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Methodology, human factors, and incidence of intraoperative awareness

Sir, We wish to comment on the article by Xu et al. recently published in Acta Anaesthesiologica Scandinavica reporting the incidence of intraoperative awareness with recall (AWR) during general anesthesia in adults in China. We congratulate the authors for successfully completing such a well-designed study with a very large number of patients recruited. Their results showed an incidence of awareness of 0.41%. In the Discussion, the authors state that this incidence was three times higher than that generally quoted from studies performed in Western countries and they discuss possible reasons for the observed differences. In a similar study by one of the authors of this letter, an incidence of awareness of 0.6% in elective surgery and 0.8% for all surgeries was observed. This incidence is similar to that observed by Xu et al. Important factors that may explain the differences in the reported incidences are: (a) the definition of awareness used, (b) the study methodology including the number of patient interviews and especially the time elapsed after surgery when the patients were interviewed, (c) the characteristics of the patients including whether elective or emergency, and (d) the number of patients evaluated. We examined studies with sufficient sample sizes and analyzed the effect of including (or not) cases of possible awareness on the reported AWR incidence. We also examined the definitions used in each of these studies in order to evaluate whether these agree, or if this was not the case, whether that could have influenced the incidence of awareness. In Xu et al., the incidence of awareness increases from 0.41% to 0.88% if possible awareness cases are included. In most of the awareness studies in our analysis, the actual incidence increases at least by a factor of two to three if the definitions used by Xu et al. or in Errando et al. are used (Table 1). Thus, there is more consistency in the literature on careful examination and reanalysis than apparent at first.