Children were eligible for the study if they were 16 years of age or younger, weighed between 3 and 60 kg, had two-ventricle circulation, had severe heart failure despite optimized medical treatment, and were on a waiting list for cardiac transplantation. Children who had already been receiving another form of mechanical circulatory support were allowed to participate, except for those who had received circulatory support with ECMO for 10 days or more (see Table S1 in the Supplementary Appendix for a complete list of inclusion and exclusion criteria). After enrollment, participants were stratified according to body-surface area. Cohort 1 included all participants with a body-surface area of less than 0.7 m2, and cohort 2 all participants with a body-surface area of at least 0.7 m2 but less than 1.5 m2.