Narrow QRS Tachycardia with Alternate Wide QRS Beats: What is the Mechanism?

A 44-year old lady underwent electrophysiology study for recurrent palpitations and documented narrow QRS regular tachycardia. The baseline ECG showed subtle preexcitation that was easily manifest on atrial pacing. The retrograde atrial activation sequence during ventricular pacing was eccentric suggesting retrograde conduction over the accessory pathway. A regular narrow QRS tachycardia with cycle length 280 ms was easily inducible on programmed atrial stimulation. The earliest ventricular activation during sinus rhythm & atrial pacing, and the retrograde atrial activation during ventricular pacing & tachycardia were diagnostic of left free wall accessory pathway and orthodromic atrioventricular reciprocating tachycardia (AVRT). During an episode of tachycardia, an ablation catheter was placed in the region of the lateral mitral annulus using retrograde trans-aortic approach. Once the ablation catheter was stabilized in that region, an interesting change in the AVRT was seen with appearance of wide QRS complexes of right bundle branch block (RBBB) morphology and left axis deviation in the alternate beats (Figure 1). The atrial activation sequence during both narrow and wide QRS beats was same with earliest activation in the distal coronary sinus. What is the mechanism of the alternate wide QRS beats during the AVRT?