Effect of overdrive pacing on spontaneous automaticity of Purkinje fiber

The more rapid discharge from the sinus node produces overdrive suppression of Purkinje fiber automaticity. Microelectrode recording from a Purkinje fiber at baseline shows a maximum diastolic potential of -77 mV. During overdrive pacing, the membrane becomes hyperpolarized and maximum diastolic potential increases to -85 mV, resulting in part from the accumulation of sodium ions, activation of the sodium-potassium pump, and an increased influx of potassium as sodium is extruded. After the cessation of overdrive pacing, the onset of the next spontaneous action potential is delayed since the slow of phase 4 diastolic depolarization is reduced and the more negative diastolic potential increases in the time required to reach threshold.