The investigators will prospectively enroll 30 neonatal and pediatric ECMO patients at a single pediatric Level 1 trauma center. The patients will be randomized into two arms; one arm will receive unfractionated heparin and the other arm will receive bivalirudin. There will be 15 patients in each arm for a total of 30 patients.

Primary aim will be to compare the efficacy of bivalirudin to unfractionated heparin.

The investigators will prospectively enroll 30 neonatal and pediatric ECMO patients at a single pediatric Level 1 trauma center. The patients will be randomized into two arms; one arm will receive unfractionated heparin and the other arm will receive bivalirudin. There will be 15 patients in each arm for a total of 30 patients.

The investigators hypothesize that neonatal and pediatric ECMO patients receiving bivalirudin will spend more time at goal anticoagulation and will experience less hemorrhagic and thrombotic complications when compared to patients receiving unfractionated heparin. Primary aim will be to compare the efficacy of bivalirudin to unfractionated heparin. The investigators secondary aim will be to define the incidence of hemorrhagic and thrombotic complications in patients receiving bivalirudin during ECMO.

Inclusion Criteria:

• Age 1 day to less than 18 years
• Cared for in the pediatric intensive care unit or pediatric cardiac intensive care unit
• receiving venovenous or venoarterial ECMO

Exclusion Criteria:

• Patients with known or suspected heparin induced thrombocytopenia prior to consent
• Patients with hepatic failure defined as coagulopathy with elevated transaminases more than three times normal values
• Patients with plan to decannulate from ECMO within 48 hours
• Known or suspected pregnant women
• Previous enrollment in this study
• Primary language spoken that is not English or Spanish

From Baseline, daily until the end of study, an average of up to 0.1 year