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The mechanism of the AV nodal Wenckebach phenomenon was studied in 25 dogs by multiple atrial and His bundle electrogram (HBE) recordings. During ventricular stimulation with 1:1 retrograde conduction, the interval from the stimulus artifact to the retrograde His deflection (S-H) interval) remained constant. Decreasing the cycle length of stimulation (CLS) resulted in retrograde AV nodal Wenckebeach cycles. When retrograde AV nodal delay reached a critical value, the Wenckebach cycles were terminated by a reciprocal beat (manifest reentry). The His bundle and ventricles were antegradely depolarized by the reentrant impulse. A further decrease in CLS produced what electrocardiographically looked like ordinary Wenckebach cycles. However, the HBE tracing revealed that reentry was occurring but was concealed by the CLS. This was confirmed by noting that the His deflection of the last ventricular paced beat of the Wenckebach cycle was antegradely depolarized and had a shorter S-H interval then all other beats of that cycle. The reentrant and retrograde impulses collided in the bundle-branch system. Further decreases in CLS masked the reentrant phenomenon on both standard ECG and HBE tracings. The S-H interval remained constant for all ventricular paced beats of the Wenckebach cycle. Under these circumstances, reentry could still be uncovered by turning off the stimulator at an appropriate time during the Wenckebach cycle. This maneuver exposed the reciprocal beat. Thus collision of the reentrant and retrograde impulses occurred within the AV node. These findings provide a satisfactory explanation for why the last beat of the Wenckebach cycle is not conducted retrogradely to the atria.

Manifest and Concealed Reentry: A MECHANISM OF AV NODAL WENCKEBACH PHENOMENON