1. From the Arrhythmia Service (P.L.-S.), Department of Medicine, London Health Science Hospital, London, Ontario, Canada; and the Section of Cardiac Electrophysiology (E.Z., D.J.C., F.G., D.L., S.D., R.B., M.P.R., M.D.H., J.C., E.P.G., F.E.M.), Cardiovascular Division, Department of Medicine, Hospital of the University of Pennsylvania, Philadelphia, Pa.