How were the studies combined?
The odds ratio (OR) of individual trials, pooled ORs, and 95% confidence intervals (CIs) were calculated using a fixed-effect model (see Other Publications of Related Interest no.1). The relative risk was obtained from the pooled ORs, and the risk reduction was calculated.
How were differences between studies investigated?
A chi-squared test of heterogeneity was calculated. Subgroup analysis was carried out on the following groups:
studies confined to adult women;
paediatric patients;
isoflurane or enflurane anaesthetics;
intravenous anaesthesia, with nitrous oxide being used in the nitrous oxide group and no other anaesthetic gas or vapour being used in the comparison group;
studies performed exclusively on patients having laparoscopic surgery;
studies on patients having intra-abdominal surgery; and
studies performed on adult patients having dental, general surgical, neurosurgical and miscellaneous procedures.