A 62-year-old man presented to the hospital following an episode of syncope and implantable cardiovertor defibrillator (ICD) shock. He was known to have ischemic cardiomyopathy, prior coronary artery bypass grafting in 2007, and a dual-chamber ICD, a Current Accel DR (St. Jude Medical, Inc., Minneapolis, MN, USA) implanted in 2009 following a ventricular fibrillation (VF) arrest. Device interrogation revealed a VF episode that was successfully terminated with a 36-J shock. Testing of the atrial and ventricular leads demonstrated adequate pace/sense function and normal lead impedances. Device parameters were as follows: DDD mode with a base rate of 40 beats per minute (rate response sensor was passive), postventricular atrial refractory period (PVARP) was 275 ms, ventricular intrinsic preference (VIP) was enabled and programmed to 100-ms extension (a setting to prolong the atrioventricular [AV] delay and permit intrinsic conduction), and premature ventricular contraction (PVC)/pacemakermediated tachycardia responses were enabled. Tachycardia detection was set to three zones, with the ventricular tachycardia 1 (VT1) zone starting at 375 ms, VT2 starting at 320 ms, and VF zone starting at 280 ms. Therapy was programmed with antitachycardia pacing followed by shocks in the first two zones and shocks only in the VF zone. The presenting episode is shown in Figure 1. What is the most likely mechanism of initiation of this VF episode; is it likely related to the underlying