there is better preservation of serine protease proteins and platelets. Unfortunately, no pharmaceutical company has decided to pursue this line of research to the point that it would be either commonplace or economically feasible to add large amounts of AT III to CPB and, therefore, avoid consumptive coagulopathies. Congenital AT III deficiency can lead to in utero fetal destruction if the fetus is homozygous for the abnormal AT III. However, patients who are heterozygous for AT III abnormalities have about 40% to 60% of normal AT III activity. They have a particularly high risk for deep vein thrombosis. Low AT III levels have been described during extracorporeal membrane oxygenation, and the addition of AT III to the extracorporeal membrane oxygenation circuit has been effective in improving outcome and decreasing bleeding in some circumstances.¹⁶ It is not known how useful it would be in all patients on CPB because only small trials have been performed, but these have been encouraging.^17,¹⁸ Both human AT III concentrate (harvested from multiple plasma donors and pasteurized) and a pharmaceutically engineered, goat milk–produced AT III (slightly different structure than human AT III) are commercially available.

Heparin cofactor II also inhibits thrombin, once it is activated. Although large doses of heparin activate heparin cofactor II, dermatans on endothelial cell surfaces activate it far more effectively, suggesting dermatans as alternative drugs to heparin.¹⁹ Dermatan sulfates are not available for use today in the United States.

Another serpin, protein C, degrades factors Va and VIIIa. Like other vitamin K–dependent factors, it requires Ca²⁺ to bind to phospholipid. Its cofactor, termed protein S, also exhibits vitamin K dependence. Genetic variants of protein C are less active and lead to increased risk for deep vein thrombosis and pulmonary embolism. When endothelial cells release thrombomodulin, thrombin then accelerates by 20,000-fold its activation of protein C ²⁰ (see Figure 31-6). Activated protein C also promotes fibrinolysis through a feedback loop to the endothelial cells to release t-PA.²¹

Regulation of the extrinsic limb of the coagulation pathway occurs via tissue factor pathway inhibitor (TFPI), a glycosylated protein that associates with lipoproteins in plasma.²² TFPI is not a serpin. It impairs the catalytic properties of the factor VIIa-tissue factor complex on factor X activation. Both vascular endothelium and platelets appear to produce TFPI.^23,²⁴ Heparin releases TFPI from endothelium, increasing TFPI plasma concentrations by as much as sixfold, which should be viewed as a biologic indicator of how poor heparin is as an anticoagulant. TFPI is not tested for in routine coagulation testing. It may be that some individuals with certain types of TFPI or who have very large amounts of circulating TFPI could be at risk for severe adverse bleeding after cardiac surgery.²⁵^–³⁰ This area is just beginning to be examined today both in terms of whether TFPI is responsible for abnormal bleeding and whether its genetic variants have abnormal bleeding or thrombosis.

von Willebrand factor (vWF), a massive molecule composed of disulfide-linked glycosylated peptides, associates with factor VIII in plasma, protecting it from proteolytic enzymes. It circulates in the plasma in its coiled inactive form.³¹ Disruption of the endothelium either allows for binding of vWF from the plasma or allows for expression of vWF from tissue and from endothelial cells. Once bound, vWF uncoils to its full length and exposes a hitherto cryptic domain in the molecule. This A-1 domain has a very high affinity for platelet GPs. Initially, vWF attaches to the glycoprotein Iα (GPIα) platelet receptor, which slows the platelet forward movements against the shear forces of blood flow. Shear forces are activators of platelets. As the platelet’s forward movement along the endothelial brush border is slowed (because of vWF attachment), shear forces actually increase; thus, the binding of vWF to GP1 acts to provide a feedback loop for individual platelets, further activating them. The activation of vWF and its attachment to the platelet are not enough to bind the platelet to the endothelium, but it creates a membrane signal that allows for early shape change and expression of other GPs, GPIb and GPIIb/IIIa. Then, secondary GPIb binding connects to other vWF nearby, binding the platelet and beginning the activation sequence. It bridges normal platelets to damaged subendothelium by attaching to the GPIb platelet receptor. An ensuing platelet shape change then releases thromboxane, β-thromboglobulin, and serotonin, and exposes GPIIb/IIIa, which binds fibrinogen.

