UNMASKING THE TRIFASCICULAR LEFT INTRAVENTRICULAR-CONDUCTION SYSTEM BY ABLATION OF THE RIGHT BUNDLE-BRANCH

Twenty-five patients underwent transcatheter right bundle oblation eit her for bundle branch reentrant tachycardias or inadvertent or deliber ate right bundle ablation during atrioventricular junctional oblation for rate control. Electrophysiologic data and 12-lead electrocardiogra ms before and after right bundle ablation were available in all patien ts. Eleven of the patients had no significant intraventricular conduct ion abnormalities by surface electrocardiograms (group I), whereas 14 patients had underlying intraventricular conduction delays (group II). AII group I patients had typical electrocardiographic changes of righ t bundle branch block after right bundle ablation, with minimal change s in initial or mean QRS axis. In group II, 5 patients had an initial 40 ms QRS axis shift of > 45 degrees, in 7 patients the mean QRS axis changed significantly (leftward in 4 and rightward in 3), and a qR pat tern in V-1 was seen in 12 of 14 patients including 2 with structurall y normal hearts. These changes, namely new Q waves, and rightward and leftward axis shifts are most likely the result of septal fascicular, left posterior fascicular, anti left anterior fascicular delay/block, which were exposed by exclusive conduction via a diseased left bundle and its fascicles. The trifascicular nature of left intraventricular c onduction is more apparent when diseased and unmasked by concomitant b lock in the right bundle branch.