1. From the Cardiac Rhythm Management Laboratory, Division of Cardiovascular Diseases, Departments of Medicine (C.R.K., C.-C.W., L.J.D., M.A.K., R.E.I., G.P.W.), Biomedical Engineering (W.M.S., R.E.I.), Veterans Affairs (L.J.D.), and Physiology (R.E.I., L.J.D.), University of Alabama at Birmingham, and the Department of Pharmacology (J.L.A.), East Tennessee State University, Johnson City.