The primary objective of the Primary Pediatric Arm of the study is to evaluate whether the 50cc TAH-t can safely support, and provide probable benefit to, transplant-eligible pediatric patients at imminent risk of death from biventricular failure without experiencing permanent disabling, stroke-related deficits. Probable benefit will be measured as transplanted during the first six months, or survival at six months and continuing on support on the initially placed 50cc TAH-t, without experiencing permanent disabling stroke-related deficits.