To determine whether results from studies investigating memory for events during anaesthesia provide any support for the view that positive suggestions and specific information presented during anaesthesia can lead to significant post-operative effects.

The following were reviewed: reference lists of three recent reviews, the reference lists of all cited studies in these reviews, all papers and reference lists published in the proceedings from two international symposia on memory and awareness in anaesthesia and all papers presented at the Third International Symposium on Memory and Awareness in Anaesthesia, held in 1995. The PsycLIT and MEDLINE databases were searched.

Study designs of evaluations included in the review

The included studies had to fulfil the following criteria: the study had to include one or more of the four selected outcome measures; the study had to have reported sufficient data to estimate an effect size or the investigators had to make the data available in reply to the authors' request.

Specific interventions included in the review

The intervention studied was positive suggestion and specific information presented during general anaesthesia supervised by a trained anaesthesiologist.

Participants included in the review

The participants were adult surgical patients. Excluded were non-surgical volunteers and patients undergoing cardiac surgery.

Outcomes assessed in the review

The following outcomes were assessed: duration of post-operative hospitalisation, morphine administered via patient-controlled analgesia systems, direct tests of memory and indirect tests of memory over three time periods after anaesthesia (less than 12 hours, 12 to 36 hours and more than 36 hours).

How were decisions on the relevance of primary studies made?

The authors do not state how the papers were selected for the review, or how many of the authors performed the selection.

The authors do not state that they assessed validity.

The methods used to extract data are not stated. An effect size estimate for each outcome reported was calculated for each study using the product-moment correlation coefficient (r) as the effect size estimate. The effect size indicated the degree of association between a manipulated variable and a measure of memory. The effect sizes were calculated from the reported means and standard deviations or from the reported statistics used to compare the relevant conditions in the experiments. Where multiple versions of the same type of test were reported, the average of the individual effect sizes was estimated.

How were the studies combined?

A composite effect size across studies was calculated after weighting individual studies by the sample size. Significance levels of the composite effect size was calculated using the Stouffer method described by Rosenthal (see Other Publications of Related Interest). A fail-safe N was computed for each significant combined effect size.

How were differences between studies investigated?

The authors do not state how differences between the studies were investigated.

Forty-four studies were included (2,517 patients).

Positive suggestions: 14 studies were used to assess the effect on duration of hospitalisation (1,092 patients); 4 studies were used to assess the effect on patient-controlled analgesia (234 patients).

Memory for specific information: less than 12 hours: 11 studies were used to assess the effect of anaesthesia using indirect tests (548 patients), one study was used to assess the effect of anaesthesia using direct tests (160 patients); 12 to 36 hours: 6 studies were used to assess the effect of anaesthesia using indirect tests (270 patients), 5 studies were used to assess the effect of anaesthesia using direct tests (338 patients); more than 36 hours: 7 studies were used to assess the effect of anaesthesia using indirect tests (224 patients), 2 studies were used to assess the effect of anaesthesia using direct tests (72 patients).

Positive suggestions: duration of hospitalisation r = 0.06 (P<0.0934); patient-controlled analgesia r = 0.20 (P<0.0011), fail-safe N = 10.

Memory for specific information: less than 12 hours: indirect tests r = 0.22 (P<0.000001), fail-safe N = 96, direct tests r = 0.24 (P<0.0012), fail-safe N = 2; 12 to 36 hours:indirect tests r = 0.02 (P<0.3105); direct tests r = 0.15 (P<0.0016), fail-safe N = 18; more than 36 hours: indirect tests r = 0.06 (P<0.1711), direct tests r = -0.30 (P<0.3821).

Positive suggestions presented during anaesthesia have little or no effect on post-operative recovery, but specific information is remembered following surgery as long as testing is not delayed longer than 36 hours. Studies of memory for events during anaesthesia provide a useful avenue for exploring unconscious cognition.

Although the inclusion criteria are clearly stated, and several outcomes were assessed, there are a number of limitations to this review. Details of the search strategy used for finding articles in the databases are not stated. Methods used to select studies and extract data are not reported. Details of the individual studies are not given and there is no report of the validity of the included studies. In view of the lack of reporting of results from individual studies it is not possible to assess the heterogeneity among studies. Heterogeneity among trials is not discussed.

Given the lack of detail of included studies, the lack of assessment of validity and lack of mention of heterogeneity among trials, it is not possible to assess the evidence offered on the memory for events during anaesthesia.

Natural Sciences and Engineering Research Council of Canada.

Rosenthal R. Meta-analytic procedures for social research. Revised ed. Newbury Park (CA): Sage Publications; 1991.

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.