1. From the Advanced Development in Arrhythmia Mechanisms and Therapy Laboratory, Centro Nacional de Investigaciones Cardiovasculares Carlos III (CNIC), Madrid, Spain (J.G.Q., J.J., D.F.R.); Arrhythmia Unit, Cardiovascular Institute, Instituto de Investigación Sanitaria del Hospital Clínico San Carlos (IdISSC), Madrid, Spain (J.G.Q., J.P.V., N.P.C, D.F.R.); and Center for Arrhythmia Research, Department of Internal Medicine-Cardiology, University of Michigan, Ann Arbor (S.V.P., O.B., J.J.).