Fascicular parasystole associated with tachycardia-dependent right bundle branch block

A 43-year-old female with a chief complaint of palpitation was subjected to clinical electrophysiological studies. Initial standard 12-lead ECG revealed that her palpitation was caused by fascicular parasystole firing at the basic cycle length of 1.25-1.40 seconds, and that both sinus and parasystolic beats were associated with left anterior fascicular block and tachycardia-dependent RBBB. His-bundle electrocardiogram suggested that the parasystolic focus was located in the proximal portion of the anterior fascicle of the left bundle branch and that the site of tachycardia-dependent conduction block was located in the main right bundle branch. These findings suggest that diffuse pathological changes in the intraventricular conducting system were responsible for both the conduction block and automatic impulse formation in the present case.