IN-VITRO EFFECT OF HEMODILUTION ON ACTIVATED CLOTTING TIME AND HIGH-DOSE THROMBIN TIME DURING CARDIOPULMONARY BYPASS

Background. Extreme dilution of clotting factors, as may occur during pediatric or neonatal cardiopulmonary bypass, often leads to inadequat e monitoring of anticoagulation with activated dotting time (ACT). In this study we postulate that the high-dose thrombin time (HiTT) is les s influenced by extreme dilution of clotting factors because it stimul ates clotting through the common pathway. Methods. Heparinized prebypa ss blood was obtained from 30 adult cardiac surgical patients and was diluted in a laboratory setting with saline solution to mimic the clin ical clear prime solution (group I; n = 10), with saline solution cont aining similar heparin as in the prebypass blood (group II; n = 10), a nd with fresh frozen plasma to substitute clotting factors in the dilu ted blood (group III; n = 10). Blood was diluted to four different deg rees: a control without dilution, 25%, 50%, and 75% dilution. The ACT and HiTT were measured and compared. Results. Tn group I, significant prolongation of ACT was observed in blood diluted to 75% as compared w ith the nondiluted blood (p < 0.01). In contrast, HiTT was not prolong ed at any degree of dilution but reduced proportionally to dilution up to 75%, reflecting the concomitant reduction of heparin. In group II, ACT increased at 25% dilution (p < 0.01) whereas HiTT increased at 50 % dilution (p < 0.01). In group III, no prolongation of ACT or HiTT wa s found in any degree of dilution. Furthermore, adding fibrinogen to t he diluted blood (n = 4) did not cause ACT to recover at 75% dilution, suggesting that dilution of other factors in the early clotting casca de rather than fibrinogen alone increases ACT. Conclusions. These resu lts imply that when blood is extremely diluted during cardiopulmonary bypass with a clear prime without substituted clotting factors, HiTT i s a better test than ACT for anticoagulation monitoring.