To the Editor:
We read with great interest the article “Concurrence of Intraoperative Hypotension, Low Minimum Alveolar Concentration, and Low Bispectral Index Is Associated with Postoperative Death” by Willingham et al.1 This study is based on data from three previous publications by the same group where they determined the incidence of awareness with the use of end-expired gas monitoring and bispectral index.2–4 In the first study, non–age-adjusted minimal alveolar concentration (MAC) was used (n = 2,000),2 while in the last two studies, age-adjusted MAC was used (n = 6,041 and 21,601).3,4 In this study, Willingham et al. converted age-adjusted MAC to non–age-adjusted MAC, and they defined low MAC as less than 0.8. The authors do not give a rationale for doing so, and we think that using non–age-adjusted MAC values may have influenced their conclusions.
Indeed, it is generally accepted that MAC is age dependent: MAC is lower in elderly patients.5–7 Therefore, using non–age-adjusted MAC values underestimates the depth of anesthesia in their elderly patients; thus, these patients may have been overdosed; this is not uncommon even today.8 For example, in an 80-yr-old patient, an end-tidal sevoflurane concentration of 1.6% would be 0.8 non–age-adjusted MAC, but this would be about 1.23 age-adjusted MAC,7 which is simply not a “low MAC.” We are curious whether the conclusions of this study would be the same if age-adjusted MAC values were used. Is it possible that many patients in this study have “a double low (mean arterial pressure and bispectral index) and one high (MAC)” instead of a “triple low”?
The authors declare no competing interests.