The resting membrane potential (RMP) determines the availability of the sodium channels. As extracellular K rises, RMP becomes less negative, and this leads to a decrease in the availability of the sodium channels which, in turn, results in a decrease in the sodium inward current (iNa) and a decrease in the maximum rate of rise (dV/dt max) of the action potential upstroke. This is the major factor responsible for the slowing of conduction velocity in the atria and ventricles that is expressed on the body surface electrocardiogram as an increase in the duration of the P wave and the QRS complex. As shown in this graph, a change in RMP from -90 to -65 mV results in a 50% decrease in dV/dt max and the cell becomes virtually inexcitable at RMPs less negative than -55 mV. It is for this reason that patients rarely survive a rise in serum potassium levels to above 10 mM.