Deficiency States

Decreased amounts of coagulation proteins may be inherited or acquired. Deficiencies of each part of the coagulation pathway are considered in turn. Table 31-1 summarizes the coagulation factors, their activation sequences, and vehicles for factor replacement when deficient.

TABLE 31-1 The Coagulation Pathway Proteins, Minimal Amounts Needed for Surgery, and Replacement Sources

Contact Activation

Although decreased amounts of factor XII, PK, and HMWK can occur, these defects do not have clinical sequelae. The autosomal dominant deficiency of factor XI is very rare. However, its incidence among Ashkenazi Jews is as high as 0.1% to 0.3%.³² Most of these patients require factor replacement with fresh-frozen plasma (FFP) for surgery. Spontaneous bleeding does not occur, but increased bleeding after a surgical event or trauma is possible. Factor XI concentrations do not directly correlate with bleeding after trauma or surgery, suggesting that factor XI deficiency can be easily overcome by activation of platelets, factor IX, and other signaling mechanisms. An FFP dose of 10 mL/kg will yield target concentrations of 20% activity, and it is often given for this rare deficiency.

Intrinsic

Hemophilia occurs worldwide, with a prevalence of 1 in 10,000. Hemophilia A, which constitutes about 80% to 85% of cases, originates from decreased activity of factor VIII. Because platelet function remains normal, minor cuts and abrasions do not bleed excessively. Joint and muscle hemorrhages ensue from minor trauma or, seemingly, spontaneously. Airway issues include epistaxis and obstruction from bleeding into the tongue. The bleeding time and prothrombin time (PT) remain normal, whereas the activated partial thromboplastin time (aPTT) is prolonged.³³ Desmopressin, a synthetic analog of vasopressin, will increase factor VIII activity by releasing vWF from endothelial cells, except in patients with severe hemophilia A who have too little functional factor VIII available for vWF.³⁴ Major surgery requires replenishment of factor VIII functional activity to greater than 80% of normal with FFP, cryoprecipitate, or factor VIII concentrate.³⁵ Factor VIII concentrate is the preferred method today. After surgery, factor VIII concentrations should be maintained greater than 30% for 2 weeks with repeat doses. Current plasma-derived concentrates are solvent detergent and heat-treated to remove lipid-coated viruses (human immunodeficiency virus [HIV], hepatitis B, human T-lymphotropic virus [HTLV]. However, this was not historically the case, and in Europe during the early parts of the HIV/acquired immune deficiency syndrome (AIDS) crisis, most individuals with hemophilia contracted HIV/AIDS from contaminated products. A recombinant product also is available but costs about three times that of the plasma-derived one.

Factor IX deficiency manifests as hemophilia B, constituting 15% to 20% of all hemophilia cases. Patients present with symptoms identical to those with hemophilia A. No study has demonstrated a salutary effect of desmopressin here. Prothrombin complex (factor IX) concentrates will replenish levels, but consumptive coagulopathy remains a possible complication, stemming from the presence of activated coagulation factors, principally factor VIIa, in the preparation.³⁶ Purified factor IX concentrate, a plasma-derived, solvent detergent and heat-treated product, currently constitutes the replacement vehicle of choice for patients with hemophilia B.³⁷ Recombinant pure factor IX concentrate will be available, but at considerable expense (factor VIIa is now available as well). Consultation with an experienced hematologist aids in the care of patients with hemophilia undergoing surgery.

Extrinsic

Inheritance of factor VII deficiency follows an autosomal recessive pattern, with a prevalence of 1 in 500,000. Although factor VII deficiency may mimic hemophilia in presentation, most often, clinical bleeding is absent and surgery is well tolerated without replacement. The PT is elevated, whereas the PTT is normal. When necessary, replacement of factor VII levels to 10% to 20% of normal with FFP suffices.

