Bispectral Index May Not Prevent Intraoperative Awareness

Laurie Barclay, MD

August 17, 2011

August 17, 2011 — Unintended intraoperative awareness occurs in more patients receiving general anesthesia with the bispectral index (BIS) protocol vs end-tidal anesthetic-agent concentration (ETAC), according to the results of a prospective, randomized trial reported in the August 18 issue of the New England Journal of Medicine.

"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," write Michael S. Avidan, MB, BCh, from the Department of Anesthesiology, Washington University School of Medicine in St. Louis, Missouri, and colleagues from the BIS or Anesthetic Gas to Reduce Explicit Recall (BAG-RECALL) Research Group. "We tested the hypothesis that a protocol incorporating the electroencephalogram-derived ...BIS is superior to a protocol incorporating standard monitoring of ...ETAC for the prevention of awareness."

In this assessor-blinded trial at 3 medical centers, 6041 patients at high risk for awareness to BIS-guided anesthesia were randomly assigned to receive BIS-guided anesthesia or ETAC-guided anesthesia. For BIS-guided anesthesia, an audible alert was sounded if the BIS value was less than 40 or more than 60, on a scale of 0 to 100, where 0 reflected the suppression of detectable brain electrical activity, and 100 reflected the awake state. For ETAC-guided anesthesia, there was an audible alert if the ETAC was less than 0.7 or more than 1.3 minimal alveolar concentration.

Anesthesia protocols for patients in both groups included structured education and checklists. A one-sided Fisher's exact test was used to evaluate superiority of the BIS protocol. Definite intraoperative awareness was the primary outcome of the study; definite or possible awareness was the prespecified secondary outcome.

Postoperative interviews revealed definite intraoperative awareness in 7 (0.24%) of 2861 patients in the BIS group and in 2 (0.07%) of 2852 patients in the ETAC group (a difference of 0.17 percentage points; 95% confidence interval [CI], −0.03 to 0.38; P = .98). There were 19 cases of definite or possible intraoperative awareness (0.66%) in the BIS group and 8 (0.28%) in the ETAC group (a difference of 0.38 percentage points; 95% CI, 0.03 - 0.74; P = 0.99). Both of these failed to show superiority of the BIS protocol. The amount of anesthesia given and the rate of major postoperative adverse outcomes were similar in both groups.

"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," the study authors write.

Limitations of this study include lack of generalizability to other populations, evaluation of the protocol against only one of many available electroencephalogram-derived monitors, failure to evaluate a protocol based on both BIS and ETAC alerts, and some missing data. In addition, it was difficult to determine if practitioners would become desensitized to either protocol, and unidentified risk factors such as genetic resistance to anesthetic agents could have confounded the results.

Editorial: Disappointing Findings

An accompanying editorial by Gregory Crosby, MD, from the Department of Anesthesiology, Perioperative and Pain Medicine at Brigham and Women's Hospital in Boston, Massachusetts, calls the findings "disappointing but not surprising."

"Monitors are meant to supplement, not supplant, clinical decision making, and depth-of-anesthesia monitors that reduce complex neurobiology to simple numbers are no exception," Dr. Crosby writes. "...Whether these devices add value in this way remains to be seen, but when minding the mind during sedation and general anesthesia, a little insight into how the brain is reacting is apt to be better than none, especially if it challenges historical ways of gauging anesthetic depth and catalyzes the search for something better."

This study was supported by the Foundation for Anesthesia Education and Research, the American Society of Anesthesiologists, the Winnipeg Regional Health Authority, the University of Manitoba Department of Anesthesia, the Department of Anesthesiology at Washington University in St. Louis, and the Department of Anesthesiology at the University of Chicago. The study authors have disclosed no relevant financial relationships.

N Engl J Med. 2011;365:591-600, 600-601.

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