MECHANISM OF ATRIOVENTRICULAR NODAL FACILITATION IN RABBIT HEART - ROLE OF PROXIMAL AV NODE

The phenomenon of atrioventricular (AV) nodal ''facilitation,'' descri bed in traditional ''black box''-functional studies, implies enhanced AV nodal dromotropic function. We investigated the role of atrial prem aturities in the modulation of the nodal cellular responses in the mec hanism of AV nodal facilitation. Atrial and His (H) bundle electrogram s and microelectrode recordings from proximal AV nodal cells were anal yzed 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 (St; coupling intervals S-1-S- 2 of 300, 200, 150, and 130 ms) followed by the test beat (S-3; coupli ng 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 const ructed. Facilitation (i.e., shortening of S-3-H-3 when S-1-S-2 was sho rtened) was demonstrated in all preparations using the H-2-S-3 (P < 0. 001) but not the S-2-S-3 format. Microelectrode recordings revealed a causal relationship between the improved proximal AV nodal cellular re sponses in facilitation and the prolonged S-2-S-3 interval. There was no evidence for enhanced nodal dromotropic function directly resulting from the introduction of the facilitating beats. Thus facilitation is based on inherent cycle-length-dependent properties of the AV node du ring application of a complex pacing protocol and primarily reflects t he uncontrolled modulation of the proximal cellular response.