Whole blood heparin concentrations do not correlate with plasma antifactor Xa heparin concentrations in pediatric patients undergoing cardiopulmonary bypass

This study was designed to test the validity of whole blood heparin concentration (WHBC) measurements using an on-site protamine titration assay with the Hepcon instrument (Medtronic Blood Management, Parker, CO, USA) in pediatric patients less than 1 year old undergoing cardiopulmonary bypass (CPB). The validity of the Hepcon measurements was examined via a test of correlation with the gold standard plasma antifactor Xa activity (anti-Xa) assay. Fifty-one patients (23 females and 28 males) under 1 year old (mean age 5.3 months) were studied prospectively. Blood samples were taken at 5 min into CPB and at the end of CPB for the WBHC, plasma anti-Xa activity, and hematocrit (Hct). The WBHC was converted to plasma heparin level using a formula taking into account the collection of blood into citrate solution and the effect of the citrate on the hematocrit. While a nonparametric statistical analysis revealed that the mean corrected values from the Hepcon instrument were not significantly different from the mean anti-Xa values (p = 0.070 at 5 min on CPB, p = 0.518 at the end of CPB), there was no significant correlation between these values at either 5 min into CPB (r = 0.113, p = 0.429) or at the end of CPB (r = -0.247, p = 0.080). The lack of correlation between the two assays may be related to the extreme hemodilution observed during CPB in small children, which leads to very low concentrations of coagulation proteins. Due to this discrepancy, whole blood heparin monitors should be further evaluated in children undergoing CPB.