Defibrillation-shock excitation in a single cardiac fiber

This modeling study examines the morphology of the action potentials generated in diastole by defibrillation-strength extracellular stimulation. A single cardiac strand of length 4 mm is considered. The membrane kinetics is represented by the Luo-Rudy II model. Stimulation is performed with a S1-S2 protocol of 300 ms coupling interval. Shocks of various strengths are delivered, and action potential magnitudes and upstroke velocities are monitored along the fiber. Under the cathode, the calculated S2 action potentials have larger magnitudes and upstroke velocities than paced activation. Under the anode, a multiphasic transmembrane potential change was observed that resembled a puzzling experimental recording by Zhou et al. (1996). A possible explanation of the observed behavior is provided