Epicardial Conductors Can Lower the Defibrillation Threshold in Rabbit Hearts

During a defibrillation shock, epicardial conductors can introduce anti-stimulatory effects due to lowering of the voltage gradient in myocardial tissue under the conductor and stimulatory effects due to membrane polarization near edges. We hypothesized that increasing the area of conductors increases the defibrillation threshold (DFT), while increasing the amount of stimulatory edge of conductors decreases the DFT. To test this, we measured the DFT in excised rabbit hearts with and without sets of rectangular conductors having 250 or 500 mm2 area and 100, 200 or 400 mm length of edges perpendicular to the line intersecting the shock electrodes. Unlike previous reports in which conductors increased or did not change DFT, present results indicate a conductor geometry having area of 250 mm2 and edge of 200 mm decreases the DFT. This result is consistent with the hypothesis that stimulatory effects of the edge of a conductor can enhance defibrillation shock efficacy

Stimulatory Current at the Edge of an Inactive Conductor in an Electric Field: Role of Nonlinear Interfacial Current–Voltage Relationship

Cardiac electric field stimulation is critical for the mechanism of defibrillation. The presence of certain inactive epicardial conductors in the field during defibrillation can decrease the defibrillation threshold. We hypothesized this decrease is due to stimulatory effects of current across the interface between the inactive conductor and the heart during field stimulation. To examine this current and its possible stimulatory effects, we imaged transmittance of indium-tin-oxide (ITO) conductors, tested for indium with x-ray diffraction, created a computer model containing realistic ITO interfacial properties, and optically mapped excitation of rabbit heart during electric field stimulation in the presence of an ITO conductor. Reduction of ITO to indium decreased transmittance at the edge facing the anodal shock electrode when trans-interfacial voltage exceeded standard reduction potential. The interfacial current-voltage relationship was nonlinear, producing larger conductances at higher currents. This nonlinearity concentrated the interfacial current near edges in images and in a computer model. The edge current was stimulatory, producing early postshock excitation of rabbit ventricles. Thus, darkening of ITO indicates interfacial current by indium reduction. Interfacial nonlinearity concentrates current near the edge where it can excite the heart. Stimulatory current at edges may account for the reported decrease in defibrillation threshold by inactive conductors