Influence of a perfusing bath on the foot of the cardiac action potential

Recently, Spach et al (Circ Res. 1998;83:1144-1164) measured the transmembrane action potential 150 to 200 microm below the tissue surface during longitudinal and transverse propagation. They found that "during longitudinal propagation there was initial slowing of V(m) [action potential] foot that resulted in deviations from a simple exponential. " (p 1144). They attributed this behavior to the effects of capillaries on propagation. The purpose of this commentary is to show that the perfusing bath plays an important role in determining the time course of the action potential foot, even when the transmembrane potential is measured 150 microm below the tissue surface. Using numerical simulations based on the bidomain model, we find that the action potential foot for transverse propagation is nearly exponential (tau(foot)=314 micros). For longitudinal propagation, the action potential foot is not exponential because of an initial slowing (best-fit tau(foot)=483 micros). We conclude that the perfusing bath must be taken into account when interpreting data showing differences in the shape of the action potential foot with propagation direction, even if the transmembrane potential is measured 150 microm below the tissue surface.