Relation of the Atrial Input Sites to the Dual Atrioventricular Nodal Pathways:

INTRODUCTION: The usually accepted definition of the dual pathway electrophysiology requires the presence of conduction curves with a discontinuity ("jump"). However, AV nodal reentrant tachycardia has been observed in patients with "smooth" conduction curves, whereas discontinuity of the conduction curve does not guarantee induction of stable reentry. We hypothesize that the duality of AV nodal conduction can be revealed by careful choice of stimulation sites during the generation of AV nodal conduction curves. METHODS AND RESULTS: In 21 rabbit heart atrial-AV nodal preparations, programmed electrical stimulation with S1-S2-S3 pacing protocol was applied either posteriorly at the crista terminalis input site (CrT) or anteriorly at the lower interatrial septum input site (IAS), or (in 8 preparations with surgically divided input sites) at both. We found that in intact preparations with "smooth" conduction curves, pacing at long coupling intervals produced shorter AV nodal conduction times from the IAS (56 +/- 9.8 msec vs 69 +/- 10.1 msec; P < 0.01). At short coupling intervals, in contrast, shorter conduction times were obtained from the CrT (173 +/- 21.8 msec vs 188 +/- 22.8 msec; P < 0.01). This resulted in a characteristic crossing of the superimposed IAS and CrT conduction curves. After division of the inputs, the IAS site had rapid conduction to the His bundle but a longer refractory period, whereas the CrT site had long conduction times and shorter refractory periods. Wavefronts entering the AV node from these two inputs can summate, resulting in improved conduction. CONCLUSION: Pacing protocols designed to accentuate the asymmetry between the AV nodal inputs can help to reveal the functional difference between the dual pathways and thus to better assess the properties of AV nodal conduction.