Roles of Electric Field and Fiber Structure in Cardiac Electric Stimulation

AbstractThis study investigated roles of the variation of extracellular voltage gradient (VG) over space and cardiac fibers in production of transmembrane voltage changes (ΔVm) during shocks. Eleven isolated rabbit hearts were arterially perfused with solution containing Vm-sensitive fluorescent dye (di-4-ANEPPS). The epicardium received shocks from symmetrical or asymmetrical electrodes to produce nominally uniform or nonuniform VGs. Extracellular electric field and ΔVm produced by shocks in the absolute refractory period were measured with electrodes and a laser scanner and were simulated with a bidomain computer model that incorporated the anterior left ventricular epicardial fiber field. Measurements and simulations showed that fibers distorted extracellular voltages and influenced the ΔVm. For both uniform and nonuniform shocks, ΔVm depended primarily on second spatial derivatives of extracellular voltages, whereas the VGs played a smaller role. Thus, 1) fiber structure influences the extracellular electric field and the distribution of ΔVm; 2) the ΔVm depend on second spatial derivatives of extracellular voltage