A 69-year-old woman was admitted to our hospital after she had received several shocks without warning from a model C154DWK ConcertoTM cardiac resynchronization therapy + defibrillation (CRT-D; Medtronic Inc., Minneapolis, MN, USA), implanted 2 years earlier for management of sustained ventricular tachycardia (VT) and congestive heart failure due to dilated cardiomyopathy. At the time of device implantation, the sensing and pacing characteristics of a tined atrial lead placed in the right atrial appendage were within normal limits. Interrogation of the device retrieved several episodes of tachycardia, unsuccessfully treated by antitachycardia pacing (Fig. 1A), followed by a successful direct cardioversion shock. Recordings stored in the device memory during VT revealed regular, nearly simultaneous ventricular and atrial electrograms (Fig. 1A). The 12-lead electrocardiogram during AAI pacing by the device revealed a marked latency between the pacing spike and the P-wave and a normal PQ interval (Fig. 1B). What is the mechanism of this tachycardia?