1. From the Cardiovascular Division, Electrophysiology Section, Hospital of the University of Pennsylvania, Philadelphia (D.S.F., F.E.M.); Cardiovascular Division, Electrophysiology Section, Lenox Hill Heart and Vascular Institute, New York, NY (S.E.M.); and Cardiovascular Division, Electrophysiology Section, Spectrum Health Medical Center, Grand Rapids, MI (M.I.D.).