How well do three clotting-time monitors reflect circulating heparin levels during heart bypass surgery?

Details

Automated activated clotting time (ACT) is the primary means of assessing anticoagulation status for cardiopulmonary bypass (CPB). Hypothermia, hemodilution, and platelet dysfunction during CPB are known to affect ACT. The recently introduced Thrombolytic Assessment System (TAS) had been reported to be less sensitive to hemodilution and hypothermia during CPB than conventional ACT devices.

This comparative device-correlation study evaluated how well TAS and two other commercially available automated ACT systems, HemoTec and Hemochron, correlated with circulating heparin levels. An assay measuring inactivation of factor Xa provided the reference standard for circulating heparin. The subjects were 19 patients undergoing moderate hypothermic CPB. Blood was sampled repeatedly at four time points: baseline (control); after heparin administration (300–400 U/kg), before CPB; during CPB; and after protamine.

HemoTec and Hemochron were highly correlated with each other (r = 0.99). The heparin dose response was greater during CPB than before CPB (p < 0.05), and device measurements showed a significant negative correlation with patient hematocrit during CPB (p < 0.05). At the second and third sampling periods—after heparin but before CPB, and during CPB—all three devices showed negative correlations with the anti-Xa assay.

The researchers concluded that none of the automated devices tested could predict circulating heparin at the levels necessary for CPB, and that these discrepancies became magnified during CPB.