Acute opioid tolerance - Intraoperative remifentanil increases postoperative pain and morphine requirement

Background: Rapid development of acute opioid tolerance Is well established in animals and is more likely to occur with large doses of short-acting drugs. The authors therefore tested the hypothesis that intraoperative remifentanil administration results in acute opioid tolerance that is manifested by increased postoperative pain and opioid requirement. Methods: Fifty adult patients undergoing major abdominal surgery were randomly assigned to two anesthetic regimens: (1) desflurane was kept constant at 0.5 minimum alveolar concentrations and a remifentanil Infusion was titrated to autonomic responses (remifentanil group); or (2) remifentanil at 0.1 mu g . kg(-1) . min(-1) and desflurane titrated to autonomic responses (desflurane group). All patients were given a bolus of 0.15 mg/kg morphine 40 min before the end of surgery. Morphine was initially titrated to need by postanesthesia care nurses blinded to group assignment. Subsequently, patients who were also blinded to group assignment-controlled their own morphine administration, Pain scores and morphine consumption were recorded for 24 postoperative h. Results: The mean remifentanil infusion rate was 0.3 +/- 0.2 mu g . kg(-1) . min(-1) in the remifentanil group, which was significantly greater than in the desflurane group. Intraoperative hemodynamic responses mere similar in each group. Postoperative pain scores were significantly greater in the remifentanil group. These patients required morphine significantly earlier than those in the desflurane group and needed nearly twice as much morphine in the first 24 postoperative h: 59 mg (25-75% Interquartile range, 43-71) versus 32 mg (25-75% interquartile range, 19-59; P < 0.01). Conclusion: Relatively large-dose intraoperative remifentanil increased postoperative pain and morphine consumption. These data suggest that remifentanil causes acute opioid tolerance and hyperalgesia.