Heparin binding proteins. Contribution to heparin rebound after cardiopulmonary bypass

Heparin rebound, the reappearance of anticoagulant activity after adequate neutralization with protamine, can lead to excessive postoperative bleeding after cardiac surgery. We investigated the mechanism of heparin rebound by using chemically modified heparin that lacks anticoagulant activity (low-affinity heparin) but that is able to displace protein-bound anticoagulantly active heparin.

Sixteen patients undergoing elective cardiac surgery were given heparin (400 U/kg) to achieve an activated clotting time (ACT) > 400 seconds. After cardiopulmonary bypass, protamine sulfate was given (by heparin-ACT dose-response curve) to return the ACT to prebypass times (preoperative, 160 +/- 9 seconds; postoperative, 156 +/- 17 seconds). Blood samples were obtained serially for 24 hours and assayed for thrombin clotting time (TCT) and heparin activity using an anti-factor Xa assay. The TCT and anti-factor Xa activity were consistently and abnormally elevated for the first 6 hours after surgery. The anti-factor Xa activity increased fourfold after the addition of low-affinity heparin (essentially free of anti-factor Xa activity), indicating that anticoagulantly active heparin persisted in the circulation after protamine neutralization bound nonspecifically to plasma proteins. Blood loss correlated with postoperative TCTs.

Our findings demonstrate that heparin anticoagulant activity persists for up to 6 hours after surgery despite apparent protamine neutralization. The observation of the marked increase in plasma anti-factor Xa activity after the addition of low-affinity heparin suggests that after its administration, a large proportion of the heparin binds to plasma proteins and is incompletely removed by protamine. After protamine is cleared, the protein-bound heparin dissociates slowly and binds to anti-thrombin III to produce an anticoagulant effect.