Bagliani G, Meniconi L, Raggi F, Corea L. Foligno General Hospital, Italy. Pacing Clin Electrophysiol 1998 Jan;21(1 Pt 1):18-24. Clear atrial depolarizations from inside the esophagus have always been recorded in electrocardiology, but their precise origin is still under discussion. Though atrial signals are recorded along most of the esophagus, pacing of the atria is possible only in a short tract, probably where the esophagus is in contact with the posterior left atrium wall. In order to ascertain which portion of atria gives rise to the esophageal atrial signal recorded in the atrial pacing segment, we examined 37 patients with normal P waves on the standard ECG by inserting esophageal and endocavitary catheters. The interval between the earliest start of the P wave and the bipolar atrial deflection, was measured both through the esophagus (PA-Eso) and the Hisian region (PA-His) (the latest depolarization of interatrial septum). The former was longer than the latter (P < 0.001) in 36 of 37 patients, showing that the esophagus recorded atrial signal, at the site of effective pacing, originates outside the interatrial septum. As the atrial depolarization recorded through the esophagus is significantly delayed compared with the Hisian region recording, a pure left origin of the esophageal signal can be hypothesized. This is supported by the well-known delayed depolarization, during sinus rhythm, of the left atrium posterior wall compared with the right atrium and interatrial septum. Measuring the interval between the standard ECG P wave and atrial depolarization recorded through esophagus in the site of effective pacing, provides a reliable noninvasive estimate of interatrial time conduction.