Quantitation of dynamic AV nodal properties and application to predict rate-dependent AV conduction.

A number of functional properties of the atrioventricular (AV) node have been described in response to changes in the atrial input rate. The purpose of this study was 1) to develop quantitative descriptors of these properties, and 2) to determine whether they can account for rate-dependent changes in AV nodal conduction. The delay in AV nodal conduction of single premature beats (recovery) was found to be an exponential function of coupling interval with a time constant of 66 +/- 2 (+/- SE) ms. A single abbreviated (facilitation) cycle did not alter the time constant of recovery or basal conduction for a subsequent beat but shifted its recovery curve to the left to an extent exponentially related to the facilitation cycle length. The induction of a tachycardia with HA interval fixed so as to control the recovery and facilitation variables resulted in a first-order onset of AV conduction slowing (fatigue). The fatigue process had a time constant in the range of 70 beats and a magnitude that was a decaying exponential function of HA interval. An equation incorporating quantitative descriptors of recovery, facilitation, and fatigue accurately predicted rate-dependent changes in AH interval. We conclude that 1) the AV nodal properties of recovery, facilitation, and fatigue are amenable to quantitative characterization, and 2) rate-dependent changes in AV nodal conduction time can be well described in terms of these underlying properties.