Simulation of the electrocardiographic U wave in heterogeneous myocardium: effect of local junctional resistance

The electrocardiogram (ECG) was reconstructed from extracellular fields generated by action potentials propagating along a 2-cm linear cable comprising 200 myocardial cells coupled through gap junctions (GJs). Intrinsic APD was varied by altering the maximal conductance of the delayed rectifier (G_x). A longer APD was simulated in the M region by a uniform decrease of G_x. APD prolongation in the M region, no matter how great, failed to yield a distinct U wave in the ECG, as expected for a globally homogeneous medium. To account for the sharp APD transitions observed experimentally, the authors imposed a resistive barrier on either side of the M region by increasing GJ resistance to levels 3-30 times normal. Conduction was little affected under these conditions, yet a separate U wave was seen to follow the T wave. U wave amplitude was directly proportional to the size of the M region and displayed sigmoid dependence of the log of the barrier resistance