Plasma heparin activity and antagonism during cardiopulmonary bypass with hypothermia.

Plasma heparin activity in 11 patients undergoing open-heart surgery was measured by comparing thrombin time of patient plasma to thrombin time of plasma containing known heparin concentrations. Although all patients received 300 units/kg of heparin, their initial plasma heparin levels varied significantly, from 1.8 units/ml in lighter petients to 3 units/ml in heavier patients. During hypothermia (25 degrees C), heparin decay was insignificant. At 37 degrees C, heparin decayed at a rate between 0.37 and 2.01 units/ml/hr. This decay was significantly faster in those patients with higher initial posthypothermia plasma heparin levels. When heparin was reversed with a protamine dose based on circulating plasma heparin levels, the mean difference between the predicted and the actual residual heparin activity was 0.025 units/ml. Heparin levels vary widely becuase of the influence of temperature on decay rates and because the space into which heparin is distributed is not simply proportional to weight. Evaluation and reversal of plasma heparin activity require ongoing analysis rather than any 1 dosage protocol.