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A safe and effective alternative is needed for patients in whom unfractionated heparin (UFH) or protamine is contraindicated (e.g., those with heparin-induced thrombocytopenia or allergy to protamine). Furthermore, choice of anticoagulant may influence graft patency in coronary surgery and may therefore be important even when there is no contraindication to UFH. Direct thrombin inhibitors have several potential advantages over UFH, demonstrated in acute coronary syndromes. However, there are also potential difficulties with their use related to lack of reversal agents and paucity of clinical experience in monitoring their anticoagulant activity at the levels required for cardiac surgery with cardiopulmonary bypass (CPB).In the first prospective randomized trial of an alternative to heparin in cardiac surgery, we compared bivalirudin (a short-acting direct thrombin inhibitor) with UFH in 100 patients undergoing off-pump coronary artery bypass (OPCAB) surgery. Blood loss for the 12 hours following study drug initiation in the bivalirudin group was not significantly greater than in the heparin group. Median graft flow was significantly higher in the bivalirudin group. We concluded that anticoagulation for OPCAB surgery with bivalirudin was feasible without a clinically important increase in perioperative blood loss. A larger study is needed to investigate the impact of improved graft patency on other clinical outcomes after cardiac surgery.