Maintenance of adequate anticoagulation is of critical importance when patients are placed on extracorporeal life support, such as ECMO. During ECMO, a patient's entire blood volume is constantly exposed to the artificial surfaces of the ECMO circuit. This exposure activates the clotting cascade, and not only is the circuit at risk for clot formation, but the patient is also at risk for thromboembolic events. Hence anticoagulation is vital in allowing the ECMO circuit to support a patient's entire cardiopulmonary system for an extended period of time. Anticoagulation on ECMO is achieved primarily by the use of a heparin continuous infusion. Heparin's main mechanism of action is to bind to and activate an enzyme called antithrombin III (AT III). AT III is the principal inhibitor of the blood coagulation serine proteases - Thrombin and Factor Xa. In the presence of, and when bound to heparin, the inhibitory activity of AT III is greatly enhanced. AT III deficiency has been shown to be a common finding in pediatric patients requiring ECMO. Our center has reported that most neonatal patients requiring ECMO also have a significant degree of ATIII deficiency prior to, and during ECMO bypass. This deficiency may lead to challenges in achieving adequate anticoagulation while on ECMO. For this reason, our institution already supplements antithrombin during pre-ECMO circuit priming and for maintaining AT III levels above 70% during the entire length of ECMO support. Optimizing AT III levels may then improve anticoagulation while on ECMO. This study aims to determine the pharmacokinetics of recombinant antithrombin (ATryn®) supplementation. To date, the use of ATryn® has primarily been studied in adults. There is no published data to date, and no current studies on the pharmacokinetics of antithrombin III in neonates, and hence no published data on the pharmacokinetics of antithrombin III in the neonatal ECMO population. Since the use of antithrombin III in the neonatal ECMO patient population has not been thoroughly studied, the use of antithrombin III supplementation varies greatly among different neonatal intensive care units. Therefore, studies that aim at determining the pharmacokinetics of antithrombin III, particularly in ECMO patients, are important because of the lack of data and published studies in this patient population. By determining the pharmacokinetics of ATryn® in our neonatal ECMO patient population, we will aid in establishing standardized dosing and administration protocols for the supplementation of antithrombin in this specific patient population.