Electrophysiological features of complete AV block within the His bundle.

The site of block in 3 patients with complete heart block was localized within the His bundle with the aid of the His bundle electrogram.Two patients had normal QRS morphology while the third had a pattern of right bundle-branch block.Split His bundle potentials, i.e.H and H' denoting the activity of the proximal and distal His bundle segments, were seen in all 3 patients.Atrial pacing in 2 patients produced a gradual increase in the AH interval, consistent with normal AV nodal function.In one of these, His bundle pacing was also successful and produced QRS complexes similar to the spontaneous beats.Intravenous atropine was used in 2 patients.In one it did not increase the heart rate, while in the other the heart rate increased slightly.It is concluded that heart block may occur within the main His bundle and that the surface electrocardiogram is of no use in predicting the site of block in such patients.Response to intravenous atropine may be inadequate or absent.