1 From the Division of Cardiology, Section for Cardiac Electrophysiology, Hospital of the University of Pennsylvania, 3400 Spruce St, Founders 9118, Philadelphia, PA 19104 (T.M.M., S.N.); Division of Cardiology, Section for Cardiac Electrophysiology (H.R.H., S.N.), and Departments of Radiology and Biomedical Engineering (H.R.H.), The Johns Hopkins University, Baltimore, Md.