Regular ventricular rate and "reverse bigeminy" in 3:2 Wenckebach periodicity

One of the more common causes of bigeminy at the ventricular level is type 1 second-degree atrioventricular (AV) block with 3:2 conduction ratio. In 3:2 Wenckebach, the shorter cycles reflect the consecutively conducted impulses and the longer cycles coincide with the blocked P waves. Theoretically, however, depending on the degree of conduction delay between the first and second transmitted impulses, other types of spacing of the QRS complexes may become possible. In this retrospective study of 180 patients who underwent electrophysiologic studies for symptomatic arrhythmias, atrial pacing-induced 3:2 Wenckebach periodicity resulted in a regular ventricular rate and/or in "reverse bigeminy" in 16 cases (8.9%). Reverse bigeminy was characterized by the shorter R-R intervals including both the blocked P waves and the first conducted beats of the subsequent cycles, and the longer R-R intervals coinciding with the second conducted beats during 3:2 Wenckebach. In 14 cases, regular ventricular rate and reverse bigeminy was triggered by marked conduction delay in the AV node and in 2 cases, the conduction delay was in the His-Purkinje system. Reverse bigeminy appeared to be related to dual AV nodal physiology in 8 patients. In 2 cases, sophisticated maneuvers such as termination of atrial pacing at critical intervals during the AV Wenckebach were required to expose the true conduction pattern. This study demonstrates that during rapid atrial rhythms, one cannot always be sure which P wave is responsible for which QRS complex. Rarely, extreme conduction delays can result in P waves conducting across the subsequent ventricular beats and be responsible not for the first, but for the following QRS complexes.