1. From the Division of Cardiology, The Children's Hospital of Philadelphia, Philadelphia, PA (D.J.G., M.G.M., N.M., B.D.H., G.W., S.M.P., J.R.); Departments of Pediatrics (D.J.G., B.D.H., G.W., S.M.P., J.R.), and Biostatistics and Epidemiology (B.F.), University of Pennsylvania School of Medicine, Philadelphia; and Divisions of Cardiology and Critical Care Medicine, Cincinnati Children's Hospital Medical Center, Cincinnati, OH (B.S.M.).