Correlation between bispectral index and age-adjusted minimal alveolar concentration

Abstract

Editor—We read with interest the article of Ni and colleagues1Ni K. Cooter M. Gupta D.K. et al.Paradox of age: older patients receive higher age-adjusted minimum alveolar concentration fractions of volatile anaesthetics yet display higher bispectral index values.Br J Anaesth. 2019; 123: 288-297Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar on age-dependent changes of minimum alveolar concentration (MAC) of volatile anaesthetics and bispectral index (BIS™, Medtronic, Minneapolis, MN, USA). The analysis is based on automatically recorded data recorded with the EPIC (Verona, WI, USA) data management system. The data set is of a respectable size and represents and reports the clinical use of volatile anaesthetics together with BIS in the authors' institution. Although the main finding that the age-dependent decrease of end-tidal expiratory concentration per decade in the BIS monitored patients is different from the decrease predicted by age-adjusted MAC is interesting, we believe that this is not sufficient evidence that the ‘BIS algorithm is inaccurate in older patients’ as stated in the conclusion. Firstly, it is relevant to understand why the authors assume that MAC (as a measure drug effect) and BIS should decrease proportionally with age. MAC as a measure of suppression of motor response is quite different from the processed EEG as a measure of cortical drug effect. Although it cannot be excluded that the age effects of the two measures are linearly correlated, we are not aware of any evidence for this. Secondly, in the publication by Ni and colleagues,1Ni K. Cooter M. Gupta D.K. et al.Paradox of age: older patients receive higher age-adjusted minimum alveolar concentration fractions of volatile anaesthetics yet display higher bispectral index values.Br J Anaesth. 2019; 123: 288-297Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar it looks like a substantial fraction of the patients had BIS values below the recommended value of 40 (Figure 2b in that article); that is, they appeared to have been ‘overdosed’. Visually, this fraction of patients with BIS below 40 is even bigger than the 35.7% reported recently by Gross and colleagues2Gross T. Feliot E. Gayat E. et al.Bispectral index during maintenance of total intravenous anesthesia: frequency of out of recommended range and impact of patients’ characteristics: a brief report.Anesth Analg. 2019; 123: 804-811Google Scholar during robotic surgery. Although the 3% difference in age-dependent decrease of end-tidal expiratory concentration per decade offers some opportunity to improve dosing in the elderly, this should be put into perspective with the number of patients where BIS indicated that they were overdosed. At least in the reported cohort, BIS could have been helpful in preventing anaesthetic overdosing. Thirdly, when the discussion about a possible relationship between anaesthetic depth and outcome began years ago, it was based on BIS as a measure of drug effect. As of 2016,3Leslie K. Short T.G. Anesthetic depth and long-term survival: an update.Can J Anaesth. 2016; 63: 233-240Crossref PubMed Scopus (27) Google Scholar none of the data were referring to the relationship between long-term survival and MAC as the measure of drug effect. In urging caution about overdosing, the authors also cite a study based on BIS.4Chan M.T.V. Cheng B.C.P. Lee T.M.C. Gin T. CODA Trial GroupBIS-guided anesthesia decreases postoperative delirium and cognitive decline.J Neurosurg Anesthesiol. 2013; 25: 33-42Crossref PubMed Scopus (464) Google Scholar We do not know of any evidence suggesting that when anaesthetics are titrated to the recommended BIS range, they would ‘suffer from adverse postoperative events’ because the concentrations were higher than predicted by age-adjusted MAC. We congratulate the authors for their interesting report based on real world data and believe that important insight into clinical anaesthesia can be gained from such data. Maybe the results rather indicate that the ‘formula’ for age-adjusted MAC needs correction (see accompanying paper by Cooter and colleagues in this issue5Cooter M. Ni K. Thomas J. et al.Age-dependent decrease in minimum alveolar concentration of inhaled anaesthetics: a systematic search of published studies and meta-regression analysis.Br J Anaesth. 2019; 124: e4-e7Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar), and that anaesthetists do not make as much use of the processed EEG information as they could or should. This is an interesting finding in itself; to quote Amerigo Vespuci (1454–1512) ‘Let it be said in a whisper, experience is certainly worth more than theory.’ The authors declare that they have no conflicts of interest. Response to ‘Correlation between bispectral index and age-adjusted minimal alveolar concentration’ (Br J Anaesth 2020; 124:e8)British Journal of AnaesthesiaVol. 124Issue 3PreviewEditor—We thank Schnider and colleagues1 for their interest and thoughtful discussion of our recent papers, and we appreciate this opportunity to comment on four specific points they raised. Full-Text PDF Open Archive