His electrogram alternans (Zhang's phenomenon) and a new model of dual pathway atrioventricular node conduction.

Abstract

In contrast to the current textbook model and the current clinical index of dual pathway atrioventricular (AV) nodal conduction, here we summarize the discovery and validation of Zhang's phenomenon (originally His electrogram alternans) as a new index of dual pathway conduction. We also describe the new findings of transverse-versus-longitudinal electrical propagation within the AV node as the electrophysiological basis underlining this new index. Thus, a new index and a new model of dual pathway AV conduction are being developed. We have reviewed current literature and provided evidence supporting a new index and a new model of dual pathway AV conduction. Recent data revealed that during fast pathway conduction, electrical excitation in the AV node propagates in a superior to inferior direction across AV conduction axis and fiber orientation to reach first the superior His bundle fibers. However, this transverse conduction can fail easily within the superior nodal domain at fast rates. The failing of transverse propagation permits electrical excitation formed at the posterior/inferior nodal region to propagate longitudinally along fiber orientation in a posterior to anterior direction through the inferior nodal domain to reach the inferior His bundle (slow pathway conduction). This transverse-versus-longitudinal electrical propagation within the AV node results in a functional dissociation in the distal node and formation of dual inputs into the His bundle, providing the electrophysiological basis for the formation of Zhang's phenomenon (His electrogram alternans). Based on strong experimental data, a new index and a new model of dual pathway AV nodal conduction are emerging, although they are still awaiting clinical validation.