Common Pathway

Deficiency of either factor V or factor X, both extremely rare autosomal recessive disorders, increases both the PT and PTT. The bleeding time is normal in factor X deficiency but prolonged in one third of patients with factor V deficiency. The bleeding time prolongation arises from the role of factor V in platelet function.³⁸ Prothrombin complex concentrate or FFP supplies prothrombin, factor V, and factor X. Numerous inherited abnormalities (polymorphisms) of prothrombin and fibrinogen occur, with varying characteristics. Cryoprecipitate, which contains 250 mg fibrinogen and 100 units factor VIII per 10-mL bag, as well as vWF and factor XIII, treats inherited or acquired disorders of fibrinogen. Many of these polymorphisms are associated with hypercoagulability and, perhaps, accelerated atherosclerosis rather than bleeding.

Liver Disease

Hepatic compromise decreases the circulating amounts of factors II, VII, and X, but the level of factor IX is often normal. Decreases in factor V are variable. Factor VIII levels, in contrast, can reach as much as five times normal in acute hepatitis. Factors XIII, XII, and XI, HMWK, and PK suffer mild decreases. Administration of FFP restores these factors to normal levels. Liver disease also leads to decreases in the production of AT III and protein S. Therefore, the buffering capacity of coagulation is thrown off. A small change in activation could, therefore, lead to a large and diffuse whole-body event such as consumptive coagulopathy.

Warfarin

Administration of this vitamin K antagonist affects plasma levels of factors II, VII, IX, and X, as well as proteins C and S.³⁹ Protein C has the shortest half-life, followed by factors VII (6 hours), IX (24 hours), X (2 days), and II (3 days).³⁶ Substantial PT prolongation and some PTT prolongation accompany warfarin therapy. For immediate restoration of clotting function, FFP is given. Otherwise, parenteral vitamin K or cessation of warfarin (Coumadin) suffices. Clinicians should be extremely careful in administering these compounds if a patient is suspected of having heparin-induced thrombocytopenia (HIT). Treatment with commercially available factor VIIa restores PT to normal and appears to stop bleeding when warfarin therapy has not had time to be reversed. The use of factor VIIa for this intervention before surgery is both effective and, perhaps, worthwhile as it avoids the use of FFP. The time scale of factor VIIa effectiveness may not be as long as if one normalized circulating levels through FFP administration. It makes sense, therefore, that if the PT is prolonged again approximately 8 to 12 hours after a dose of factor VIIa, then redosage of the drug be given.

Inherited Thrombotic Disorders

A number of genetic abnormalities lead to thrombosis. The most prevalent (2% to 5%) in European-derived populations is factor V Leiden, in which a point mutation at residue 1691 on factor V renders it resistant to inactivation by activated protein C.⁴⁰ Venous thromboembolism risk increases 7-fold in heterozygotes and 80-fold in homozygotes, but episodes are less severe than in other thrombotic disorders. Pregnancy and oral contraceptives greatly exacerbate the thrombotic tendency.⁴¹

Congenital AT III deficiency (1:1000 patients) causes venous thromboembolism and heparin resistance. This autosomal dominant disorder involves three types: absence of AT (type I), dysfunctional AT (type II), and AT with dysfunction limited to a reduced response to heparin (type III). Clinical presentation begins at age 15 or later, with venous thrombosis occurring with surgery, pregnancy, or bed rest. Replacement AT III is now available for use in the United States.

Protein C or S deficiencies, if homozygous, present at birth as neonatal purpura fulminans. Protein C deficiency heterozygotes demonstrate 40% to 60% protein C activity and present with venous thrombosis beginning in adolescence. The role of reduced concentrations of protein S in causing thrombosis has come into question.⁴¹ Together, deficiencies of AT, protein C, and protein S account for 10% to 15% of inherited thrombosis.⁴²

Deficiency of heparin cofactor II is rare. Its role in thrombosis is uncertain. Other conditions that cause thrombosis are dysfibrinogenemia with lysis-resistant fibrinogen, plasminogen deficiency, t-PA deficiency, excess plasminogen activator inhibitor 1 activity (plasminogen activator inhibitor 1 inhibits t-PA), and homocysteinemia.

