Propagation versus delayed activation during field stimulation of cardiac muscle.

This modeling study seeks to explain the experimentally detected delay between the application of an electric field and the recorded response of the transmembrane potential. In this experiment, conditions were deliberately set so that the field should excite all cells at once and so that no delay should be caused by a propagating wave front. The explanation of the observed delay may lie in the intrinsic properties of the membrane. To test this hypothesis, the strength latency curves were determined for three cases: (1) for a membrane patch model, in which the membrane is uniformly polarized and its intrinsic properties can be studied; (2) for the cardiac strand directly excited by the electric field; and (3) for the cardiac strand excited by a propagating wave front. The models of the membrane patch and the directly excited strand yield excitation delays that are comparable to those observed experimentally in magnitude and in the overall shape of the strength latency curves. The delays resulting from propagation are, in general, dependent on the position along the strand, although for some positions the strength latency curves for propagation are similar to those obtained from the directly activated strand and from the patch model. Therefore, the delay in excitation does not necessarily imply the presence of propagating wave fronts and can be attributed to intrinsic membrane kinetics.