His electrogram alternans reveal dual-wavefront inputs into and longitudinal dissociation within the bundle of his

Abstract

Background - His electrogram (HE) amplitude and morphology changes were observed in our previous studies during transition from "fast" to "slow" atrioventricular nodal (AVN) conduction. This phenomenon and its significance for the dual-AVN electrophysiology are not well recognized and have not been studied. Methods and Results - Experiments were performed on 17 healthy rabbit atrial-AVN preparations during standard programmed electrical pacing. HEs were mapped along the His bundle with roving surface electrodes, along with recording of cellular action potentials (APs). HEs recorded from the superior margin of the His bundle were of greater amplitude during basic beats and decreased substantially, by 42 ± 19% (P<0.01), when premature A1A2 shortened to 178 ± 20 ms. In contrast, the HEs from the inferior margin increased dramatically, 2.9 ± 1.7 times (P<0.01), during short A1A2 and remained high until AVN block occurred. In addition, during long A1A2, the superior HEs consistently preceded the inferior by 1.9 ± 0.7 ms. In contrast, at short A1A2, the superior HEs occurred 2.7 ± 0.8 ms after the inferior. Cellular AP recordings demonstrated clearly the presence of and the transition between early (fast) and late (slow) excitation wavefronts that accompanied HE alternans. Conclusions - The morphological-electrophysiological evidence from the AV junction suggests that fast and slow wavefronts reach the His bundle differently, producing functional longitudinal dissociation into 2 domains. The characteristic HE alternans recorded from these domains are a new sensitive tool to determine the presence of distinctly different wavefronts and their participation in the conduction during reentrant or other arrhythmias. These findings provide further understanding of the mechanisms of dual-AVN electrophysiology.