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Pacemaker Tachycardia with Changing Rates

A 60-year-old man with ischemic cardiomyopathy was admitted with cardiogenic shock. He had a Boston Scientific Teligen 100 E110 dualchamber implantable defibrillator (Boston Scientific, Natick, MA, USA) placed 6 months prior to admission. During his stay in the cardiac care unit, multiple episodes of paced wide-complex tachycardia with accompanying hemodynamic deterioration were noted. The bradycardia parameters were programmed as follows: DDD, base rate: 50 beats per minute (bpm); upper tracking rate: 120 bpm; paced or sensed atrioventricular (AV) delay: 200 ms/200 ms; postventricular atrial refractory period (PVARP): 300 ms; PVARP after premature ventricular contraction (PVC): 400 ms; AV search+: on with search AV delay set at: 400 ms; and the search interval set: 32 ms. There were numerous episodes noted on telemetry: the initiation of a typical episode is shown in Figure 1. One such episode occurred during interrogation, but midway through the recording, the cycle length abruptly changed from 505 ms to 648 ms (Fig. 2). What is the tachycardia and what is the mechanism of the change in cycle length?