P waves during ectopic atrial rhythms in man: a study utilizing atrial pacing with fixed electrodes.

Threshold bipolar pacing was performed from one of 12 selected atrial sites with temporary implanted electrodes in 69 patients following open-heart surgery in order to study P wave polarity and morphology and the P-R interval during paced ectopic atrial rhythms. A negative P wave was recorded in lead I only with pacing the left atrium and only when pacing near the left pulmonary veins. A positive bifid P wave in V1 was recorded only with left atrial pacing and only when pacing was near the inferior pulmonary veins and coronary sinus. P wave polarity and morphology were otherwise of no use in localization of the origin of the impulse in these studies. The pacing stimulus to P wave interval was found to vary between 10 and 54 msec, making the duration of the P-R interval an unreliable indicator of the site of origin of the paced impulse. Although the relation of these paced rhythms to spontaneously occurring ectopic rhythms is unclear, the previously published criteria for localizing ectopic atrial rhythms ...