Letter to Editor: Electrocardiographic diagnostic dilemma: gradual QRS widening recorded by rhythm Holter monitoring

Figure 1 demonstrates one of the frequent episodes of gradual QRS widening during sinus rhythm recorded by Holter monitoring of a young lady, who has been complaining of palpitation for a few years. She has no structural heart disease and echocardiographic findings are normal. In Figure 1, the first beat has normal QRS duration, whereas the subsequent group beats demonstrate gradual widening of QRS complexes ending up with complete left bundle branch block (LBBB) at the last beat associated with emerging secondary ST-T wave changes. Heart rate and PR intervals do not substantially change during the episode. What is the probable underlying mechanism responsible for this type of conduction behavior?