1. From the Cardiovascular Division, Electrophysiology Section, Hospital of the University of Pennsylvania, Philadelphia (J.J.L., D.M., R.K.P., P.S., E.S.Z., D.S.F., G.E.S., R.D.S., R.D., F.C.G., D.L., M.D.H., M.P.R., D.J.C., F.E.M., S.D.); and Department of Medicine, Johns Hopkins University, Baltimore, MD (M.A.E.).