Homocysteinemia is the mild heterozygous state of cystathione β-synthetase deficiency, known as homocystinuria in its more serious homozygous form. Increased plasma concentrations of homocysteine induce endothelial cell tissue factor activity, stimulate factor V activation, and impair protein C activation, all of which contribute to thrombosis. Folic acid and vitamins B₆ and B₁₂ reduce homocysteine plasma concentrations.⁴³

Platelet Function

Most clinicians think first of the coagulation proteins when considering hemostasis. Although no one element of the many that participate in hemostasis assumes dominance, platelets may be the most complex.⁴⁴ Without platelets, there is no coagulation and no hemostasis, so it could be argued that they are most important. Without the proteins, there is hemostasis, but it lasts only about 10 to 15 minutes as the platelet plug is inherently unstable and breaks apart under the shear stress of the vasculature. Platelets provide phospholipid for coagulation factor reactions; contain their own microskeletal system and release coagulation factors; secrete active substances affecting themselves, other platelets, the endothelium, and other coagulation factors; and alter shape (through active actin-myosin contraction) to expose membrane GPs essential to hemostasis. Their cell signaling is highly regulated, is present in other cell lines (RBCs, leukocytes, and endothelial cells), and has been intensively studied. Platelets have perhaps as many as 30 to 50 different types of cell receptors, with many ways of these being activated and inhibited.

The initial response to vascular injury is formation of a platelet plug. Good hemostatic response depends on proper functioning of platelet adhesion, activation, and aggregation (Figure 31-7). This section first discusses these aspects and then follows with the effects of platelet disorders and platelet-inhibiting pharmaceuticals. Clinicians talk about platelet dysfunction, which is largely overarching and grossly too general a term. The complexity that is platelet function really needs careful study.

Figure 31-7 Platelet function in hemostasis.

Glycoproteins Ib and IX and von Willebrand factor (VWF) mediate adhesion to the vessel wall. Glycoproteins IIb and IIIa and integrin molecules (FG [fibrinogen], TSP [thrombospondin]) mediate platelet aggregation. ADP, adenosine diphosphate.

(From George J, Shattil SJ: The clinical importance of acquired abnormalities of platelet function. N Engl J Med 324:27, 1991.)

Platelet Adhesion

Capillary blood exhibits laminar flow, which maximizes the likelihood of interaction of platelets with the vessel wall. Red cells and white cells stream near the center of the vessels and marginate platelets. However, turbulence causes reactions in endothelium that lead to the secretion of vWF, adhesive molecules, and tissue factor. Shear stress is high as fast-moving platelets interact with the endothelium. When the vascular endothelium becomes denuded or injured, the platelet has the opportunity to contact vWF, which is bound to the exposed collagen of the subendothelium. A platelet membrane component, GPIb, attaches to vWF, thus anchoring the platelet to the vessel wall. Independently, platelet membrane GPIa and GPIIa and IX may attach directly to exposed collagen, furthering the adhesion stage.⁴⁴^–⁴⁶

After activation (see later), additional adhesive mechanisms come into play. Release of selectin GPs from α-granules allows their membrane expression, thus promoting platelet-leukocyte adhesion. This interaction ultimately may allow expression of tissue factor on monocytes, thus amplifying coagulation.⁴⁷

The integrin GPs form diverse types of membrane receptors from combinations of 20 α and 8 β subunits.⁴⁸ One such combination is GPIIb/IIIa, a platelet membrane component that initially participates in platelet adhesion. Platelet activation causes a conformational change in GPIIb/IIIa, which results in its aggregator activity.

