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 characteriz ed 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 decreas ing and increasing cycle length was determined with specifically desig n stimulation protocols. Nodal hysteresis was found to be entirely acc ounted for by variations in the contribution of nodal recovery and fat igue properties observed at corresponding cycle lengths. The study est ablishes how this contribution varies on a beat-to-beat basis as a res ult of cycle length history. This holds true for the numerous changes in hysteresis observed in response to changes in the sequence and slop e of the ramps. Facilitation clearly affected NCT during these respons es but did not contribute to the hysteresis. Moreover, the study demon strates that there is no inherent change in the characteristics of nod al function with the direction of the ramp that could account for the hysteresis. Thus nodal hysteresis arises from nodal functional propert ies of recovery and fatigue but does not constitute a distinct indepen dent intrinsic property of the node.