A new far-field cardiac defibrillation mechanism


Introduction. Experimental research activity has recently focused on a promising new method for low-energy defibrillation. Called far-field defibrillation, the method imposes electric field pulses that engage the bulk of the heart tissue, in contrast to other methods that deliver electrical energy locally through implanted electrodes. The effectiveness of this method can potentially depend on the timing of the delivery of the pulses. Here we describe a new mechanism by which these electric field pulses might terminate reentrant waves that operates independently of shock timing. Methods. A three-dimensional finite-difference inonodomain computer simulation, which includes a full ion channel model and resistive gap junction coupling, is run in rectangular domains of different widths, designed to represent heart walls of varying thicknesses. Once a reentrant action potential scroll wave is established in the system, an electric field stimulus is delivered with varying field vector orientations through the imposition of its effect on the domain boundary conditions. Results. We find that, once the surface perpendicular to the scroll wave filament is depolarized by the electric field, termination of the scroll wave always results. Termination is nearly immediate in the case of thin walls (0.5 cm). In thicker walls (e.g., 2.0 cm), interaction of the induced wave with the scroll wave results in an L-shaped filament, which then shrinks and disappears by the same mechanism by which scroll wave rings terminate. Termination thus occurs independently of wall thickness, timing, and electric field orientation, as long as the latter has a normal component sufficient (about 1 V/cm) to elicit a wave. This new mechanism will likely operate alongside other mechanisms, and thus has the potential to lower the defibrillation threshold.