Platelet adhesion begins rapidly—within 1 minute of endothelial injury—and completely covers exposed subendothelium within 20 minutes.⁴⁵ It begins with decreased platelet velocity when GPIb/IX and vWF mediate adhesion, followed by platelet activation, GPIIb/IIIa conformational change, then vWF binding and platelet arrest on the endothelium at these vWF ligand sites.^44,⁴⁶

Adhesion requires margination of platelets; high hematocrits concentrate RBCs in the central regions of a vessel, promoting marginal placement of platelets. Dilute hematocrit (e.g., post-CPB) impairs this effect, thus adversely affecting platelet adhesion. Low hematocrit also may affect platelet prostaglandin levels as RBCs are required for preprocessing of arachidonic acid before platelets make thromboxane. Because of rheology, RBC transfusions have been thought by some to improve hemostasis. However, transfusion should not be used primarily to achieve this goal because RBC concentrates carry a high concentration of cytokines and platelet-activating factor, which may contribute to platelet dysfunction or consumptive coagulopathy. Some of the most recent data-based studies looking at only several units of blood transfusion have noted that when transfusion is used, the postoperative chest tube output is greater. That observation is, however, fundamentally different from one in which if a patient is bleeding and has a particularly low hematocrit, the use of RBCs may well increase margination of platelets.

Platelet Activation and Aggregation

Platelet activation results after contact with collagen, when adenosine diphosphate (ADP), thrombin, or thromboxane A₂ binds to membrane receptors, or from certain platelet-to-platelet interactions. Platelets then release the contents of their dense (δ) granules and α granules. Dense granules contain serotonin, ADP, and Ca²⁺; α granules contain platelet factor V (previously termed platelet factor 1), β-thromboglobulin, platelet factor 4 (PF4), P-selectin, and various integrin proteins (vWF, fibrinogen, vitronectin, and fibronectin). Simultaneously, platelets use their microskeletal system to change shape from a disk to a sphere, which changes platelet membrane GPIIb/IIIa exposure. Released ADP recruits additional platelets to the site of injury and stimulates platelet G protein, which, in turn, activates membrane phospholipase. This results in the formation of arachidonate, which platelet cyclooxygenase converts to thromboxane A₂. Other platelet agonists besides ADP and collagen include serotonin, a weak agonist, and thrombin and thromboxane A₂, both potent agonists. Thrombin is by far the most potent platelet agonist, and it can overcome all other platelet antagonists, as well as inhibitors. In total, more than 70 agonists can produce platelet activation and aggregation.

Agonists induce a graded platelet shape change (the amount based on the relative amount of stimulation), increase platelet intracellular Ca²⁺ concentration, and stimulate platelet G protein. In addition, serotonin and thromboxane A₂ are potent vasoconstrictors (particularly in the pulmonary vasculature).⁴⁹ The presence of sufficient agonist material results in platelet aggregation. Aggregation occurs when the integrin proteins (mostly fibrinogen) released from α granules form molecular bridges between the GPIIb/IIIa receptors of adjacent platelets (the final common platelet pathway).

Platelet Disorders

Dysfunctional vWF produces von Willebrand disease (vWD), an autosomal dominant disorder of variable expressivity.³⁴ With an incidence of 1.4 to 5 cases per 1000 population, vWD is the most common inherited coagulopathy. Patients present with mucocutaneous hemorrhages rather than hemarthroses. Common symptoms include epistaxis, ecchymoses, and excessive bleeding after trauma, with surgery, or during menses. Because vWF concentrations vary greatly with time, symptoms have variable expressivity. Desmopressin reverses the prolonged bleeding time in patients with mild vWD.⁵⁰ As with hemophilia A, severe cases of vWD do not benefit from desmopressin therapy. In one rare class of vWD (type IIB, 3% to 5% of vWD), desmopressin aggregates platelets, inducing thrombocytopenia and worsening rather than helping hemostasis. Table 31-2 summarizes features of the more common types of vWD. When blood products are needed, cryoprecipitate constitutes the replacement vehicle of choice in vWD, although recent factor VIII concentrates retain vWF activity and have been used successfully during cardiac surgery.⁵¹

TABLE 31-2 Major Types of von Willebrand Disease