Progressive widening of the QRS complex during tachycardia: What are the underlying rhythms?

摘要

A 24-year-old male was admitted because of a 3-year-long history of recurrent tachycardia refractory to a beta-blocker agent and propafenone. There was no abnormality on transthoracic echo and an electrophysiologic study was performed after medications were discontinued for at least 5 half-lives. A narrow QRS complex tachycardia was repro-ducibly induced by a right atrial premature beat. The QRS morphology of this tachycardia showing an incomplete right bundle branch block-like pattern spontaneously changed into a wider QRS complex with a different QRS configuration. The transition toward this wider QRS complex was gradual until its width became stable (Fig. 1). An intracardiac tracing (Fig. 2) also shows the transition from a narrow to a wide QRS. The tachycardia cycle length remained the same at 310 milliseconds at the beginning of the QRS transition but gradually shortened thereafter, reaching a value of 280 milliseconds by the time the QRS complex morphology stabilized (Fig. 3). What is the mechanism of the arrhythmia?