Mechanisms of conduction time hysteresis in rabbit atrioventricular node

The functional origin of atrioventricular nodal hysteresis was studied in isolated rabbit heart preparations. This hysteresis is characterized by asymmetric changes in nodal conduction time (NCT) occurring for symmetric changes in cycle length. The respective contribution of the nodal properties of recovery, facilitation, and fatigue to the beat-to-beat changes in NCT observed during paired symmetric ramps of decreasing and increasing cycle length was determined with specifically design stimulation protocols. Nodal hysteresis was found to be entirely accounted for by variations in the contribution of nodal recovery and fatigue properties observed at corresponding cycle lengths. The study establishes how this contribution varies on a beat-to-beat basis as a result of cycle length history. This holds true for the numerous changes in hysteresis observed in response to changes in the sequence and slope of the ramps. Facilitation clearly affected NCT during these responses but did not contribute to the hysteresis. Moreover, the study demonstrates that there is no inherent change in the characteristics of nodal function with the direction of the ramp that could account for the hysteresis. Thus nodal hysteresis arises from nodal functional properties of recovery and fatigue but does not constitute a distinct independent intrinsic property of the node.