Correlations between activated clotting time values and heparin concentration measurements in young infants undergoing cardiopulmonary bypass.

BACKGROUND: Monitoring heparin concentration along with the activated clotting time (ACT) may provide a more accurate guide for the administration of heparin to infants during cardiopulmonary bypass (CPB). However, standard laboratory assays of heparin concentration (antifactor Xa heparin concentration) require plasma instead of whole blood, and results are not immediately available to clinicians. Alternatively, measurements of whole blood heparin concentration may be performed at the bedside using an automated protamine titration device, the Hepcon instrument (Hepcon Hemostasis Management System Plus; Medtronics, Minneapolis, MN). The purpose of this investigation was to compare ACT measurements from 3 commercially available instruments and bedside measurements of whole blood heparin concentration using the Hepcon instrument with laboratory measurements of antifactor Xa plasma heparin concentration in infants younger than 6 months of age undergoing CPB.
METHODS: Forty-four pediatric patients younger than 6 months of age scheduled for elective cardiac surgery requiring CPB were enrolled in this prospective study. Blood samples were drawn 3 minutes after the initial heparin bolus and immediately before the termination of CPB to obtain measurements of heparin anticoagulation. Kaolin-activated ACTs were performed with the Hemochron (International Technidyne Corporation, Edison, NJ), Hepcon, and i-STAT (i-STAT Corporation, East Windsor, NJ) instruments. Whole blood heparin concentration was measured using the Hepcon instrument. Plasma heparin concentration was measured using an antifactor Xa chromogenic substrate assay.
RESULTS: Immediately after the initial heparin bolus, none of the ACT values correlated with plasma heparin concentration. When measured immediately before the termination of CPB, only the i-STAT ACT showed a moderate correlation. Conversely, bedside measurements of whole blood heparin concentration showed satisfactory agreement with laboratory measurements of plasma heparin concentration at both time points (concordance correlation coefficients 0.30 and 0.67, respectively). There is a bias in that antifactor Xa-measured plasma heparin concentration tends to be higher than Hepcon-measured whole blood heparin concentration.
CONCLUSIONS: In infants younger than 6 months old undergoing CPB, caution is warranted when using ACT values as the sole indication of adequate heparin anticoagulation. In general, ACT prolongation correlates poorly with plasma heparin concentration. Only i-STAT ACT values showed a moderate correlation when measured immediately before the termination of CPB. Alternatively, bedside measurements of whole blood heparin concentration measured by the Hepcon instrument agreed well with antifactor Xa laboratory measurements. Our data support the clinical utility of bedside measurements of heparin concentration to provide timely, convenient, and accurate measurements of heparin concentration in these infants.