The Value of Auditing the Frequency of Inhalational Anesthetics With a Combination of Very Low Bispectral Index and High Fraction of the Age-Adjusted Minimum Alveolar Concentration.

BACKGROUND

Previously, a depth of anesthesia bispectral index (BIS™) <45 was considered lowand found to have no clinical benefit. A BIS <35 was considered very low and was not only without evident clinical benefit but also associated with a greater risk of postoperative delirium. We considered the association between BIS and the anesthetic dose of inhalational agents, quantified using the minimum alveolar concentration (MAC) fraction, which was the patient's end-tidal inhalational agent concentration divided by the agent's altitude- and age-adjusted minimum alveolar percentage concentration. The MAC fraction was displayed on the anesthesia machine. When the MAC fraction >1.0 and the BIS <35, it implies that the inhalational anesthetic agent concentration can be reduced without harmful clinical effects. We hypothesized a substantive percentage of cases (>10%) have long (≥15 minutes) periods having the combination of MAC fraction >1.0 and BIS <35.

METHODS

The retrospective database cohort study included N = 8,566 cases from September 13, 2016, to August 12, 2024 that met the following criteria: (1) ≥100 minutes of BIS monitoring (BIS minutes); (2) use of general anesthesia; (3) tracheal intubation and extubation performed in the operating room where the anesthetic was administered; and (4) absence of prone positioning. The latter three were characteristics of studies examining prolonged extubation, defined as ≥15 minutes from the end of surgery. From the = 8,566 cases studied, 1,862,022 BIS minutes were automatically recorded in the electronic health record. Comparisons were made with the matching MAC fractions. A Clopper-Pearson two-sided exact 95% confidence interval was calculated for the planned primary endpoint, the percentage of cases with ≥15 minutes wherein simultaneously the BIS <35 and MAC fraction >1.0. Post hoc, we added a 97.5% confidence interval for the incidence with ≥30 minutes to compare with 10%.

RESULTS

Among the 8,566 cases with ≥100 minutes of BIS monitor use, 29.5% (2,527) had prolonged extubation. There were contemporaneously 152,443 other cases without BIS monitoring. Those other cases had a nearly identical incidence of prolonged extubations (29.3%, 44,675). A total of 375 distinct anesthesia practitioners used the BIS monitor during the 8,566 cases, with each contributing, on average, only 0.27% (standard deviation [SD] = 0.33%) of anesthetic minutes. Among the = 7,031 cases with BIS measured when the MAC fraction ≥0.6, 25% (1,780/7,031) had ≥15 minutes of very low BIS and MAC fraction >1.0. The 95% confidence interval was 24% to 26%. Being considerably larger than 10% ( < 0.0001), post hoc, we repeated the calculations using the threshold of ≥30 minutes of very low BIS and MAC fraction >1.0. The estimated incidence was 15%, with a 97.5% confidence interval of 14% to 16%, also significantly exceeding 10% ( < 0.0001).

CONCLUSIONS

If department practices are such that processed electroencephalographic monitoring (e.g., BIS) is commonly used during general anesthesia to assess anesthetic depth, our results recommend auditing the volatile agent concentration and the corresponding anesthetic depth index, as we did in this study. We recommend that feedback and education regarding suitable titration of anesthetic agent concentrations relative to the index should be provided at the departmental level.