Low activated coagulation time during cardiopulmonary bypass does not increase postoperative bleeding.

The activated coagulation time (ACT) is widely used to monitor adequacy of anticoagulation during cardiopulmonary bypass despite absence of data establishing an ACT below which adverse outcomes occur. For anticoagulation before cardiopulmonary bypass, we administered a single dose of heparin (300 U/kg) to 193 patients and measured ACT and heparin levels at intervals after administration. No additional heparin was administered to any patient. Clot formation in the cardiopulmonary bypass circuit and excessive postoperative chest tube drainage were considered outcomes indicating inadequate anticoagulation. Cardiopulmonary bypass averaged 59 +/- 23 minutes (range, 30 to 138 minutes). Activated coagulation time values at every sampling period were normally distributed. In 51 patients (26.4%) ACT values were less than 400 seconds, including 4 less than 300 seconds, at some sampling time after heparinization. Patients with low ACT values did not bleed more postoperatively than those with high ACT values, nor was bleeding related to heparin level. No clots were found in any perfusion circuit. We conclude that a minimum ACT value for adequacy of heparinization is not yet defined but that it is less than 400 seconds.