INTRA-HIS BUNDLE BLOCK CORRESPONDS WITH INTERRUPTION OF THE BRANCHINGPORTION OF THE HIS-BUNDLE

We compared His-bundle electrograms with pathological findings of the atrioventricular conduction system in four patients with complete atri oventricular intra-His block with narrow QRS complexes on ECG. Split H is electrograms were recorded at the time of electrophysiological stud y. The patients died from noncardiac causes at 10 days, 1 year, 4 year s, and 9 years, respectively, after the pacemaker implantation. Serial sections through the atrioventricular conduction system revealed stri ctly localized more than 50% reduction of conducting cells replaced by fibrosis at the branching portion of His bundle. The proximal portion s of the bundle branches also exhibited decrease of the conducting cel ls showing a rough positive relation with the patient's age. Therefore , we considered that the H-1 spikes seen on His-bundle electrograms or iginated from the penetrating portion of His, which was virtually inta ct in our cases, and that the H-2 spikes originated from the right sid e of the distal branching portion of His.