Abstract: We included 160 studies with 54,109 enrolled participants; 53,713 participants started the studies and 50,034 completed the studies or data analysis (or both). We could not use 115 RCTs in meta-analytic comparisons because they had zero awareness events. We did not merge 27 of the remaining 45 studies because they had excessive clinical and methodological heterogeneity. We pooled the remaining 18 eligible RCTs in meta-analysis. There are 10 studies awaiting classification which we will process when we update the review. The meta-analyses included 18 trials with 36,034 participants. In the analysis of anaesthetic depth monitoring (either Bispectral Index or M-entropy) versus standard clinical and electronic monitoring, there were nine trials with 34,744 participants. The overall event rate was 0.5%. The effect favoured neither anaesthetic depth monitoring nor standard clinical and electronic monitoring, with little precision in the odds ratio (OR) estimate (OR 0.98, 95% confidence interval (CI) 0.59 to 1.62). In a five-study subset of Bispectral Index monitoring versus standard clinical and electronic monitoring, with 34,181 participants, 503 participants gave awareness reports to a blinded, expert panel who adjudicated or judged the outcome for each patient after reviewing the questionnaires: no awareness, possible awareness, or definite awareness. Experts judged 351 patient awareness reports to have no awareness, 87 to have possible awareness, and 65 to have definite awareness. The effect size favoured neither Bispectral Index monitoring nor standard clinical and electronic monitoring, with little precision in the OR estimate for the combination of definite and possible awareness (OR 0.96, 95% CI 0.35 to 2.65). The effect size favoured Bispectral Index monitoring for definite awareness, but with little precision in the OR estimate (OR 0.60, 95% CI 0.13 to 2.75). We performed three smaller meta-analyses of anaesthetic drugs. There were nine studies with 1290 participants. Wakefulness was reduced by ketamine and etomidate compared to thiopental. Wakefulness was more frequent than awareness. Benzodiazepines reduces awareness compared to thiopental, ketamine, and placebo., Also, higher doses of inhaled anaesthetics versus lower doses reduced the risk of awareness. We graded the quality of the evidence as low or very low in the 'Summary of findings' tables for the five comparisons. Most of the secondary outcomes in this review were not reported in the included RCTs. Anaesthetic depth monitors may have similar effects to standard clinical and electrical monitoring on the risk of awareness during surgery. In older studies comparing anaesthetics in a smaller portion of the patient sample, wakefulness occurred more frequently than awareness. Use of etomidate and ketamine lowered the risk of wakefulness compared to thiopental. Benzodiazepines compared to thiopental and ketamine, or higher doses of inhaled anaesthetics versus lower doses, reduced the risk of awareness.

Plain-language summary: We found 160 randomized controlled trials with 54,109 participants. Eighteen studies with 36,034 participants contributed evidence about devices and drugs to prevent premature waking up during surgery. Nine studies compared anaesthetic depth monitoring versus other methods to adjust drugs. Nine studies compared different drugs. There are 10 studies awaiting classification, which we will process when we update the review. In the largest studies of anaesthetic depth monitors (five studies with 31,181 participants) there were 152 participants with possible or definite awareness (recall of surgery events after surgery). The use of anaesthetic depth monitors to adjust drugs during anaesthesia may have similar effects on the risk of awareness when compared with standard clinical and electrical monitoring. Wakefulness is reduced by ketamine and etomidate compared to thiopental. Benzodiazepines reduces awareness compared to thiopental, ketamine, and placebo. Also higher doses of inhaled anaesthetics versus lower doses reduced the risk of awareness. The quality of the evidence was low or very low because the studies the results were not similar across studies, and there were not enough data.