Bleeding is the most feared complication during extracorporeal membrane oxygenation (ECMO) and is associated with high dosing of heparin. There is no consensus on antithrombin (AT) supplementation during ECMO. However, AT is needed by heparin to properly anticoagulate. We hypothesize that maintaining normal antithrombin levels during ECMO is associated with a less heparin dosing and more adequate level of anticoagulation.