This review found that avoiding the intra-operative use of nitrous oxide reduced the risk of postoperative nausea and vomiting, especially in women, but that the overall impact was modest. These conclusions are supported by the data, but should be interpreted with some caution due to the unclear quality of the included trials.

To compare the incidence of postoperative nausea and vomiting in adults following general anaesthesia with or without nitrous oxide.

MEDLINE, EMBASE and Cochrane Central Register of Controlled Trials (CENTRAL) were searched in June 2009 for studies published in English, French or Spanish. Search terms were reported.

Randomised controlled trials that compared anaesthesia with and without nitrous oxide in adults (18 years or over), and reported data on postoperative nausea and vomiting, were eligible for inclusion. Trials in which nitrous oxide was used as a sedative or analgesic agent were excluded, as were trial with no cases of nausea and vomiting in any group.

Included trials compared the non-nitrous oxide anaesthetics alfentanil, desflurane, enflurane, fentanyl, isoflurane, ketamine, morphine, propofol, sevoflurane and sufentanil (alone or in varying combinations) with the same anaesthetic combined with nitrous oxide. Types of surgery included oral, gynaecological, laparoscopy, general, neurological, vascular, orthopaedic, and ear-nose-throat.

Two reviewers independently assessed studies for inclusion; disagreements were resolved through discussion.

Trial quality was not formally assessed.

Two reviewers independently extracted data to calculate relative risks (RR) and odds ratios (ORs) together with 95% confidence intervals (CIs); disagreements were resolved through discussion.

Summary relative risks and odds ratios, together with 95% confidence intervals, were estimated using the DerSimonian and Laird random-effects model. The number needed to treat (NNT) was also calculated. Heterogeneity was assessed using the X² and I² statistics.

Sensitivity analysis was used to investigate the influence of individual studies on summary estimates. Subgroup analysis was used to assess the influence of gender, use of remifentanil as an alternative to nitrous oxide, propofol alone for maintenance compared with propofol plus nitrous oxide, laparoscopic surgery, and date of publication.

Publication bias was assessed using funnel plots and the tests described by Harbord, and by Begg and Mazumdar.

Thirty-three trials reported in 30 publications were included in the review (n=4,598 patients).

Anaesthetic alone significantly reduced postoperative nausea and vomiting (RR 0.80, 95% CI 0.71 to 0.90) compared with anaesthetic plus nitrous oxide. The number of patients needed to treat with a non-nitrous oxide anaesthetic to result in one fewer case of post-operative nausea and vomiting was 17 (95% CI 12 to 31). There was significant heterogeneity (p<0.05). The exclusion of two trials reduced the heterogeneity, but had no effect on summary estimates.

Subgroup analysis showed similar results (RRs all <1) suggesting reduced post-operative nausea and vomiting; this was statistically significant in women.

There was no evidence of publication bias.

Avoiding nitrous oxide reduced the risk of postoperative nausea and vomiting, especially in women, but the overall impact was modest.

The review addressed a clear question and inclusion criteria were defined. The literature search only involved three databases with no apparent attempts to access other data sources or screen bibliographies to locate relevant studies. The review was restricted to studies published in three languages, so there was a possibility of language and publication bias. Publication bias was assessed in the review and no evidence was found. Appropriate steps were taken to minimise bias and errors when selecting studies and extracting data.

Trial quality was not formally assessed, so the reliability of the included trials was unclear. Methods used to pool data were appropriate and included the assessment and investigation of heterogeneity.

The authors' conclusions are supported by the data, but should be interpreted with some caution due to the unclear quality of the included trials.

Practice: The authors did not state any implications for practice.

Research: The authors stated that future studies should try and determine those groups of patients in whom the omission of nitrous oxide may be most beneficial.

Not stated.

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.