1. From the Cardiovascular Branch (A.J.D., V.K.R., B.S.S.P., J.M.H., R.J.L.) and Laboratory of Cardiac Energetics, Division of Intramural Research (M.A.G., D.C.P., E.R.M.), National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Md, and Boston Scientific Corporation, Plymouth, Minn (S.S., G.S.).