Nociception during surgery

Noxious stimuli, of which signals ascend from nociceptors in the peripheral organs to the brain, evoke changes in autonomic responses via somato-sympathetic and viscero-sympathetic reflexes in addition to changes in the electroencephalogram. Surgical invasion induces nociception, and anesthetics suppress it, namely antinociception. Although pain is not recognized under general anesthesia, nociception during surgery induces these responses even under general anesthesia. Nociception monitoring estimates the balance between nociception and antinociception by using variables evoked by autonomic responses or in the electroencephalogram during noxious stimuli. The nociceptive response (NR), which includes the variables of heart rate, systolic blood pressure, and perfusion index, is increased by nociceptive stimuli, and decreased by anesthetics during general anesthesia. Higher levels of the averaged values of NR from the start to end of surgery (mean NR) are associated with postoperative complications. The usefulness of monitoring nociception in the awake state is, however, controversial.