Study designs of evaluations included in the review
It would appear that the efficacy studies had to be randomised controlled trials (RCTs), although this was not directly stated as a condition. It was stated that the studies were excluded on the grounds of not being randomised or controlled. Studies had to have 'low to moderate risk of bias' to be included, while studies with self-selected participants or other selection bias were excluded. All of the included efficacy trials were RCTs.
The design of the studies evaluating side-effects were not subject to inclusion criteria. Controlled and uncontrolled trials, observational studies and case reports were eligible. Some observational studies were subsequently excluded on the grounds of bias. Unpublished data were excluded. In addition, some other studies on occupational exposure were reported but not systematically reviewed.
Specific interventions included in the review
The criterion for inclusion was analgesia using N2O. Some studies assessed continuous administration of N2O through a nasal catheter; some assessed intermittent administration, titrated by an anesthesiologist in two studies but with the mode of delivery and control not stated for others. The mean duration of use, where stated, varied from 10 to 137 minutes. The concentrations of N2O varied from 30 to 80%, with the majority of studies using 50%. Four studies compared N2O concentrations (50 to 80%). N2O was also compared with other analgesics, including methoxyflurane, enflurane, isoflurane, desflurane, trichloroethylene and non-inhaled analgesics, including epidurals. Some studies allowed prior and concurrent use of opioids, while others excluded such use.
Participants included in the review
The participants included were women in labour. Three studies only included women in second stage labour, and three only women in first stage when labour was established; the remainder of the studies were in both first and second stage labour, or unspecified.
Outcomes assessed in the review
The outcomes assessed were a measure of efficacy for pain relief or a measure of side-effects on the labour, the mother, and/or the foetus or newborn baby. Studies in which the efficacy assessment was delayed, or the assessment was not by the parturient woman, were excluded. Pain relief was measured by a categorical scale or by a visual analogue score. The side-effects experienced by the women were self-reported after labour, and concerned experiences such as nausea, vomiting, consciousness and memory. Effects on labour were measured by haemodynamic changes, oxygen saturation levels, diffusion hypoxia, intra-uterine pressure and contraction frequency. Effects on the baby were measured using the Neurologic and Adaptive Capacity Score (NACS) and Early Neonatal Neurobehavioral Scale (ENNS) score at points from birth to 24 hours of life, and the 1-minute Apgar scores.
How were decisions on the relevance of primary studies made?
The author does not state how the papers were selected for the review, or how many of the reviewers performed the selection. Since there is only a single author, it would appear that only one person was involved.