Understanding "Heparin Resistance" in Cardiac Surgery

This study will explore altered heparin responsiveness (AHR) in cardiac surgical patients undergoing cardiopulmonary bypass (CPB) requiring systemic anticoagulation with heparin. The investigators will evaluate the hypothesis that AHR may be directly related to, modulated or mediated by interactions between heparin, antithrombin (AT), the heparin-AT complex, and one or more acute phase proteins. The investigators are particularly interested in identifying patients with "true heparin resistance", that is, patients who demonstrate AHR even after antithrombin-replenishment in the presence of an adequate systemic dose of heparin.