Functional anatomy of AV conduction: changing concepts in the ablation era

Transitional cell populations (TCs) are located adjacent to the compact AV node in the rabbit. TCs located superior to the compact AV node and inferior to the interatrial septum (midpathway) have the longest Wenckebach cycle length (WBCL) (208 +/- 12 ms), but fail to determine AV WB when the posterior and/or anterior inputs remain intact. When all 3 TC populations remain intact, AV WB is determined at a shorter CL (195 +/- 15 ms) by TCs of the posterior input. With transection of the posterior TC input into the compact AV node, AV conduction is maintained at a longer AV WBCL consistent with WB within TCs located superior and anterior to the compact AV node. AV conduction remains intact in most hearts after transection of posterior and anterior TC populations from the compact AV node, with AV WB determined by TC input from the midpathway at a prolonged CL (214 +/- 10 ms) consistent with the pretransection midpathway WBCL. In a separate set of experiments, surgical separation of the 3 TC inputs from each other was performed while maintaining connection of the 3 individual TC inputs to the compact AV node remained intact. Stimulation of the proximal input from any 2 of the 3 TC inputs summated to produce a longer AV WBCL than observed from either AV nodal input alone. Data provided by the ablation of AV nodal inputs show the existence of 3 anatomically and functionally distinct populations of TCs providing independent and summated atrial input into the compact AV node.