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Narrow QRS Tachycardia with Transition: What Is the Mechanism?

A 50‐year‐old woman underwent radiofrequency catheter ablation of narrow QRS tachycardia that was terminated with intravenous adenosine. Twelve‐lead electrocardiogram was normal during sinus rhythm. The electrophysiological study showed normal atriohisian and hisioventricular intervals during sinus rhythm. Atrial burst pacing reproducibly induced two narrow QRS tachycardias. Figure 1 shows spontaneous switch of tachycardia 1 to tachycardia 2. What are the likely mechanisms of the tachycardia and the transition?