1. From the Department of Cardiology, Children's Hospital Boston (T.P.S., C.A., G.P., K.G.); the Department of Cardiology, Massachusetts General Hospital (M.J.S.); Department of Pediatrics (T.P.S., C.A.) and Department of Medicine (M.J.S.), Harvard Medical School; and Department of Biostatistics, Harvard School of Public Health (K.G.), Boston, MA.