5.10 Adjuvants in anesthesia and postoperative pain
A number of adjuvants have been added to the intrathecal local anaesthetics for
supplementation of intraoperative anaesthesia and postoperative analgesia. They have
advantages as they reduce the dose of local anaesthetic; provide long lasting postoperative
analgesia with reduced incidence of central nervous system depression, motor effects or
hypotension.
Ketamine is an intravenous (IV) anesthetic with analgesic properties in subanesthetic doses,
secondary to its action on the NMDA receptor. Several trials are investigating the use of
low-dose ketamine for managing postoperative pain. The effect of adding ketamine to
morphine for postoperative patient-controlled analgesia (PCA), compared with using
morphine alone was investigated in six trials involving 330 patients. Only one trial in
patients undergoing lumbar microdiscectomy39 showed a beneficial effect of adding
ketamine. There were no analgesic effects of ketamine in the other five studies in patients
undergoing abdominal surgery 41. Adverse effects including vivid dreams, hallucinations,
dysphoria, and disorientation were increased in ketamine-treated patients in two studies.