The maximum rate of rise of the action potential upstroke (dV/dt max) is at its peak value when the resting membrane potential (RMP) is approximately -75mV and does not increase at more negative values (i.e. from -75 to -90mV). dV/dt max begins to decrease when the RMP becomes less negative than -75mV and conduction velocity does not slow until dV/dt max has fallen by about 25%. This occurs when the RMP is between -65 and -70mV and this corresponds to an extracellular potassium concentration of approximately 7.0mM. This explains why the duration of the P wave and the QRS complex do not increase until the serum K level exceeds 7.0mM.