Alerting thresholds for the prevention of intraoperative awareness with explicit recall: a secondary analysis of the Michigan Awareness Control Study

Abstract

Background: Intraoperative awareness with explicit recall is a potentially devastating complication of surgery that has been attributed to low anaesthetic concentrations in the vast majority of cases. Past studies have proposed the determination of an adequate dose for general anaesthetics that could be used to alert providers of potentially insufficient anaesthesia. However, there have been no systematic analyses of appropriate thresholds to develop population-based alerting algorithms for preventing intraoperative awareness.

Objective: To identify a threshold for intraoperative alerting that could be applied for the prevention of awareness with explicit recall.

Design: Secondary analysis of a randomised controlled trial (Michigan Awareness Control Study).

Setting: Three hospitals at a tertiary care centre in the USA.

Patients: Unselected patients presenting for surgery under general anaesthesia.

Interventions: Alerts based on end-tidal anaesthetic concentration or bispectral index values.

Main outcome measures: Using case and outcomes data from the primary study, end-tidal anaesthetic concentration and bispectral index values were analysed using Youden's index and c-statistics derived from a receiver operating characteristic curve to determine a specific alerting threshold for the prevention of awareness.

Results: No single population-based threshold that maximises sensitivity and specificity could be identified for the prevention of intraoperative awareness, using either anaesthetic concentration or bispectral index values. The c-statistic for anaesthetic concentration was 0.431 ± 0.046, and 0.491 ± 0.056 for bispectral index values.

Conclusion: We could not derive a single population-based alerting threshold for the prevention of intraoperative awareness using either anaesthetic concentration or bispectral index values. These data indicate a need to move towards individualised alerting strategies in the prevention of intraoperative awareness.

Trial registration: Primary trial registration (Michigan Awareness Control Study) ClinicalTrials.gov identifier: NCT00689091.

Conflict of interest statement

Conflicts of interest: G.A.M. has a patent pending (through the University of Michigan) on the use of connectivity measures to assess levels of consciousness.

Figures

Fig. 1. Flow diagram. Parent trial to secondary analysis with outcome of intraoperative awareness with explicit recall (AWR)

MACS, Michigan Awareness Control Study; MAC minimum alveolar concentration; BIS, Bispectral Index.

Fig. 2. Comparison of end-tidal anaesthetic concentration (ETAC) alerting and ‘no intervention’ group behaviour

Figure shows plot depicting the mean percentage of the case to trigger an alert ± 2 × SEM. Quarter refers to the eight 3-month periods over 2 years.

Fig. 3. End-tidal anaesthetic concentration (ETAC) alerting trend analysis

Figure shows plot depicting the mean percentage of the case to trigger an alert ± 2 × SEM.

Fig. 4. Bispectral Index (BIS) alerting trend analysis

Figure shows plot depicting the mean percentage of the case to trigger an alert ± 2 × SEM.