1. From the Department of Cardiovascular Medicine (P.A.R., S.P.H., F.Z.K., D.P.D.), University of Cambridge, and Department of Medical Physics and Clinical Engineering (P.A.W.), Addenbrooke's Hospital; and Department of Cardiology (P.A.R., S.P.H., P.A.W., F.Z.K., M.O., N.E.J.W.), Papworth Hospital, Cambridge, UK.