William E. Ladd Professor of Surgery, Harvard Medical School; Cardiovascular Surgeon in Chief, Department of Cardiac Surgery, Children's Hospital, Boston, Associate Professor of Pediatrics, Harvard Medical School; Senior Associate in Cardiology, Department of Cardiology, Children's Hospital, Boston, Bronson Crothers Professor of Neurology, Harvard Medical School; Neurologist-in-Chief, Department of Neurology, Children's Hospital, Boston