Atrioventricular nodal accommodation: rate- and time-dependent effects.

To characterize intrinsic rate- and time-dependent properties of the atrioventricular (AV) node, we examined whether AV interval (AVI) would be comparable at identical heart rates (HR) reached using different types of stimulation paradigms. We compared changes in AVI during five consecutive 30-s, 20 beat/min increases in HR from control with AVI changes induced by 30-s single-step protocols to each of the same levels. In addition, HR was maintained at each level for 30, 60, 90, 120, and 150 s to control for the influences of time. Chloralose-anesthetized dogs (n = 16) were autonomically decentralized and instrumented to record electrocardiogram, blood pressure, and multiple intracardiac electrograms. Computer-generated HR steps were begun 20 beats/min above control, while continuously recording AVI. The beat-by-beat changes in AVI within each step were quantitated for all protocols. Differences between AVI during single- and multiple-step protocols were most pronounced during the first 15 s of atrial pacing. Accommodation in AVI (change in AV conduction time associated with an increased but constant heart rate) was evident during both protocols. The degree of accommodation during multiple-step protocols was modulated by the cumulative effects of earlier HR; however, a similar degree of accommodation occurred at higher HR irrespective of protocol used. Finally, the time to onset of loss of 1:1 AV conduction was significantly shorter when HR was increased using a multiple-step protocol. Thus the intrinsic response of the AV node to HR change is dependent on absolute level of HR, duration of rate change, and the potential cumulative effects of any earlier HR steps.