Conduction slows because the rate at which the individual cells depolarizes decreases. This occurs because of the changes in the resting transmembrane potential associated with the rise in extracellular potassium and the effect of this change in resting potential on the sodium inward current, the ionic current primarily responsible for the upstroke of the action potential in atrial and ventricular myocardial cells and in the cells which comprise the bundle branches and the His-Purkinje network. In the example shown here, the increase in extracellular potassium from 4.2 to 8.3mM depolarizes the resting membrane from -85 mV to -67mV. This causes a decrease in the availability of the sodium channels thereby decreasing the sodium inward current and slowing the rate of rise of the action potential upstroke.