Assessing heparin dosing in neonates on venoarterial extracorporeal membrane oxygenation

We studied 12 consecutive neonates placed on venoarterial extracorporeal membrane oxygenation (ECMO) in 2004-2005. Activated clotting times (ACT) and anti-factor Xa levels were measured, and the corresponding heparin drip rate was noted. The mean heparin drip rate was 42.2 +/- 10.9 (SD) U/kg/hour (range 20.0-69.5 U/kg/hour). There were 55 simultaneous ACT and anti-factor Xa samples drawn. The mean ACT was 167 +/- 20 seconds (range 128-227 seconds). There was no correlation between ACT levels and heparin dose (r = 0.21; p = 0.12). The mean anti-factor Xa activities were 0.73 +/- 0.19 U/ml (range 0.1-1.0 U/ml). There was a correlation (r = 0.75; p < 0.0001) between anti-factor Xa and heparin dose. We also examined the effect of day on ECMO on heparin drip rate, ACT, and anti-factor Xa. There was no correlation between day on ECMO and either heparin drip rate (r = 0.21, p = 0.12) or ACT (r = 0.002, p = 0.99). However, there was a positive correlation (r = 0.46, p < 0.0005) between day on ECMO and anti-factor Xa activities. In these neonatal patients on venoarterial ECMO, ACT was not a reliable indicator of heparin effect. Furthermore, the increase in anti-factor Xa levels with time on ECMO suggests that heparin accumulates and/or that anti-thrombin III levels decrease with time on ECMO.