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KEY CONCEPTS

KEY CONCEPTS

  • Image not available. Hemophilia is an inherited bleeding disorder resulting from a congenital deficiency in factor VIII or IX.

  • Image not available. The goal of therapy for hemophilia is to prevent bleeding episodes, and as a result their long-term complications, and to arrest bleeding if it occurs.

  • Image not available. Recombinant factor concentrates usually are first-line treatment of hemophilia as they have the lowest risk of infection.

  • Image not available. Inhibitor formation is the most significant treatment complication in hemophilia. It is associated with significant morbidity and decreased quality of life.

  • Image not available. Recombinant factor VIIa is effective for the treatment of acute bleeds in patients with hemophilia A or B that has developed inhibitors.

  • Image not available. The goal of therapy for von Willebrand disease (vWD) is to increase von Willebrand factor (vWF) and factor VIII levels to prevent bleeding during surgery or arrest bleeding when it occurs.

  • Image not available. Factor VIII concentrates that contain vWF are the agents of choice for treatment of type 3 vWD and some type 2 von Willebrand disease, and for serious bleeding in type 1 von Willebrand disease.

  • Image not available. Desmopressin acetate often is effective for treatment of type 1 vWD. It also may be effective for treatment of some forms of type 2 vWD in addition to mild to moderate hemophilia A.

The coagulation system is intricately balanced and designed to stop bleeding at the site of vascular injury through complex interactions between the vascular endothelium, platelets, procoagulant proteins, anticoagulant proteins, and fibrinolytic proteins. Hemostasis stops bleeding at the site of vascular injury through the formation of an impermeable platelet and fibrin plug. Three key mechanisms facilitate hemostasis including vascular constriction, primary platelet plug formation (primary hemostasis), and clot propagation through fibrin formation (secondary hemostasis). Derangements in this system can lead to either bleeding or thrombosis. Bleeding disorders are the result of a coagulation factor defect, a quantitative or qualitative platelet defect or enhanced fibrinolytic activity.

COAGULATION FACTORS

Secondary hemostasis facilitates propagation and stabilization of the initial platelet plug formed in primary hemostasis through the formation of fibrin on the activated platelet surface. This step is initiated via the tissue factor pathway and is vital for adequate hemostasis. Coagulation factors circulate as inactive precursors (zymogens). Activation of these coagulation proteins leads to a cascading series of proteolytic reactions (Fig. 19-2). At each step, a clotting factor undergoes limited proteolysis and becomes an active protease (designated by a lowercase “a,” as in Xa).

The coagulation factors can be divided into three groups on the basis of biochemical properties: vitamin K-dependent factors (II, VII, IX, and X), contact activation factors (XI and XII, prekallikrein, and high-molecular-weight kininogen), and thrombin-sensitive factors (V, VIII, XIII, and fibrinogen). Biologic half-life and blood product source varies by coagulation factor (Table 101-1).

TABLE 101-1Blood Coagulation Factors

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