Unidirectional Block in Cardiac Fibers; Effects of Discontinuities in Coupling Resistance and Spatial Changes in Resting Membrane Potential in a Computer Simulation Study

The mechanisms for conduction and unidirectional block (UDB) in cardiac tissue under spatial changes in cell-to-cell coupling resistivity (R,) and resting potential (Vrest) were studied. Cable theory was used to simulate the cardiac fiber, and the Beeler and Reuter [1] model, or a modified model based on the Ebihara-Johnson [2] formulation was used to describe the ionic currents. The effects of discontinuities in Ri as would result from collagenous or fibrotic tissue on propagation characteristics were studied. We were especially interested in the effects on propagation characteristics of discontinuities in Ri in the border zone between normal and ischemic tissue. We found that conduction block is more likely to occur when an abrupt decrease in Ri is encountered as compared to an abrupt increase in Ri. Discontinuities in Ri were found to cause changes in propagation characteristics, changing regions of bidirectional block to UDB or bidirectional propagation. Spatial changes in Vrest were also studied. We found that when alone was altered, block was not likely to occur, while discontinuities in Ri superimposed with gradients increased the likelihood of block. We also found that Ri discontinuities located in the border zone between normal and ischemic tissue can create exit block or propagation of a parasystolic focus.