(1) Physiology. Within cardiac tissue, the period of time directly following depolarization, during which a stimulus, regardless of strength, cannot initiate a new depolarization. This period of time constitutes the period for tissue repolarization. The refractory period of ventricular tissue starts at the beginning of the QRS complex and ends at approximately the peak of the T wave. The refractory period for atrial tissue starts at the beginning of the P wave and ends approximately 200 ms later. (2) Devices. For a pacemaker, the time period directly following a sensed or paced event during which all activity is ignored by the sense amplifier. This period is immediately followed by the relative refractory period. The total refractory period (absolute plus relative) is typically an independently programmable value for the atrial and ventricular channels (in dual-chamber pacemakers).