An irregular supraventricular tachycardia: What is the mechanism?

A 28-year-old woman underwent radiofrequency catheter ablation of narrow QRS tachycardia (NQT) that was terminated with intravenous adenosine. Twelve-lead ECGwas normal during sinus rhythm.On electrophysiological study, an atrial programmed stimulation reproducibly induced a NQT. Figure 1 shows a spontaneous initiation of NQT after AH jump with varying VA interval. What are the likely mechanisms of the tachycardia?

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