1. Department of Medicine (Cardiology) and the Feinberg Cardiovascular Research Institute, Northwestern University Feinberg School of Medicine, Chicago, IL 60611, USA;
2. The Center for Arrhythmia Research, University of Michigan School of Medicine, Ann Arbor, MI 48109, USA;
3. Masonic Medical Research Laboratory, Utica, NY 13501, USA;
4. Department of Physics, California State University Northridge, Northridge, CA 91330, USA