The relation between extracellular potassium concentration increase and action potential duration decrease is well established. In this work, potassium concentration changes in relation with various activation patterns in an inhomogeneous tissue were in focus and effort was paid to investigate spatial patterns and draw quantitative conclusions about their effects. A series of simulations were performed with different combinations of short stimulus periods and increased extracellular potassium concentrations. The effect of these perturbations on the cellular and overall tissue activation and wave propagation characteristics was investigated.