Myocardial stimulation: the electrochemistry of electrode-tissue coupling

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Abstract
Experiments have been performed on dog and cat myocardium to investigate the electrochemical phenomena which occur during electrical stimulation with extracellular metal electrodes. These studies have shown that only a portion of the pulse energy delivered to a tissue-electrode system is available to initiate tissue depolarization. The remaining energy is dissipated in overcoming electrochemical impedances at the electrode-tissue interface which are not directly associated with tissue depolarization. In regions where depolarization occurs, tissue contributes only ohmic impedances to the total impedance in the stimulation circuit. The resultant ohmic potentials are identical in instantaneous contour to the current in the system. The amplitude, time course, and distribution of these potentials depend on the pulse current density, the distribution of volume resistivity in the tissue medium, the distance away from the electrode surface, and the type of stimulus-pulse source. These ohmic potentials are constant during a constant-current pulse, but decay as the current decays during a constant-voltage pulse. Because of this decay, applied voltage is not a precise measure of tissue stimulation thresholds.

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