Atrioventricular nodal gap conduction as a manifestation of dual nodal pathways

Electrophysiologic studies were performed in a 76 year old patient for evaluation of sinus bradycardia. Atrial extrastimuli were induced during sinus rhythm at progressively decreasing coupling (A1-A2) intervals. At an A1-A2 interval of 420 msec, right bundle branch block developed, and at 370 msec conduction failed below the His bundle. When the interval was reduced to 320 msec, conduction resumed with a normal QRS pattern with an abrupt increase in A-H intervals from 165 to 305 msec. These findings are interpreted as type I or atrioventricular (A-V) nodal gap conduction physiologically related to conversion from a rapid to a slow A-V nodal conduction mode.