1. From the Cardiovascular Branch (A.N.R., J.D.T., C.O., M.J., V.K.R., V.J.W., W.H.S., R.D.S., R.J.A., R.J.L.) and the Laboratory of Cardiac Energetics (M.A.G., R.B.T., E.R.M.), Division of Intramural Research, National Heart Lung and Blood Institute, National Institutes of Health, Bethesda, Md; Division of Pediatric Cardiology (J.D.T., M.C.S.), Children’s National Medical Center, Washington, DC; and Department of Biomedical Engineering (R.B.T.), University of Alberta, Edmonton, Canada.