Cardiac activation during external cardiac pacing

To determine which cardiac chambers are stimulated during external cardiac pacing we performed external pacing in eight normal volunteers while recording the esophageal electrogram. All subjects demonstrated evidence of pacing, which was ventricular in each case. The atrium was frequently activated retrogradely during pacing. We conclude that external cardiac pacing selectively stimulates the ventricle and is thus not of value for noninvasive atrial pacing studies.