Nitrous oxide added at the end of isoflurane anesthesia hastens early recovery without increasing the risk for postoperative nausea and vomiting: a randomized clinical trial

Nitrous oxide (N₂O) has been reported to increase the risk of postoperative nausea and vomiting (PONV) in a dose-dependent manner. We investigated the effect of adding N₂O at the end of isoflurane inhalational anesthesia on the recovery and incidence of PONV. Our hypothesis was that N₂O would reduce the time to early recovery without increasing the incidence of PONV.

After obtaining ethics committee approval and written informed consent, 100 women at American Society of Anesthesiologists physical status I-III and scheduled for laparoscopic-assisted vaginal hysterectomy were randomized into two groups (G) according to the carrier gas: GO₂ (air in 30% oxygen) and GN₂O (the same mixture until the last 30 min of surgery, when 70% N₂O in 30% oxygen was used). No PONV prophylaxis was given. Anesthesia was induced with thiopental 5 mg·kg⁻¹, vecuronium 0.1 mg·kg⁻¹, and fentanyl 1-2 μg·kg⁻¹ iv and maintained with isoflurane. Indicators of early recovery (time to extubation, eye opening, following commands, orientation) were assessed by an anesthesiologist unaware of the group assignment. The incidence and severity of PONV was measured at two and 24 hr postoperatively.

Altogether, 82 participants completed the study (42 in GO₂, 40 in GN₂O) and were analyzed. The mean (SD) time of N₂O administration in GN₂O patients was 27.1 (10.1) min. The mean (SD) time to extubation was faster in GN₂O patients [5.4 (2.9) min] than in GO₂ patients [7.5 (3.7) min] (mean difference, 2.0 min; 95% confidence interval [CI], 0.6 to 3.4, P = 0.009). The ability to open eyes, follow commands, and being oriented were all faster in GN₂O patients than in GO₂ patients (differences of 3.9 min, 95% CI, 1.6 to 6.1, P = 0.001; 3.4 min, 95% CI, 1.0 to 5.7, P = 0.006; 3.8 min, 95% CI, 0.9 to 6.7, P = 0.010, respectively). The incidence of PONV was not different between the groups, but the rescue antiemetic was required less often in the GN₂O patients (mean difference in metoclopramide dose between the GN₂O and GO₂ groups, 5.1 mg; 95% CI, 0.8 to 9.4, P = 0.019).

Adding N₂O during the last 30 min of an isoflurane-based inhalational anesthetic reduced the time to extubation, eye opening, and orientation.