Re-entrant excitation initiated in models of inhomogeneous atrial tissue

We demonstrate that a shift of the vulnerable window caused by tissue inhomogeneity can play a role in the generation of re-entrant excitation. The Earm-Hilgemann-Noble equations were incorporated into one- and two-dimensional inhomogeneous partial differential equation models of atrial tissue. Inhomogeneity was produced by a reduction of gNa over part of the medium and the vulnerable window for initiating re-entrant activity in homogeneous models determined from numerical integrations. Forty percent reduction of gNa produced little effect on the width of the vulnerable window, but the onset of the vulnerable window was delayed. The delay of the vulnerable window facilitates the initiation of re-entry at junctions between tissue with normal and reduced excitability, even though there is hardly any change in action potential duration.