Nitrous oxide (N2O) reduces postoperative opioid-induced hyperalgesia after remifentanil-propofol anaesthesia in humans

Abstract

Background. The aim of this study was to test if intraoperative administration of N2O during propofol-remifentanil anaesthesia prevented the onset of postoperative opioid-induced hyperalgesia(OIH).

Methods. Fifty adult ASA I-II patients undergoing elective open septorhinoplasty under general anaesthesia were studied. Anaesthesia was with propofol, adjusted to bispectral index (40-50), and remifentanil (0.30 mu g kg(-1) min(-1)). Patients were assigned to one of the two groups: with N2O (70%) and without N2O (100% oxygen). Mechanical pain thresholds were measured before surgery and 2 and 12-18 h after surgery. Pain measurements were performed on the arm using hand-held von Frey filaments. A non-parametric analysis of variance was used in the von Frey data analysis. P<0.05 was considered statistically significant.

Results. Baseline pain thresholds to mechanical stimuli were similar in both groups, with mean values of 69 [95% confidence interval (CI): 50.2, 95.1] g in the group without N2O and 71 (95% CI: 45.7, 112.1) g in the group with N2O. Postoperative pain scores and cumulative morphine consumption were similar between the groups. The analysis revealed a decrease in the threshold value in both groups. However, post hoc comparisons showed that at 12-18 h after surgery, the decrease in mechanical threshold was greater in the group without N2O than the group with N2O (post hoc analysis with Bonferroni's correction, P<0.05).

Conclusions. Intraoperative 70% N2O administration significantly reduced postoperative OIH in patients receiving propofol-remifentanil anaesthesia.