ELECTROPHYSIOLOGICAL MECHANISM OF THE CHARACTERISTIC ELECTROCARDIOGRAPHIC MORPHOLOGY OF TORSADE-DE-POINTES TACHYARRHYTHMIAS IN THE LONG-QT SYNDROME - DETAILED ANALYSIS OF VENTRICULAR TRIDIMENSIONAL ACTIVATION PATTERNS

Background The long-QT syndrome (LQTS) is an electrophysiological (EP) entity characterized by prolongation of cardiac repolarization and th e occurrence of polymorphic ventricular tachyarrhythmias (VTs), someti mes with a twisting QRS morphology, better known as torsade de pointes (TdP). In the present study, detailed analysis of ventricular tridime nsional activation patterns during nonsustained TdP V-T was performed to provide an EP mechanism of the periodic transition in QRS axis. Met hods and Results The studies were conducted with the anthopleurin-A ca nine model of LQTS. Tridimensional isochronal maps of ventricular acti vation were constructed from 256 bipolar electrograms obtained from th e use of 64 plunge needle electrodes. In 26 episodes of nonsustained T dP VT, detailed activation maps could be accurately constructed during QRS-axis transitions in surface ECGs. The initial bed of all VTs cons istently 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 e pisodes, the transition in QRS axis coincided with the transient bifur cation of a predominantly single rotating scroll into two simultaneous scrolls involving both the right ventricle and left ventricle separat ely. The common mechanism for initiation or termination of bifurcation was the development of functional conduction block between the anteri or 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 var ied its location and orientation from beat to beat, with the majority of ventricular myocardium being activated in a centrifugal pattern. Co nclusions The study provides for the first time an EP mechanism for th e characteristic periodic transition of the QRS axis during TdP VT in the LQTS.