Virtual source separation modifies cardiac tissue response to field stimulation

In recent years, the concept of virtual electrode sources, in locations remote from the stimulating electrodes, has become increasingly important in our understanding of the mechanisms of cardiac defibrillation. One class of virtual electrodes - that of oppositely polarized virtual elecvode pairs - tends to appear in bounded regions of cardiac tissue. We have examined the stimulatory effects of oppositely polarized pairs of virtual electrodes by modeling an active strip of bounded cardiac tissue. To examine the impact which these electrode source pairs have on one another, we characterized field stimulation at several tissue lengths. At large length scales (1.5 cm) we found that field stimulation strongly resembled unipolar current injection. Both the time constant and the threshold for stimulation were consistent with those for point stimulation. However, on short length scales (150 p),fie ld stimulation behaved very differently than point stmulation. Specifically, we found that the close proximity of oppositely polarized sources acted both to increase the stimulation threshold and to decrease the membrane charging time constant, boosting the tissue frequency response.