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.