Heparin monitoring during cardiac surgery. Part 2: Calculating the overestimation of heparin by the activated clotting time.

Activated clotting time (ACT) values were converted to heparin concentration, enabling an assessment of the accuracy of the ACT and a quantification of the prolongation imposed by bypass. Blood samples were obtained from 42 adult cardiopulmonary bypass (CPB) patients before and during bypass surgery. Samples were analysed for ACT (HemoTec ACT) and anti-factor Xa (anti-Xa) plasma heparin concentration. The mean heparin concentration calculated before bypass was an accurate reflection of plasma heparin; however, calculated values rose to around 170% of anti-Xa values upon connection to bypass. By adjusting for this rise, for 95% of cases the calculated heparin concentration would vary between 0.60 and 1.65 times anti-Xa values. Without accounting for artificial prolongation or individual sensitivities, the ACT may give values between 0.8 and 3.0 times that indicated by the anti-Xa assay. When both individual heparin sensitivities and the effects of bypass are considered, the ACT may provide a more suitable indication of heparin levels; however, typical use may overestimate heparin up to threefold.