Fascicular block within the His bundle.

The findings in a patient with complete atrioventricular block and normal morphology and duration of the QRS complex are presented. A His bundle electrogram was obtained, which led to the location of the atrioventricular block within the His bundle. A careful review of the electrocardiograms obtained during the seven years preceding the onset of complete atrioventricular block showed a QRS complex with the features of left anterior hemiblock and a progressive impairment of atrioventricular conduction. From these data, we inferred that the different degrees of atrioventricular block and the left anterior hemiblock were caused by lesions within the His bundle involving the fibers destined for the left anterior division of the left branch. After the onset of complete atrioventricular block, with the subsidiary pacemaker located in the His bundle distal to the lesions, the QRS complex became normal, indicating the integrity of the bundle branches and fasciculi. The atrial and proximal His potential intervals and those between distal His and ventricular potentials were normal.