[Paroxysmal block in phase 4 of the bundle of His]

Five cases of chronic paroxysmal atrio-ventricular block are reported, being unusual in the siting of the conduction defect within the trunk of the bundle of His, and in the "paradoxical" mode of onset of the block. The ventriculograms which were carried out, and the escapes, were of normal duration in four cases and the intracavitary recordings showed a double His potential in all patients. In one case, the block during phase 4 seemed to occur at the height of the trunkal Wenkebach periods, and in another it could be brought on by manipulation of the vagus. In all five cases, a favorable conduction zone could be calculated, the block to the P waves in phase 4 occuring in an arithmetical fashion after a critical lengthening of the PR interval. The return to sinus rhythm was always related to an escape (or to an electrically induced ventricular complex), but the explanation of the initial capture on electrophysiological grounds is obscure.