NMS. Surgery

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Part I ♦ Foundations B. Intrinsic pathway: Factor XIIa activates XI, and then XIa activates IX. IX then converges with the extrinsic pathway by activating factor X. This pathway can be initiated either by exposure to a negatively charged surface (exposed collagen from a damaged vessel) or thrombin itself activates factor IX. C. Both pathways converge at factor X: Factor Xa then mediates conversion of prothrombin to thrombin with factor Va as a cofactor. Thrombin mediates fibrinogen conversion to fibrin. Finally, factor XIIIa mediates cross-linking of fibrin. III. Regulation and fibrinolysis: A. The coagulation system is a cascade; i.e., each activated factor is able to activate many of the factors in subsequent steps. B. Thrombin itself acts as a positive feedback loop by activating factor IX. Tissue factor pathway inhibitor (TFPI) may inhibit TF–VIIa complexes. Coagulopathy I. History: Lab studies should not be routinely ordered preoperatively. A. Identify any patient-perceived coagulopathy: Bruising, petechia, easy bleeding/nosebleeds, history of bleeding from other procedures (dental/ surgical). B. Family history C. Medical conditions/risk factors: Liver disease (cirrhosis), renal failure (uremia). II. Physical: Evidence of bruising or petechia. III. Laboratory evaluation. A. Platelet count: Normal is 150,000–400,000/mL blood. B. Bleeding time (BT): Measures platelet function. Disorders of platelets include uremia, drugs (aspirin, clopidogrel, Gp II B/III A inhibitors), and von Willebrand disease. C. Prothrombin time (PT): Because factors II (thrombin), VII, and X are produced by the liver, PT represents a good measure of vitamin K-dependent coagulation factors and is therefore used to monitor warfarin therapy. The international normalized ratio (INR) is a normalization factor to equate lab values between labs. D. Activated partial thromboplastin time (aPTT): Measures the intrinsic cascade and is useful for following patients on IV unfractionated heparin therapy. E. Thrombin time (TT): Tests the conversion of fibrinogen to fibrin via thrombin; elevated when fibrinogen is depleted and in the presence of heparin. 1. Protein C and protein S degrade factors V and VIII. 2. Antithrombin III inhibits thrombin-Xa complexes. 3. Fibrinolysis: Tissue-type plasminogen activator (t-PA) and urokinase- type plasminogen activator (uPA) mediate conversion of plasminogen to plasmin.