Pacing the Human Cardiac Conduction System

Specialized fiber electrograms were recorded from selected sites along the His bundle and bundle branches during open-heart surgery in 26 patients. The heart was then paced from the same recording sites. Stimuli applied to the proximal His bundle recording site always resulted in His bundle pacing, which was characterized by a stimulus artifact-to-QRS interval which equaled or very nearly equaled the previously recorded His bundle electrogram-to-QRS interval, and no change in QRS duration or waveform from that recorded with a conducted atrial beat. Wher stimuli were applied to distal His bundle recording sites, the response was variable: in some instances, ventricular pacing occurred; in other instances, His bundle pacing occurred; and in still other instances, ventricular pacing occurred when high stimulus amplitudes were applied, while His bundle pacing occurred when the stimulus amplitudes were reduced below a range of 0.5-3.5 ma. Stimuli applied to the right or left bundle branch recording sites always resulted in ventricular pacing. Therefore, when stimuli are applied to specialized fiber recording sites and His bundle pacing results, it is certain that the site of origin of the previously recorded specialized fiber electrogram is the His bundle, but when ventricular pacing results, the site of origin of the previously recorded specialized fiber electrogram could be the distal His bundle or the bundle branches.