Abstract: We included six NRS with 1,563,820 participants. Five were large retrospective cohort studies using routinely collected hospital or administrative data from the United States (US). The sixth was a smaller cohort study based on emergency medical care in Haiti. Two were restricted to obstetric patients whilst the others included a range of surgical procedures. It was not possible to combine data as there was a degree of heterogeneity between the included studies. Two studies failed to find a difference in the risk of death in women undergoing caesarean section when given anaesthesia by NPAs compared with physician anaesthetists, both working independently. One study reported there was no difference in mortality between independently working provider groups. One compared mortality risks between US states that had, or had not, 'opted-out' of federal insurance requirements for physician anaesthetists to supervise or direct NPAs. This study reported a lower mortality risk for NPAs working independently compared with physician anaesthetists working independently in both 'opt-out' and 'non-opt out' states. One study reported a lower mortality risk for NPAs working independently compared with supervised or directed NPAs. One reported a higher mortality risk for NPAs working independently than in a supervised or directed NPA group but no statistical testing was presented. One reported a lower mortality risk in the NPA group working independently compared with the supervised or directed NPA group in both 'opt-out' and 'non-opt out' states before the 'opt-out' rule was introduced, but a higher mortality risk in 'opt-out' states after the 'opt-out' rule was introduced. One reported only one death and was unable to detect a risk in mortality. One reported that the risk of mortality and failure to rescue was higher for NPAs who were categorized as undirected than for directed NPAs. Three studies reported the risk of anaesthesia-related complications for NPAs working independently compared to physician anaesthetists working independently. Two failed to find a difference in the risk of complications in women undergoing caesarean section. One failed to find a difference in risk of complications between groups in 'non-opt out' states. This study reported a lower risk of complications for NPAs working independently than for physician anaesthetists working independently in 'opt-out' states before the 'opt-out' rule was introduced, but a higher risk after, although these differences were not tested statistically. Two studies reported that the risk of complications was generally lower for NPAs working independently than in the NPA supervised or team group but no statistical testing was reported. One reported no evidence of increased risk of postoperative complications in an undirected NPA group versus a directed NPA group. The risk of bias and assessment of confounders was particularly important for this review. We were concerned about the use of routine data for research and the likely accuracy of such databases to determine the intervention and control groups, thus judging four studies at medium risk of inaccuracy, one at low and one, for which there was insufficient detail, at an unclear risk. Whilst we expected that mortality would have been accurately reported in record systems, we thought reporting may not be as accurate for complications, which relied on the use of codes. Studies were therefore judged as at high risk or an unclear risk of bias for the reporting of complications data. Four of the six studies received funding, which could have influenced the reporting and interpretation of study results. Studies considered confounders of case-mix, co-morbidity and hospital characteristics with varying degrees of detail and again we were concerned about the accuracy of the coding of data in records and the variables considered during assessment. Five of the studies used multivariate logistic regression models to account for these confounders. We judged three as being at low risk, one at medium risk and one at high risk of incomplete adjustment in analysis. No definitive statement can be made about the possible superiority of one type of anaesthesia care over another. The complexity of perioperative care, the low intrinsic rate of complications relating directly to anaesthesia, and the potential confounding effects within the studies reviewed, all of which were non-randomized, make it impossible to provide a definitive answer to the review question.

Plain-language summary: The evidence was current up to 13 February 2013. We found six relevant studies, five of which were large observational studies from the US with a comparison group and with study durations from two to 11 years, and one was a much smaller 12 week study from Haiti. There were over 1.5 million participants in the studies. Information for these studies was taken from American insurance databases (Medicare) and from hospital records. The small study was based on emergency medical care after the 2008 hurricanes in Haiti. Most studies stated that there was no difference in the number of people who died when given anaesthetic by either a nurse anaesthetist or a medically qualified anaesthetist. One study stated that there was a lower rate of death for nurse anaesthetists compared to medically qualified anaesthetists. One study stated that the risk of death was lower for nurse anaesthetists compared to those being supervised by an anaesthetist or working within an anaesthetic team, whilst another stated the risk of death was higher compared to a supervised or team approach. Other studies gave varied results. Similarly, there were variations between studies for the rates of complications for patients depending on their anaesthetic provider. Much of the data came from large databases, which may have contained inaccuracies in reporting. There may also be important differences between patients that might account for variation in study results, for example, whether patients who were more ill were treated by a medically qualified anaesthetist, or whether nurse anaesthetists worked in hospitals that had fewer resources. Several of the studies had allowed for these potential differences in their analysis, however it was unclear to us whether this had been done sufficiently well to allow us to be confident about the results. There was also potential confounding from the funding sources for some of these studies. As none of the data were of sufficiently high quality and the studies presented inconsistent findings, we concluded that it was not possible to say whether there were any differences in care between medically qualified anaesthetists and nurse anaesthetists from the available evidence.