Adult patients on veno-venous ECMO for respiratory failure will be randomized to maintain AT between 80% and 100% (study group) or no supplementation unless heparin resistance occurs (control group). Sample size of of 20 patients per group (n=40) is calculated upon the primary outcome measure of reduction of heparin use in the study group compared to the control group. AT and anti-Xa will be measured before ECMO and daily while on ECMO. Anticoagulation will be guaranteed with unfractionated heparin infusion with a target aPTT 1.5-2X normal. Study will end at ECMO removal.