A new mechanism for atrioventricular nodal gap-vagal modulation of conduction.

The well-known paradoxic behavior of atrioventricular conduction, the so-called gap phenomenon, that occurs when impulses within a certain range of coupling intervals are blocked while impulses with shorter coupling intervals are conducted is attributed to differences in properties of refractoriness in neighboring regions of the conduction system. In contrast, in the present study a model was developed showing a similar phenomenon, dependent on different electrophysiologic mechanisms and localized within the atrioventricular node in an isolated rabbit heart tissue preparation (n = 11). The hearts were paced at cycle length of 400-500 msec, and atrioventricular nodal conduction times (A2H2) were measured versus atrial extrastimulus (A1A2) coupling intervals by standard extrastimulus techniques. Postganglionic vagal stimulation was applied in the atrioventricular node as short bursts of subthreshold (for myocardium) stimuli with duration of 50-150 msec, amplitude of 20-800 microA, and absolute phase (delay ...