The activated coagulation time: Suitability for monitoring heparin effect and neutralization during pediatric cardiac surgery

Objectives: To determine how the stage of surgery affects the relationship between activated clotting time (ACT) and heparin effect in children undergoing cardiac surgery using cardiopulmonary bypass (CPB) and to compare the results of ACT determinations made with two different coagulation timers using different clot detection technologies and activator compositions. Design: Prospective, paired observation. Setting: Tertiary care children's hospital affiliated with an academic medical center. Participants: Fifty- eight children scheduled for nonprimary cardiac surgery. Interventions: None. Measurements and Main Results: ACTs were measured by two different commercially available automated coagulation timers (Hepcon Hemostasis Management System [HP] and the Hemochron model 801 [HM]) at four different time points over the course of cardiac surgery requiring CPB in patients ranging in age from 0.16 to 19 years. Simultaneous determinations of whole blood heparin concentration using heparin-protamine titrations were made as well. When the two methods of ACT determination were compared, baseline ACTs were not significantly different. HP ACT prolongation after heparin administration but before bypass was significantly less than HM ACT prolongation (median ACT range HM ≥999 seconds; HP, 560 to 679; p = 0.006). Twenty-one percent of the HP ACTs and none of the HM ACTs fell below the 480 seconds required at this institution for the initiation of CPB (p = 0.008). Both instruments showed a significant further prolongation of ACT after initiation of bypass (median ACT range HP ≥999 seconds; HM ≥999; p < 0.001 for both), whereas the heparin concentration decreased significantly (before, 3.5 ± 0.2 U/mL; after, 2.7 ± 0.1; p < 0.001). After termination of CPB and heparin neutralization, no significant difference between the ACTs was found. However, four HP ACTs were ≥999 seconds despite simultaneous HM baseline values and whole blood heparin concentrations of zero. Heparin concentration correlated with ACT prolongation using both the HM (Spearman ρ = 0.36; p = 0.02) and the HP (Spearman ρ = 0.57; p = 0.0025) instruments before, but not 10 minutes after, initiation of bypass. Conclusions: In pediatric cardiac surgery, the relationship between ACT and heparin concentration changes depending on when during the surgery the ACT is measured. ACT prolongation in children anticoagulated for CPB correlates poorly with heparin concentrations during CPB. HP and HM ACT tests are not interchangeable. The HM ACT is a better indicator of heparin neutralization than the HP ACT. On the other hand, continued prolongation of the HP ACT after heparin neutralization may be related to risk of postoperative hemorrhagic complications. If devices from different manufacturers are freely substituted for each other, clinical practice may be altered in an uncontrolled manner.