Remifentanil-induced postoperative hyperalgesia and its prevention with small-dose ketamine

Background: Remifentanil-induced secondary hyperalgesia has been documented experimentally in both animals and healthy human volunteers, but never clinically. This study tested the hypotheses that increased pain sensitivity assessed by periincisional allodynia and hyperalgesia can occur after relatively large-dose intraoperative remifentanil and that small-dose ketamine prevents this hyperalgesia. Methods: Seventy-five patients undergoing major abdominal surgery were randomly assigned to receive (1) intraoperative remifentanil at 0.05 mu g (.) kg(-1) (.) min(-1) (small-dose remifentail); (2) intraoperative remifentanil at 0.40 mu g (.) kg(-1) (.) min(-1) (large-dose rentifentanif); or (3) intraoperative remifentanil at 0.40 mu g (.) kg(-1) (.) min(-1) and 0.5 mg/kg ketamine just after the induction, followed by an intraoperative infusion of 5 mu g (.) kg(-1) (.) min(-1) until skin closure and then 2 mu g (.) kg(-1) (.) min(-1) for 48 h (large-dose remifentanil-ketamine). Pain scores and morphine consumption were recorded for 48 postoperative hours. Quantitative sensory tests, peak expiratory flow measures, and cognitive tests were performed at 24 and 48 h. Results: Hyperalgesia to von Frey hair stimulation adjacent to the surgical wound and morphine requirements were larger (P < 0.05) and allodynia to von Frey hair stimulation was greater (P < 0.01) in the large-dose remifentanil group compared with the other two groups, which were comparable. There were no significant differences in pain, pressure pain detection threshold with an algometer, peak flow, cognitive tests, or side effects. Conclusion: A relatively large dose of intraoperative remifentanil triggers postoperative secondary hyperalgesia. Remifentanil-induced hyperalgesia was prevented by small-dose ketamine, implicating an N-methyl-D-aspartate pain-facilitator process.