In an attempt to determine the mechanism by which heparin exerts its anticoagulant effect in children, this investigation proposes to evaluate the relationship between preoperative ATIII, HCII and 2M levels in pediatric patients of varying age groups with clinical measurements of heparin response in the operating room, specifically changes in ACT values with heparin administration and a generated heparin dose-response curve. We also propose to evaluate the adequacy of heparinization in neonates, whom are known to have a quantitative deficiency in ATIII, by measuring PT1.2 and FPA that is produced during CPB as compared to an adult population.