Search

The atria of 37 patients were paced from selected sites during cardiac surgery. When the atria were paced from endocardial sites low in the right atrium, the P waves in ECG leads II, III, and aVF were shown to be either negative, biphasic, or positive, depending on the site paced. When the endocardial sites were paced, the P-R intervals were, almost without exception, less than 0.12 sec. When those endocardial sites closest to the A-V junction were paced, the P-R intervals were always less than 0.12 sec. When the atria were paced, from the epicardial sites, the P-R intervals were always greater than 0.12 sec. Negative P waves in ECG leads II, III, and aVF were recorded when the atria were paced from the postero-inferior left atrium and the caudal right atrium. The P-R interval did not always reflect the initial period of atrial activation because an isoelectric interval, generally of 0.01 to 0.025 sec, was frequently present between the onset of atrial stimulation and the first clear evidence of the P wave in the ECG. The implications of these results are discussed.