Division of Cardiology, Department of Cardiac Electrophysiology, Northwestern University, 251 East Huron, Chicago, IL 60611, USA.
Progressive aberrancy (defined as beats with QRS duration and morphology between that of sinus rhythm and full aberrancy) occurred in 10 of 32 episodes of atrial pacing in 5 of 11 patients. The episodes resembled fusion beats but were due to bilateral bundle branch delay.