Short time constant for rate-dependent changes of atrioventricular conduction in dogs

The number of short cardiac cycles necessary to induce, and normal cycles to dissipate, a rate-dependent shortening in atrioventricular (AV) nodal functional refractory period (FRP) and conduction time (CT) was determined in six anesthetized dogs. For the induction study, the periodic premature stimulation procedure was performed at control and repeated six times while 1, ..., 6 conditioning short prepremature cycles (PPC) were introduced between the last basic cycle and the premature cycle. In all dogs, FRP was maximally shortened and the recovery curve was maximally shifted to the left when the premature cycle was preceded by 1 PPC. Adding 2, ..., 6 PPC resulted in slightly fewer but persistent shortenings of both FRP and CT. The dissipation of the FRP and CT shortenings produced by 2 and 6 PPC was studied subsequently in the same dogs by introducing 1, ..., 6 normal basic cycles between the last PPC and the test premature cycle. The FRP and CT shortenings induced by 2 or 6 PPC were almost completely dissipated after one normal basic cycle. Thus, one short cycle produces a maximum shortening in AV nodal FRP and CT, and one normal basic cycle is sufficient to dissipate these effects.