1. From the Cardiovascular Research Center, Massachusetts General Hospital, Charlestown (M.R., S.A.L., S.M., M.F.S., P.T.E.); Department of Cardiology, University Medical Center Groningen, University of Groningen, Groningen, the Netherlands (M.R., I.C.V.G.); National Heart Lung and Blood Institute's and Boston University's Framingham Heart Study, Framingham, MA (M.R., J.W.M., J.D.F., M.F.S., E.J.B.); Cardiac Arrhythmia Service, Massachusetts General Hospital, Boston (S.A.L., P.T.E.); Cardiology Section...