A model study of intramural dispersion of action potential duration in the canine pulmonary conus

Regional gradients of action potential duration (APD) due to electrophysiological differences between endocardial, midmyocardial, and epicardial myocytes may exist across the ventricular wall. In addition, activation sequence-induced gradients of APD may occur if intramural fiber rotation accelerates or decelerates the depolarization wave front. To investigate relative contributions of regional and activation sequence-induced gradients to intramural APD dispersion, we simulated action potential propagation in two-dimensional models with idealized geometries representing the canine pulmonary conus. Ionic currents for endocardial myocytes were described using the Luo-Rudy membrane equations. Modifications to I(Ks) approximated action potentials of epicardial and midmyocardial cells. Spatial coupling was modeled with a bidomain representation of tissue structure that included unequal anisotropic conductivity ratios. Activation sequence-induced gradients reached 69 ms cm(-1) during a nonuniform activation sequence where the change in orientation between endocardial and epicardial fibers accelerated the depolarization wave front. Regional gradients reached 133 ms cm(-1) at the boundary between endocardial and midmyocardial cells. When regional and activation sequence-induced gradients were oriented in opposite directions, overall APD dispersion decreased. When the gradients were oriented in the same direction, overall dispersion measured as high as 202 ms cm(-1). This gradient exceeded values previously estimated as sufficient to induce cardiac arrhythmia during premature stimulation and suggests that regional and activation sequence-induced gradients increase arrhythmia vulnerability in the presence of other arrhythmogenic conditions.