Unmasking the trifascicular left intraventricular conduction system by ablation of the right bundle branch

Twenty-five patients underwent transcatheter right bundle ablation either for bundle branch reentrant tachycardias or inadvertent or deliberate right bundle ablation during atrioventricular junctional ablation for rate control. Electrophysiologic data and 12-lead electrocardiograms before and after right bundle ablation were available in all patients. Eleven of the patients had no significant intraventricular conduction abnormalities by surface electrocardiograms (group I), whereas 14 patients had underlying intraventricular conduction delays (group II). All group I patients had typical electrocardiographic changes of right bundle branch block after right bundle ablation, with minimal changes in initial or mean QRS axis. In group II, 5 patients had an initial 40 ms QRS axis shift of >45 °, in 7 patients the mean QRS axis changed significantly (leftward in 4 and rightward in 3), and a qR pattern in V1 was seen in 12 of 14 patients including 2 with structurally normal hearts. These changes, namely new Q waves, and rightward and left-ward axis shifts are most likely the result of septal fascicular, left posterior fascicular, and 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 conduction is more apparent when diseased and unmasked by concomitant block in the right bundle branch.