Idiopathic hypertrophic subaortic stenosis with split His bundle potentials. Electrophysiologic and pathologic correlations.

A 25-year-old white female had idiopathic hypertrophic subaortic stenosis proved by catheterization. The ECG revealed left bundle branch block. Electrophysiologic studies revealed normal PA and AH intervals, but the HB interval was prolonged (70 msec). The width of the His spike was 20 msec. A clear, reproducible split His potential was demonstrated with the atrial extrastimulus technique. Because the patient was symptomatic, a permanent pacemaker was inserted. One week later the patient died suddenly. Autopsy revealed an enlarged heart with a markedly thickened and sigmoid ventricular septum and a small and coarsely trabeculated lumen. Histologically the common bundle was situated on the right side of the septum. It was intact at its origin, but showed fibrotic changes more distally. The beginning of the left bundle branch was markedly disrupted as it traversed the septum. The right bundle was moderately fibrosed. There was excellent correlation between the electrocardiographic and electrophysiologic findings and the findings in the conduction system. The cause of the sudden death was related either to the outflow obstruction or to an arrhythmia, or both.