Role of His Refractory Premature Ventricular Complexes in the Differential Diagnosis of a Left Bundle Branch Block Morphology Tachycardia.

A 65-year-old man, with no significant past medical history, presented with an episode of sudden onset palpitations and lightheadedness. He was hemodynamically stable and the ECG revealed a left bundle branch block (LBBB) morphology tachycardia at 214 beats per minute (Figure 1A). The tachycardia terminated spontaneously with conversion to normal sinus rhythm and LBBB (Figure 1B). Further evaluation included a 2-dimensional echocardiogram showing a left ventricular ejection fraction of 40% to 45%, left heart catheterization showing angiographically normal coronaries, and cardiac magnetic resonance imaging revealing atypical septal motion with left ventricular ejection fraction of 60% to 65% and no scar or ischemia. The patient underwent an electrophysiology study. At baseline, the sinus cycle length was 890 ms, atrial-His interval was 64 ms, Hisventricular interval was 108 ms, and QRS duration was 120 ms. Spontaneous onset of the LBBB morphology tachycardia after a sinus beat is shown in Figure 2. The tachycardia cycle length is 306 ms, and the His-ventricular interval is 120 ms. The QRS while similar to the LBBB seen during sinus rhythm shows change in axis and the morphology in aVR is predominantly positive. There is no ventriculo-atrial conduction during the tachycardia. Attempts at entrainment from right ventricle (RV) led to termination of the tachycardia. His refractory premature ventricular complexes (PVCs) delivered from RV apex during tachycardia after the His depolarization advances the next H and V (Figure 3). What is the mechanism of the tachycardia?