Lack of correlation between activated clotting time and plasma heparin during cardiopulmonary bypass.

The activated clotting time (ACT) with a Hemochron system for determining heparin requirements during cardiopulmonary bypass surgery, (CPB) accompanied by hemodilution and hypothermia was evaluated using plasma heparin levels as a standard. In 28 patients who were administered a standard heparin regimen (300 units/kg prebypass, 8000 units in the pump prime and 100 units/kg hourly during CPB) mean prebypass plasma heparin was 4 units/ml, and ACT was 493 seconds. During CPB mean plasma heparin decreased significantly (p < 0.001) to 3.1 units/ml, whereas mean ACT increased significantly (p < 0.001) to 674 seconds. The mean protamine requirement predicted from ACT was significantly higher (43%) than predicted from plasma heparin levels or actual protamine administered. The ACT neither accurately reflected plasma heparin during CPB nor predicted protamine requirements. The fixed-dose regimen employed, however, prevented both intraoperative thrombosis, assessed clinically in all patients, and clotting on six arterial line filters, as determined by scanning EM, despite wide variations in ACT and plasma heparin levels during surgery.