Widening of the QRS complex during the wide complex tachycardia: What is the mechanism?

A28?year?old woman underwent an electrophysiology study (EPS)because of episodes of palpitations with documented narrow (NCT)and wide QRS complex tachycardia (WCT) on a 24?hHolterECG.During the EPS, the patient was in sinus rhythm (SR) with normalatrio?His (AH) and His?ventricular (HV) intervals of 45 and 55 ms,respectively. The intracardiac electrograms of the tachycardia duringthe spontaneous transition from a WCT with a right bundle branchblock (BBB) morphology (first WCT, f?WCT) to a wider right BBB one(second WCT, s?WCT) are shown in Figure1. Spontaneous conversionbetween the s?WCT and the f?WCT has also been detected during theEPS (Figure2). What is the underlying mechanism?