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The origin of body surface P waves was evaluated in the intact dog by recording body surface isopotential maps and atrial epicardial potential maps. P waves were recorded from 150 points on the body surface, and 80 atrial unipolar electrograms were recorded by means of permanently implanted atrial electrodes during normal sinus rhythm and during atrial pacing. When the atrial pacing sites were changed, the associated shape changes in body surface P waves were highly dependent on the position of the recording sites. Throughout most of the P wave, multiple maxima were present on the body surface, and these were caused by widely separated right and left atrial excitation waves; however, the presence of several simultaneous atrial excitation waves could not be appreciated from the body surface maps when the atrial excitation waves were close to one another. In the last third of atrial excitation there were two maxima on the body surface, one due to atrial excitation waves and the other due to early atrial repolarization. In contrast to the more complicated patterns during excitation, during the P-R segment a single maximum and a single minimum were present on the body surface. The sequence of potential changes on both body surface and atrial epicardium during repolarization was similar to the sequence of excitation except that there was reversed polarity.

Isopotential Body Surface Maps and Their Relationship to Atrial Potentials in the Dog