A study of the relationship between Bispectral index and age-adjusted minimum alveolar concentration during the maintenance phase of general anesthesia in elective surgery

Bispectral index (BIS) and minimum alveolar concentration (MAC) are commonly used to monitor the depth of anesthesia. The objective was to study the correlation between BIS and age-adjusted minimum alveolar concentration (aaMAC) during the maintenance phase of anesthesia. The influence of variables affecting BIS and or aaMAC was studied to determine an equation between BIS and aaMAC. This prospective observational study was carried out after institutional ethical approval in adult patients 18–60 years of either sex, ASA I and II posted for elective surgery under general anesthesia. Five minutes after airway management, BIS values and aaMAC equivalents were noted during the maintenance phase of anesthesia. aaMAC and corresponding BIS values were recorded every minute for periods, where the anesthetic agent concentration had remained the same during preceding 5 minutes till the switching off of the anesthetic agent. Age, sex, ASA status, use of nitrous oxide, inhalational agent, dose of midazolam, and opioid used were also recorded. BIS/aaMAC showed an inverse correlation. Increasing age, ASA II status, morphine equivalent >5, and use of nitrous oxide, sevoflurane, or isoflurane were associated with a higher BIS at equivalent aaMAC. Using the exchangeable correlation structure, a generalized estimation equation was obtained as the best predictor. Factors affecting both aaMAC and BIS affect the relationship between the two, and although there are wide variations, BIS and aaMAC can be equated and values of either can be calculated if one is known using a generalized estimates equation.