Impact of Patient Age on Age-Adjusted Minimum Alveolar Concentration Fractions and Prolonged Tracheal Extubations...

Introduction Prolonged time to tracheal extubation after general anesthesia is defined as an interval of 15 minutes or longer from the recorded end of surgery to the actual removal of the tracheal tube. Thereafter, the operating room team is idle, so prolonged extubations reduce workflow and the productivity of anesthesia clinicians. During inhalational anesthesia, the overwhelming majority of prolonged times to tracheal extubation are caused by substantial residual doses at the completion of the surgical procedure. The minimum alveolar concentration (MAC) fraction is the end-tidal inhalational agent concentration divided by the agent's altitude- and age-adjusted minimum alveolar concentration for a 40-year-old. Age adjustment is essential because volatile anesthetic requirements decline markedly with increasing age, meaning that a fixed percentage concentration represents a much higher effective dose in an elderly patient than in a young adult. Previously, the authors showed a substantial positive correlation between the MAC fraction at the end of surgery and prolonged extubations. In the current study, we tested three hypotheses on the associations between MAC fraction for maintenance of anesthesia and patient age, MAC fraction at the end of surgery and patient age, and prolonged extubations and patient age.  Methods Our retrospective cohort study used a subset of the same dataset from the University of Iowa that was used in an earlier investigation of inhalational agent dosing behaviors. The anesthesia machines displayed MAC fractions. The cohort of n = 75,050 cases had general anesthesia, tracheal intubation, a mean MAC fraction > 0.6 throughout the case, and patient age > 30 years. The associations between patient age and MAC fractions were evaluated using Spearman's rank correlations for comparison with research done at another institution. The associations between prolonged extubations and MAC fractions were tested using the Wilcoxon-Mann-Whitney test. Results With care provided using anesthesia machines that display MAC fractions, there was a positive correlation (p < 0.0001) between age and MAC fraction, but an association of tiny magnitude: Spearman's rank correlation 0.021, 95% confidence interval 0.014 to 0.028. The correlation was significantly lower than the previously published rank correlation of 0.12 when machines lacked the capability to display MAC fraction (p < 0.0001). There was no significant association between age and the MAC fraction at the end of surgery (rank correlation: 0.006; 95% confidence interval: -0.001 to 0.013; p = 0.105). There was no significant association between age and prolonged extubation (area under the receiver operating characteristic curve of 0.503; p = 0.28). Conclusions Prolonged times to tracheal extubation are economically important. Our findings demonstrate that when using anesthesia workstations equipped with real-time MAC fraction displays, the confounding effect of patient age on anesthetic maintenance dosing was effectively neutralized. At the completion of surgery, there was no residual correlation between patient age and the remaining depth of anesthesia. Consequently, the incidence of prolonged extubation was independent of patient age, confirming that most delays in emergence were driven by clinician titration behaviors rather than patient demographics.