Failure to recognize a His bundle potential in complete atrioventricular block

This report concerns a patient with complete heart block, in whom electrophysiological studies showed at times an escape rhythm with narrow QRS complexes preceded by His potentials with normal HV intervals (35--40 msec) and at other times an escape rhythm of similar rate, having wide QRS complexes of left bundle branch block configuration with no preceding His bundle activity. Complexes intermediate in width and configuration and preceded by His potentials with an HV interval inversely proportional to QRS width were also recorded. These observations are explained by a site of block proximal to the His bundle and competition between two pacemaker foci having similar discharge rates, one situated in the junctional region below the site of block and the other more distally in the right bundle branch or right ventricle. It is proposed that the combination of a proximal site of block and a distally situated dominant pacemaker may be a common reason for failure to record a His potential in patients with complete heart block.