In addition, slowing of this pump allows potassium to accumulate in the extracellular space. This accumulated potassium reduces the concentration gradient across the cell membrane which also leads to depolarization. In the first 15 to 20 seconds of ischemia, extracellular potassium can increase to between 10 and 15 mM, and the membrane depolarize by 20-30mV. 2. The upstroke velocity and amplitude of the action potential are directly related to the number of Na-channels actively participating in carrying the action potential current. The fewer the number of these channels the lower the amplitude and slower the upstroke velocity.
Conduction velocity is decreased by reduced upstroke velocity or by reduced amplitude.
Depolarization of the ventricle is reflected in the ECG as the QRS complex. This complex represents the time from depolarization of the first ventricular cell to the last ventricular cell. Thus if conduction velocity is reduced, i.e. if upstroke velocity and amplitude of the action potential are reduced, then the duration of the QRS complex should increase and its amplitude decrease. The latter reflects the decrease in number of actively depolarizing cells per unit time that is intrinsic to reduced conduction velocity through a constant mass of tissue. When this occurs the amplitude of the action potential is decreased - i.e. the cells fail to depolarize fully.