Unusual induction of a very slow supraventricular tachycardia: What is the mechanism?

A 73-year-old man with fatigue and exertional dyspnea was referred to our clinic for evaluation of daily episodes of regular palpitations. His past medical history was notable for hypertension, diabetes mellitus, chronic obstructive pulmonary disease and hypothyroidism on medical therapy. Five months' prior he was implanted with a permanent bicameral pacemaker (PM) for sick sinus syndrome associated with head trauma secondary to syncope; the pacemaker was programmed in DDD mode, with a lower rate of 50 bpm, a upper rate of 130 bpm, and the AAI-DDD mode switch algorithm on to minimize right ventricular pacing. A 24-h Holter monitor showed several episodes of a regular rhythm, with no visible P waves, at a rate between 75 and 95 bpm. Of note, these paroxysms were induced by premature atrial contractions (PACs) conducted with a longer PR and often terminated by premature ventricular contractions (PVCs) with a retrograde P wave (Fig. 1). While interrogating his device, this clinical slow tachycardia was easily inducible with both atrial and ventricular pacing. Despite beta-blocker therapy (metoprolol 100 mg BID), symptoms did not resolve and the patient was brought to the electrophysiology (EP) lab for an EP study and ablation. Fig. 2 shows the response to atrial extrastimuli delivered at different phases of the tachycardia cycle. Fig. 3 shows a peculiar phenomenon occurring