Immediate cessation of, and avoidance of reexposure to, heparin are important principles underlying the management of patients with immune-mediated heparin-induced thrombocytopenia (HIT) (Chong and Berndt, 1989). Because further antithrombotic therapy is often necessary for these patients, several alternative anticoagulant strategies have been developed (see Chapters 12-17). However, patients with HIT who require cardiovascular surgery present exceptional problems. Apart from the inherent disturbances of the hemostatic system in this patient population, considerable activation of the hemostatic system results from the surgical trauma itself. Further, during cardiopulmonary bypass (CPB), there is exposure of blood to the large non-endothelial surfaces of the CPB circuit and reinfusion of tissue factor-activated blood aspirated from the operative field into the CPB system. This profound hemostatic activation requires potent high-dose anticoagulation in order to prevent thrombosis within both the CPB system and the patient (Edmunds, 1993; Slaughter et al., 1994). Anticoagulation with unfrac-tionated heparin (UFH), point-of-care monitoring by activated clotting time (ACT) systems, and reversal via the antidote protamine comprise a longstanding and well-established strategy permitting cardiovascular surgery. This approach is so universally entrenched that there is very minor experience with all other forms of anticoagulation in this patient setting.

Any alternative anticoagulant considered for HIT patients should ideally meet certain requirements. First, the agent should be effective in minimizing activation of coagulation during CPB and in preventing thrombus formation. Second, a rapid and simple method of monitoring its anticoagulant effects should be available, so as to avoid inappropriate under- or over-anticoagulation. Finally, rapid and complete reversibility of the anticoagulant effects is important to minimize postoperative bleeding complications. Unfortunately, no existing agent meets all of these requirements.

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