Averaging over depth during optical mapping of electroporation

During defibrillation, strong electric shocks applied to the heart can cause electroporation: the formation of large, nonselective pores in the cell membrane. Experiments using optical mapping indicate that electroporation occurs in cardiac tissue when the transmembrane potential, V_m, is significantly less than ±400 mV. Our hypothesis, which we test by numerical simulation, is that V_m is greater than ±400 mV at the tissue surface, but optical mapping underestimates V_m because it averages over depth.