Summary
Heparin therapy for children undergoing cardiopulmonary bypass (CPB) is monitored
in the operating room by automated whole blood activated clotting times (ACT). For
many years our institution used Hemochron (HC) ACT machines but changed to HemoTec
(HT) ACT machines because they required 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 administered to our
patients during CPB. This study was conducted to investigate the potential mechanisms
responsible for these clinical observations. First, we compared ACT values on ex vivo
blood samples from 20 consecutive pediatric patients (6 samples each) during CPB.
The HC ACT values were significantly and systematically increased over HT ACT values
(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 values. If HT ACT values had been used
for patient monitoring, all patients 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, r2 = 0.93 HT: Y = 82X + 109, r2 = 0.94). Heparin concentrations in our patients 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 concentrations 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 results, 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 altering 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 in children requires further study in clinical trials and ongoing
quality control.
