Effects of transplantation on atrioventricular nodal accomodation and hysteresis

In the present study, atrioventricular nodal accommodation and hysteresis characteristics were evaluated in awake, unsedated dogs, before (n = 10) and after cardiac transplantation (n = 10). Chronically instrumented animals were atrially paced at a cycle length (CL) of 400 ms, followed by an abrupt decrease in pacing CL to 300 ms, followed by an abrupt return in pacing CL to 400 ms (with pacing sustained for 60 s at each CL). Atrioventricular nodal conduction characteristics (assessed by AH intervals) were simultaneously monitored. Under control conditions, AH intervals lengthened rapidly after an abrupt decrease in pacing CL [mean time for AH interval lengthening to stabilize (T(onset)) = 2 ± 1 s], whereas AH intervals lengthened more slowly (P < 0.05) after transplantation (T(onset) = 41 ± 4 s). Similarly, after an abrupt increase in pacing CL, control AH intervals shortened rapidly [mean time for AH interval to return to base line (T(onset)) = 5 ± 1 s], whereas AH intervals shortened more slowly (P < 0.05) after transplantation (T(onset) = 42 ± 5 s). Thus accomodation appears to be an intrinsic atrioventricular nodal response (present after cardiac denervation by transplantation) to abrupt, sustained atrial CL changes. Furthermore equivalence (P = NS) in atrioventricular nodal accommodation responses to symmetric CL decreases and increases after transplantation suggests that hysteresis [i.e., nonequivalence (P < 0.05) in atrioventricular nodal accomodation responses], as seen under control conditions, results primarily from extrinsic (neural) modification of intrinsic atrioventricular nodal responses to symmetric CL changes.