• atrioventricular node;
  • rate-dependent function;
  • pretest conduction time;
  • recovery index;
  • tachyarrhythmias;
  • recovery;
  • fatigue;
  • frequency step

Introduction: Rate-dependent AV nodal function is often assessed with premature protocols. Conduction curves generated from nodal responses differ with the selected recovery index (atrial–atrial, AA, or His–atrial interval, HA). We propose that these differences arise from changes in pretest conduction time that affect nodal recovery time assessment.

Methods and Results: We varied the basic (S1S1), pretest (S1S2), and test (S2S3) cycle length with S1S2S3 protocols, and analyzed nodal responses as AA (A3H3 vs A2A3) and HA (A3H3 vs H2A3) curves in six rabbit heart preparations. Any A2H2 (pretest conduction time) prolongation bodily shifted AA curve rightward and HA curve leftward, regardless of prevailing basic cycle length. A prolonged A2H2 caused all A3H3 to occur at longer A2A3 and shorter H2A3, as compared with control. When corrected for these shifts, AA and HA curves displayed similar recovery and fatigue properties. To further investigate the possibility that nodal recovery time varies beyond that imposed by pacing interval, six additional preparations were subjected to 5-minute frequency step protocols during which a long cycle was introduced after every 30th short cycle. After each long cycle, nodal conduction time increased progressively despite the constant short cycle length and fatigue level.

Conclusions: Increases in the pretest conduction time play a pivotal role in apparent recovery-index-dependent differences in rate-dependent AV nodal function by shifting AA and HA curves in opposite directions along the x-axis. When corrected for pretest effects, AA and HA curves display similar rate-dependent AV nodal function with recovery and fatigue as main properties.