The tachycardia that outsmarted the SMART

A 57-year-old man presented with palpitations and shock delivery from his implantable cardioverter-defibrillator (ICD). He had a history of ischemic cardiomyopathy with an ejection fraction of 20%. One month prior to this presentation, he received a DX ICD (Iforia 5 VR-T DX, Biotronik SE & Co. KG) for primary prevention of sudden cardiac death. Two ventricular tachycardia (VT) zones (VT1 162–188 bpm; VT2 188–222 bpm) were programmed in addition to a ventricular fibrillation (VF) zone (4222 bpm). Upon device interrogation, intracardiac electrograms (EGM) during antitachycardia pacing (ATP) are shown in Fig. 1. What is the tachyarrhythmia and the reason for therapy delivery?