Wide QRS complex tachycardia with alternating right bundle branch block morphologies: What is the mechanism?

Figure 1. Surface electrocardiogram shows a wide complex tachycardia with 2 different right bundle branch block morphologies.

Figure 2. A: Intracardiac electrogram at the right paraseptal region. Fixed right bundle branch (RBB) to V interval (44 ms) was only recorded in the first conducted beat via the fast pathway while RBB signal was not observed in the following conducted beat via the slow pathway. B: Corresponding ladder diagram of the right paraseptal recording. C: Intracardiac electrogram at the left paraseptal region. Fixed left bundle branch (LBB) to V interval (54 ms) was observed during the anterograde conduction via the fast and slow pathways. The conduction of slow pathway to LBB was different from fast pathway, which resulted in different activation patterns of proximal LBB and bipolar electrogram characteristics. D: Corresponding ladder diagram of left paraseptal recording.

Figure 3. Right atrial pacing during preablation and postablation. A: Preablation atrial stimulation at 430 ms terminated the tachycardia without changing the sinus rate. Right atrial pacing reproducibly induced “1P 2QRS” (red arrows) with fixed coupling interval of slow pathway. B: Postablation atrial pacing at 430 ms showed 2:1 AV conduction. The QRS complexes during pacing are similar to the QRS complexes during sinus rhythm. This is consistent with the QRS complexes conducted via the fast pathway, and a confirmation that the slow pathway was eliminated.