Heparin kinetics during paediatric open-heart operations

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Abstract
Heparin levels before and during cardiopulmonary bypass were measured in two groups of children undergoing open heart operations. Patients in Group I weighed more than 10 kg (14 ± 3.5, SD) and those in Group II weighed less than 10 kg (6.5 ±.2.2, SD). The concentration of heparin in the pump prime was 2.40 ± 0.3 units/ml in Group I and 4.6 ± 1 .6 units/ml in Group II. (Smaller patients received relatively larger volumes of heparinized blood in the pump prime.) After a 300-units/kg loading dose of heparin, levels in Group I peaked at 5.4 ± 0.8 units/ml and declined to 4.04 ± 1.2 immediately before bypass. Group II levels peaked at 4.1 ± 1.1 and declined to 3.2 ± 0.9 units/ml. After bypass began, heparin levels in both groups approached those of the pump prime: levels in Group I fell to 2.7 ± 0.4 units/ml whereas those in Group I I rose to 4.0 ±1.1. The change in heparin level when bypass began could be predicted (r = 0.97), regardless of group, by using multiple linear regression analysis. We conclude that the concentration of heparin in pump prime is the primary determinant of heparin concentration in the paediatric patient when bypass begins because of the relatively large volume of pump prime compared with the plasma volume of the patient. This effect is most critical in patients weighing less than 10 kg.