An evaluation of two activated clotting time monitors during cardiac surgery

The activated clotting time (ACT) is a commonly used method for assessing the degree of anticoagulation during cardiac surgery. Two automated ACT monitors were evaluated in 29 adult patients undergoing cardiac surgery. The HemoTec ACT monitor (HTC; HemoTec, Inc., Englewood, CO) was evaluated using 0.8 ml of whole blood in dualchamber, high range kaolin cartridges. The hemochron ACT monitor (HCH; International Technidyne, Inc, Edison, NJ) was evaluated using 2.0 ml of whole blood in glass tubes with diatomaceous earth activator. Following sternotomy, a coagulation profile consisting of HTC, HCH, and a partial thromboplastin time (PTT) was obtained (T0). Beef lung heparin was administered in 3 consecutive doses: 40 units/kg (T1), 80 units/kg (T2), and 180 units/kg (T3). Coagulation profiles were drawn 5 minutes after each dose. Following cardiopulmonary bypass (CPB), coagulation profiles were drawn 15 minutes (T4), 2 hours (T5), and 24 hours (T6) after the protamine dose. HTC and HCH ACT values differed significantly at T1, T2, and T4 (p<0.001). In the pre-CPB period, the HCH ACT correlation with PTT divided by its control value (PTT/CTL) (r=0.73) was significantly better (p=0.02) than the correlation of HTC ACT with PTT/CTL (r=0.41). In the post-CPB period, both HCH ACT (r=0.45) and HTC ACT (r=0.30) correlated weakly with PTT/CTL. In the bias analysis, the limits of agreement (of all HCH and HTC ACT values) showed that HTC ACT is between 61% and 133% of the HCH ACT value in 95% of determinations. These data contradict the results of a previous comparison of the two monitors, a study that suggested that HTC ACT overestimated anticoagulation at low heparin doses. HCH ACT correlated better than HTC ACT with the PTT values only in the pre-CPB period. The bias analysis indicates that even though the mean ACT values are usually close, there is considerable spread (-39 to +33%) when the 95% confidence intervals are calculated.