Electrophysiological Mechanism of the Characteristic Electrocardiographic Morphology of Torsade de Pointes Tachyarrhythmias in the Long-QT Syndrome

TLDR

Long‑QT syndrome is an electrophysiological disorder characterized by prolonged cardiac repolarization and polymorphic ventricular tachyarrhythmias, including torsade de pointes. The study aims to elucidate the electrophysiological mechanism responsible for the periodic QRS‑axis transition observed during nonsustained torsade de pointes in long‑QT syndrome. Using an anthopleurin‑A canine model, the authors constructed three‑dimensional isochronal activation maps from 256 bipolar electrograms recorded with 64 plunge‑needle electrodes. In 26 episodes of nonsustained torsade de pointes, the initial beat arose from a subendocardial focus, subsequent beats were driven by reentrant scroll waves that bifurcated into two simultaneous scrolls in 22 cases—transitions linked to functional conduction block between the right‑ventricular free wall and septum—while in 4 episodes a single rotating circuit produced a fast polymorphic VT, providing the first electrophysiological explanation for the periodic QRS‑axis shifts in long‑QT syndrome.

Abstract

The long-QT syndrome (LQTS) is an electrophysiological (EP) entity characterized by prolongation of cardiac repolarization and the occurrence of polymorphic ventricular tachyarrhythmias (VTs), sometimes with a twisting QRS morphology, better known as torsade de pointes (TdP). In the present study, detailed analysis of ventricular tridimensional activation patterns during nonsustained TdP VT was performed to provide an EP mechanism of the periodic transition in QRS axis.The studies were conducted with the anthopleurin-A canine model of LQTS. Tridimensional isochronal maps of ventricular activation were constructed from 256 bipolar electrograms obtained from the use of 64 plunge needle electrodes. In 26 episodes of nonsustained TdP VT, detailed activation maps could be accurately constructed during QRS-axis transitions in surface ECGs. The initial beat of all VTs consistently arose as a subendocardial focal activity, whereas subsequent beats were due to reentrant excitation in the form of rotating scrolls. The VT ended when reentrant excitation was terminated. In 22 of 26 episodes, the transition in QRS axis coincided with the transient bifurcation of a predominantly single rotating scroll into two simultaneous scrolls involving both the right ventricle and left ventricle separately. The common mechanism for initiation or termination of bifurcation was the development of functional conduction block between the anterior or posterior right ventricle free wall and the ventricular septum. In 4 of 26 episodes, a fast polymorphic VT, with an apparent shift in QRS axis, was due to a predominantly single localized circuit that varied its location and orientation from beat to beat, with the majority of ventricular myocardium being activated in a centrifugal pattern.The study provides for the first time an EP mechanism for the characteristic periodic transition of the QRS axis during TdP VT in the LQTS.