Effects of Right Bundle Branch Block on the Antidromic Circus Movement Tachycardia in Patients with Presumed Atriofascicular Pathways

Background: The typical and most common tachycardia in patients with atriofascicular pathways is a macro reentrant tachycardia, with anterograde conduction over the decrementally conducting bypass tract and retrograde conduction over the right bundle branch‐His‐AV node axis resulting in a short V‐right bundle branch and short V‐H interval. Objectives: To report on changes in rate and QRS configuration when right bundle branch block (RBBB) develops spontaneously during antidromic tachycardia using an atriofascicular fiber. Methods: Three of 25 patients with an antidromic circus movement tachycardia using a right‐sided atriofascicular pathway showed episodes of right bundle branch block (RBBB) during ventriculo‐atrial conduction. Effect of retrograde RBBB on tachycardia rate and QRS configuration was studied using intracardiac and extracardiac recordings. Results: All 3 patients showed prolongation of their V‐A interval when retrograde RBBB occurred during tachycardia, resulting in a longer tachycardia cycle length. The VA time increase ranged from 85 to 100 msec, with a mean 346 ± 5 msec. Two of the 3 patients also showed a change in QRS configuration due to a more leftward shift of the frontal plane QRS axis. Conclusion: Rate changes in antidromic tachycardia in patients with atriofascicular fibers can be based on a shift in VA conduction from one bundle branch to the other. This may be accompanied by changes in the frontal plane QRS axis because of a change in ventricular activation sequence.