HEPARIN-THERAPY DURING CARDIOPULMONARY BYPASS IN CHILDREN REQUIRES ONGOING QUALITY-CONTROL

Heparin therapy for children undergoing cardiopulmonary bypass (CPB) i s monitored in the operating room by automated whole blood activated c lotting times (ACT). For many years our institution used Hemochron (HC ) ACT machines but changed to HemoTec (HT) ACT machines because they r equired a smaller blood sample and provided results in duplicate. When HemoTec ACT machines were introduced at our institution, the surgical team was concerned that increased amounts of heparin were being admin istered to our patients during CPB. This study was conducted to invest igate the potential mechanisms responsible for these clinical observat ions. First, we compared ACT values on ex vivo blood samples from 20 c onsecutive pediatric patients (6 samples each) during CPB. The HC ACT values were significantly and systematically increased over HT ACT val ues (HC: 750 +/- 40 vs HT: 418 +/- 26, Mean +/- SEM, p <0.01). 94% of all HC ACT values were above 450 s compared to only 27% of HT ACT valu es. If HT ACT values had been used for patient monitoring, all patient s would have received more heparin to achieve ACT values above 450 s. The two machines reported similar ACT values when heparin was added in vitro to whole blood (0.1-5.0 units/ml), (HC: Y = 98X + 104, r(2) = 0 .93 HT: Y = 82X + 109, r(2) = 0.94). Heparin concentrations in our pat ients following a bolus of 300 U/kg of heparin, but prior to CPB were 3.2 +/- 0.07 units/ml. Following the initiation of CPB, heparin concen trations decreased to 1.3 +/- 0.05, reflecting, in part, hemodilution by the pump prime (1 U of heparin/ml). In contrast to the in vitro res ults, there was no relationship between ACT values measured by either machine and plasma heparin concentrations in ex vivo samples. Finally, plasma concentrations of 8 coagulation proteins measured prior to CPB and following CPB were decreased by 27-55%, predominantly reflecting the final dilution by CPB. In conclusion: 1) HT and HC machines cannot be used interchangeably in pediatric patients without risk of alterin g clinical practice in an uncontrolled fashion; and 2) ACT values from children on CPB correlate poorly with heparin concentrations, likely due to hemodilution. Optimal use of anticoagulant therapy during CPB i n children requires further study in clinical trials and ongoing quali ty control.