Prevention of intraoperative awareness in a high-risk surgical population.
This review may be edited
Posted By: Gordon S. Doig
Posted Date: 20 Nov 2013
Title: Prevention of intraoperative awareness in a high-risk surgical population.
Authors: Avidan MS, Jacobsohn E, Glick D, Burnside BA, Zhang L, Villafranca A, Karl L, Kamal S, Torres B, O'Connor M, Evers AS, Gradwohl S, Lin N, Palanca BJ, Mashour GA; BAG-RECALL Research Group.
Reference: N Engl J Med. 2011 Aug 18;365(7):591-600. doi: 10.1056/NEJMoa1100403.
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BACKGROUND: Unintended intraoperative awareness, which occurs when general anesthesia is not achieved or maintained, affects up to 1% of patients at high risk for this complication. We tested the hypothesis that a protocol incorporating the electroencephalogram-derived bispectral index (BIS) is superior to a protocol incorporating standard monitoring of end-tidal anesthetic-agent concentration (ETAC) for the prevention of awareness.
METHODS: We conducted a prospective, randomized, evaluator-blinded trial at three medical centers. We randomly assigned 6041 patients at high risk for awareness to BIS-guided anesthesia (with an audible alert if the BIS value was <40 or >60, on a scale of 0 to 100, with 0 indicating the suppression of detectable brain electrical activity and 100 indicating the awake state) or ETAC-guided anesthesia (with an audible alert if the ETAC was <0.7 or >1.3 minimum alveolar concentration). In addition to audible alerts, the protocols included structured education and checklists. Superiority of the BIS protocol was assessed with the use of a one-sided Fisher's exact test.
RESULTS: A total of 7 of 2861 patients (0.24%) in the BIS group, as compared with 2 of 2852 (0.07%) in the ETAC group, who were interviewed postoperatively had definite intraoperative awareness (a difference of 0.17 percentage points; 95% confidence interval [CI], -0.03 to 0.38; P=0.98). Thus, the superiority of the BIS protocol was not demonstrated. A total of 19 cases of definite or possible intraoperative awareness (0.66%) occurred in the BIS group, as compared with 8 (0.28%) in the ETAC group (a difference of 0.38 percentage points; 95% CI, 0.03 to 0.74; P=0.99), with the superiority of the BIS protocol again not demonstrated. There was no difference between the groups with respect to the amount of anesthesia administered or the rate of major postoperative adverse outcomes.
CONCLUSIONS: The superiority of the BIS protocol was not established; contrary to expectations, fewer patients in the ETAC group than in the BIS group experienced awareness. (Funded by the Foundation for Anesthesia Education and Research and others; BAG-RECALL number, NCT00682825.).
Are the Results Valid?
1. Was the assignment of patients to treatments randomized? ( Was allocation concealment maintained?)
Yes, allocation concealment was maintained using sealed opaque numbered envelopes.
2. Were all patients who entered the trial properly accounted for and attributed at its conclusion?
2a. Was followup complete?
Figure 1 presents a complete patient flow diagram. Some patients were excluded from analysis after finding out they did not meet inclusion criteria or after 'technical difficulties'. If these patients received ANY attempts at BIS monitoring or ETAC, their outcomes should have been reported. Given outcome events are extremly rare, even a small number of exclusions could have influenced conclusions.
2b. Were patients analyzed in the groups to which they were randomized?
Yes, there were no 'cross-overs'.
3. Were patients, health workers, and study personnel blind to treatment?
Outcome adjudicators and patients were blinded.
4. Were the groups similar at the start of the trial?
This is a large scale trial with 6,100 patients. Table 1 demonstrates reasonable balance.
5. Aside from the experimental intervention, were the groups treated equally?
What are the Results?
1. How large was the treatment effect?
The authors reported designing this trial with an interest in using 1-tailed significance testing to investigate ONLY benefit from BIS. As the results from this trial demonstrate, this 1-tailed hypothesis is untenable as there was a strong sign of increased awareness with BIS. I cannot duplicate the authors '1-tailed p-values reported in Table 3. The p-values and confidence intervals reported below are re-calculated in SAS using Exact methods:
Primary outcome: definite awareness
Exact 2-tailed p
Exact difference (95%CI)
0.17 (-0.03 to 0.38)
Secondary outcome: definite or possible awareness
Exact 2-tailed p
Exact difference (95%CI)
0.38(0.03 to 0.74)
2. How precise was the estimate of the treatment effect?
See Exact 95% confidence intervals, reported above.
Will the Results Help Me In Caring For My Patients?
1. Can the results be applied to my patient care?
Yes, it is relative easy to set alarms for End Tidal Anesthetic-agent Concentration monitoring.
2. Were all clinically important outcomes considered?
No. The impact of awareness on patients anxiety or quality of life was not reported. Costs are not reported.
3. Are the likely treatment benefits worth the potential harms and costs?
There is no evidence of benefit from BIS. Interpretation of two-sided Exact 95% CIs suggest awareness may be higher with BIS.

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Prevention of intraoperative awareness in a high-risk surgical population.

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