Age, minimum alveolar concentration and choice of depth of sedation monitor: examining the paradox of age when using the Narcotrend monitor

BACKGROUND With an ageing global population, it is important to individualise titration of anaesthetics according to age and by measuring their effect on the brain. A recent study reported that during general surgery, the given concentration of volatile anaesthetics, expressed as a fraction of the minimum alveolar concentration (MAC fraction), decreases by around only 3% per age-decade, which is less than the 6% expected from age-adjusted MAC. Paradoxically, despite the excessive dosing, Bispectral index (BIS) values also increased. OBJECTIVE We planned to investigate the paradox of age when using the Narcotrend depth of anaesthesia monitor. DESIGN Secondary analyses of a prospective observational study. SETTING Tertiary hospital in Switzerland, recordings took place during 2016 and 2017. PATIENTS One thousand and seventy-two patients undergoing cardiac surgery entered the study, and 909 with noise-free recordings and isoflurane anaesthesia were included in this analysis. INTERVENTION We calculated mean end-tidal MAC fraction and mean index value of the Narcotrend depth of sedation monitor used in the study during the prebypass period. Statistical associations were modelled using linear regression, local weighted regression (LOESS) and a generalised additive model (GAM). MAIN OUTCOME MEASURES Primary endpoints in this study were the change in end-tidal MAC fraction and mean Narcotrend index values, both measured per age-decade. RESULTS We observed a linear decrease in end-tidal MAC fraction of 3.2% per age-decade [95% confidence interval (CI) −3.97% to −2.38%, P < 0.001], consistent with previous findings. In contrast to the BIS, mean Narcotrend index values decreased with age at 3.0 index points per age-decade (95% CI, −3.55 points to −2.36 points, P < 0.001), a direction of change commensurate with the increasing age-adjusted MAC fraction with patient age. These relationships were consistent regardless of whether age-adjusted MAC was displayed on the anaesthetic machine. CONCLUSIONS We caution that the ‘paradox of age’ may in part depend on the choice of depth of sedation monitor. TRIAL REGISTRATION ClinicalTrials.gov Identifier: NCT02976584.