Role of the Compact Node and Its Posterior Extension in Normal Atrioventricular Nodal Conduction, Refractory, and Dual Pathway Properties

AV Nodal Conduction and Dual Pathways. Introduction: The functional origin of AV nodal conduction, refractory, and dual pathway properties remains debated. The hypothesis that normal conduction and refractory properties of the compact node and its posterior nodal extension (PNE) play a critical role in the slow and the fast pathway, respectively, is tested with ablation lesions targeting these structures. Methods and Results: A premature atrial stimulation protocol was performed before and after PNE ablation in six isolated rabbit heart preparations. Discrete (<300 μm) histologically controlled PNE lesions amputated the AV nodal recovery curve from its left steep portion reflecting slow pathway conduction and prevented reentry without affecting the right smooth fast pathway portion of the curve. The ablation shortened A 2 H 2 max from 159 ± 16 ms to 123 ± 11 msec (P < 0.01) and prolonged the effective refractory period from 104 ± 6 msec to 119 ± 11 msec (P < 0.01) without affecting A 2 H 2 min (55 ± 9 msec vs 55 ± 8 msec; P = NS) and functional refractory period (174 ± 7 msec vs 175 ± 6 msec; P = NS). These results did not vary with the input reference used. In six other preparations, lesions applied to the compact node after PNE ablation shifted the fast pathway portion of the recovery curve to longer conduction times and prolonged the functional refractory period, suggesting a compact node involvement in the fast pathway. Conclusion: The normal AV nodal conduction and refractory properties reflect the net result of the interaction between a slow and a fast pathway, which primarily arise from the asymmetric properties of the PNE and compact node, respectively.