Summary: Ischemic ST-segment shifts have been modeled using scalar stationary approximations of the bidomain model. We propose an alternative simplification of the bidomain equations: a linear system modeling the resting potential, to be used in determining ischemic TP shifts. Results of 2D and 3D simulations show that the linear system model is much more accurate than the scalar model. This improved accuracy is important if the model is to be used for solving the inverse problem of determining the size and location of an ischemic region. Furthermore, the model can provide insight into how the resting potential depends on the variations in the extracellular potassium concentration that characterize ischemic regions.