Repetitive pacemaker-mediated tachycardia occurring only during left ventricular pacing: What is the mechanism?

a l ase summary 54-year-old man with an artificial aortic valve and a istory of hemodynamically unstable sustained ventricuar tachycardia was admitted for placement of an implantble cardioverter-defibrillator (ICD). The patient had an nlarged left ventricle, low ejection fraction (35%), left undle branch block, QRS 195 ms, and low exercise olerance. A biventricular ICD (Kronos LV-T, Biotronik, erlin, Germany) was implanted with the left ventricular LV) lead in the posterolateral vein. Two days later, redischarge evaluation by chest x-ray film revealed stale positions of the right ventricular (RV), LV, and atrial eads. Detailed device checkup confirmed good threshlds and sensing in all chambers. However, immediately fter the device was programmed to DDD LV pacing only basic rate 60 bpm, upper tracking rate 130 bpm), an rrhythmia appeared (Figure 1). These episodes of tachyardia took the form of repetitive salvos that were interpersed with one to a few LV-paced beats at a rate of 70 pm. This profile was repeated each time the pacing ode was changed from either DDD BiV or DDD RV to DD LV. Reprogramming to DDD BiV or DDD RV acing always abolished the arrhythmia and prevented its ecurrence. Figures 2 and 3 show intracardiac ECGs with vent markers during the arrhythmia, sinus rhythm, and V, LV, and BiV pacing. What was the mechanism of the achycardia?