Case presentation

A 76-year-old man had a surgery of aortic valve replacement, mitral valve and tricuspid valve repair, and surgical pulmonary vein isolation (for severe aortic valve regurgitation, mitral valve regurgitation and paroxysmal atrial fibrillation). After the operation, he showed a low left ventricular ejection faction (LVEF 27%) with a QRS prolongation from 98 ms to 132 ms (Figure 1A). On the fourth postoperative day, he had a wide QRS tachycardia of left bundle branch block (LBBB)-like and superior axis deviation with a heart rate of 156 bpm. (Figure 1A). The baseline AH and HV interval was 90ms and 76ms. Programed stimulation from the right ventricular apex (RVA) produced a fragmented potential (Fg) following a spike potential (spike-Fg) at the His bundle region (Figure 1B). With extra-stimulations, the fragmented component was prolonged and the sequence of these potentials became reversed (Fg-spike), thereby inducing the wide-QRS complex tachycardia. This tachycardia showed a LBBB-like morphology and superior axis deviation with the tachycardia cycle length (TCL) of 420 ms (Figure 1B) and atrioventricular (AV) dissociation. What is the mechanism of the LBBB morphology tachycardia with AV dissociation? What do the spike and Fg potentials produced by RVA stimulation indicate?