ZHANG’S PHENOMENON (HIS ELECTROGRAM ALTERNANS) AND A NEW MODEL OF ATRIOVENTRICULAR NODE DUAL PATHWAY CONDUCTION
Youhua Zhang, M.D., Ph.D., New York Institute of Technology College of Osteopathic Medicine, NY, USA
Zhang’s phenomenon (originally His electrogram alternans), a new index of dual pathway atrioventricular (AV) node conduction, indicates that there are dual inputs rather than a final common pathway from the AV node into the His bundle. Our recent data revealed that during fast pathway (FP) conduction, the electrical excitation in the AV node propagates in a superior to inferior direction across the AV conduction axis. In contrast, this superior-inferior activation fails within the superior nodal domain during slow pathway (SP) conduction. This then permits electrical excitation to proceed longitudinally along the AV conduction axis through the inferior nodal domain. This transverse versus longitudinal electrical propagation in the AV node produces superior-fast and inferior-slow dual inputs into the His bundle during dual pathway conduction (the electrophysiological basis of Zhang’s phenomenon). We believe that the peculiar anatomical location and fiber orientation of the AV node are mainly responsible for the unique electrical conduction pattern. The AV node is open only superiorly and posteriorly to atrial excitation. Fibers inside the AV node are largely aligned longitudinally along the AV conduction axis. Electrical propagation across fiber orientation (in a superior to inferior direction) is possible during FP conduction. However, this cross-fiber activation results in a longer effective refractory period and, thus, this activation will fail at short prematurities (i.e., SP conduction). Longitudinal activation along the fiber orientation results in a short effective refractory period. The failing of superior-inferior activation permits excitation to proceed along fiber orientation in the inferior nodal domain during SP conduction (i.e., at short prematurities). In summary, the transverse versus longitudinal electrical propagation within the AV node, the resulting functional dissociation in the distal node, and the superior-fast and inferior-slow dual inputs into the His bundle form the main features of the new model of dual pathway AV conduction.