Rate-dependent AV nodal function: closely bound conduction and refractory properties

The atrioventricular node (AV) node conducts slowly and filters atrial impulses. Puzzlingly, the recovery (conduction time vs atrial cycle length) and refractory curve (His bundle cycle length vs atrial cycle length) characterizing AV nodal function undergo disparate rate-dependent changes. We sought the functional origin and significance of these disparate changes.

Differences between the recovery and refractory curve were assessed in 30 steady state AV nodal responses (all potential paired combinations between 5 basic and 6 pretest cycle lengths imposed with S(1)S(2)S(3) protocols) in rabbit heart preparations. Five of these responses corresponded to standard premature protocols. Both basic and pretest cycle length shortenings increased pretest conduction time that in turn equally shortened the ensuing His-atrial interval at all test cycle lengths. This effect was mathematically predictable and tended to shift the refractory curve downward whereas not affecting the recovery curve. Moreover, increases in pretest conduction time also shifted the recovery and refractory curve equally rightward on their x-axis, thereby biasing curve comparison between steady states. This problem could be overcome with equivalent results either by accordingly correcting the atrial cycle length or by using the His-atrial interval as the recovery index.

Recovery and refractory curves from AV nodal steady state responses including standard premature protocols only differ by the His-atrial interval that decreases with the pretest conduction time. The latter also biases curve comparison between steady states. Rate-dependent AV nodal function is best assessed with recovery curves freed from changes in pretest conduction time.