Figure1. A. Wide complex tachycardia (cycle length 358ms) presented as right bundle branch block pattern (rSR’) in lead V1. The rate of the ventricle was higher than that of the atrium. The His bundle potential preceded ventricular wave with a shorter HV interval (16ms) than that (52ms) during sinus rhythm. B. During the onset of tachycardia, the atrial rate speeded up and the V-A relationship changed to 1:1, the morphology of the QRS complex changed between narrow and fused, with inconstant AV and VA interval. C. The wide complex tachycardia could occur immediately after the termination of the narrow complex tachycardia. The HV interval (52ms) during narrow complex tachycardia was identical to that during the sinus rhythm. D. The surface electrocardiogram showed that the morphology of the QRS complex alternately transformed among wide, narrow and fused. The cycle length of the tachycardia fluctuated between 341ms and 358ms.