Case presentation
A 78-year-old man was referred for the electrophysiologic (EP) study for sustained wide QRS-complex tachycardia (WCT). Transthoracic echocardiography revealed diffuse severe hypokinesis with an ejection fraction (EF) of 33%. He was referred for radiofrequency catheter ablation (RFCA). An electrocardiogram (ECG) at the beginning of the procedure showed WCT with a cycle-length of 520 ms and a left bundle branch block (LBBB) pattern, a concentric pattern with the earliest atrial activation site at His and a septal VA interval of 36 ms, an HH interval preceding a VV interval with a fixed HV interval of 75 ms, and 1:1 VA conduction (Figure 1B). The ECG during sinus rhythm (SR) showed the same QRS morphology during WCT, and AH and HV intervals were 204, and 72 ms, respectively (Figure 1A). Catheter manipulation or any stimulation easily induced the WCT. Therefore, antegrade and retrograde dual atrioventricular (AV) nodal (AVN) physiology were not confirmed. During WCT, a transient VA perturbation following AH block was observed (Figure 2). What is the diagnosis?