Electrode polarity does not alter the initial ventricular activation sequence during pacing with extracardiac electrodes.

Ventricular epicardial mapping was performed in six closed-chest anesthetized dogs to investigate the cardiac electrical response to external pacing. A right thoracotomy was performed, complete AV block was produced by formaldehyde injected into the AV node and a sock electrode array, comprised of 127 unipolar electrodes, was placed over the ventricles. Isopotential and isochronal epicardial maps were generated by computer from the unipolar electrograms. Unipolar stimulation pulses were applied between pairs of different types of cutaneous (metal, carbon) and esophageal (metal) electrodes, and recordings were performed at maximum lung inflation. Isopotential maps recorded during the stimulation artifacts showed that the epicardial electrical field was little affected by the type of electrode but depended mostly on electrode position. A reproducible and relatively uniform apex-to-base potential gradient was regularly produced with anteroposterior and anteroesophageal electrode configurations. This uniform potential gradient induced ventricular activation sequences that showed interindividual differences. Thus, for each dog, the areas of initial activation observed on the isochronal maps during pacing tended to remain the same (i.e., apical, lateral, and anterior) despite changes in the stimulation protocol. Inverting the polarity of the electrodes did not appreciably change the site of origin of activation (81% remained the same) and activation never originated from the area showing the most negative potential during the stimulation artifact. In conclusion, since electrode polarity does not seem to alter the ventricular activation sequence during cardiac pacing with extracardiac electrodes, the standard nomenclature for cutaneous patches, which defines the negative electrode as the "active" electrode, may have to be revised.