Case studies: Simultaneous intermittent intraatrial and intraventricular conduction defects mimicking trifascicular conduction delay.

Both intermittent rate related left bundle branch block and intermittent rate related trifascicular conduction delay (left bundle branch block with marked H-V interval prolongation) have been previously described. However intermittent rate related intraatrial block with left bundle branch block has not. No only does the case presented herein reveal intermittent rate related intraatrial block with left bundle branch block, but also PR prolongation secondary to the intraatrial block. This combination mimicked intermittent trifascicular conduction delay. Electrophysiological testing allowed differentiation of this pseudotrifascicular defect from true trifascicular conduction delay.