Mechanism of atrioventricular nodal facilitation in rabbit heart: role of proximal AV node

Department of Cardiology, The Cleveland Clinic Foundation, Cleveland, Ohio 44195 The phenomenon of atrioventricular (AV) nodal "facilitation," described in traditional "black box"-functional studies, implies enhanced AV nodal dromotropic function. We investigated the role of atrial prematurities in the modulation of the nodal cellular responses in the mechanism of AV nodal facilitation. Atrial and His (H) bundle electrograms and microelectrode recordings from proximal AV nodal cells were analyzed in 15 superfused rabbit AV node preparations. The pacing protocol consisted of 30 basic beats (S 1 ; coupling interval S 1 -S 1 = 300 ms) followed by a facilitating prematurity (S 2 ; coupling intervals S 1 -S 2 of 300, 200, 150, and 130 ms) followed by the test beat (S 3 ; coupling interval S 2 -S 3 scanned in 5-ms steps). Conduction curves (S 2 -H 2 vs. S 1 -S 2 , S 3 -H 3 vs. S 2 -S 3 , and S 3 -H 3 vs. H 2 -S 3 ) were constructed. Facilitation (i.e., shortening of S 3 -H 3 when S 1 -S 2 was shortened) was demonstrated in all preparations using the H 2 -S 3 ( P