Standard Beats Anesthesia EEG Technique for Preventing Surgery Awareness

A method of administering general anesthesia while monitoring EEG activity to track the patient’s level of consciousness proved to be no better than the standard anesthesia technique at preventing episodes of patient awareness during surgery, according to a study published in the Aug. 18 issue of the New England Journal of Medicine.

"The overall incidence of awareness was lower than anticipated, suggesting that both protocols were likely to have had efficacy, but the finding of fewer cases [with the standard approach than with the EEG method] was contrary to the expected result," said Dr. Michael S. Avidan of the department of anesthesiology at Washington University, St. Louis, and his associates.

"Notwithstanding major advances in our understanding of consciousness and anesthesia, until we clarify fully the mechanisms and measurement of anesthetic-induced unconsciousness and amnesia, some patients are still likely to have this complication," the researchers noted.

Unintended intraoperative awareness (defined as the experience and explicit recall of sensory perceptions during surgery) is estimated to occur in approximately 1% of at-risk patients, and can lead to posttraumatic stress disorder. Approximately 20,000-40,000 U.S. patients each year are estimated to experience such awareness while under general anesthesia.

Some factors that appear to raise the risk of unintended awareness while under general anesthesia are a history of the condition during previous surgery, aortic stenosis, pulmonary hypertension, end-stage lung disease, anticipation of difficult intubation, poor exercise tolerance, impaired cardiac ejection fraction, excess alcohol intake, and the use of benzodiazepines, opiates, or anticonvulsants.

The current standard method for monitoring intraoperative patient awareness doesn’t measure consciousness itself, but instead ensures that "enough" anesthetic is used by tracking the end-tidal anesthetic-agent concentration (ETAC). Maintaining this at 0.7 MAC (minimum alveolar concentration) or higher is thought to decrease the incidence of awareness during surgery.

An alternative method is the bispectral index (BIS), which uses a single EEG signal from a sensor on the patient’s forehead to calculate brain activity and produces a numerical readout on a scale from 0 (indicating the suppression of all brain activity) to 100 (indicating a fully awake state). Maintaining a target range of 40-60 on this scale is thought to both prevent awareness and allow reductions in the dose of anesthetic.

Two previous clinical trials comparing the two techniques have yielded conflicting results.

Dr. Avidan and his colleagues conducted the BAG-RECALL (BIS or Anesthetic Gas to Reduce Explicit Recall) clinical trial to determine whether the BIS method is superior to standard ETAC in at-risk surgical patients.

They randomly assigned 6,041 adults undergoing elective surgery during a 2-year period at three medical centers in the United States and Canada to one or the other technique. In the BIS group, an alarm sounded when the BIS value exceeded 60 or fell below 40. In the ETAC group, an alarm sounded if the ETAC fell below 0.7 or exceeded 1.3 MAC.

In both study groups, a sign was attached to the anesthesia machines reminding clinicians to check these values and consider whether the patient might be aware. In addition, these values were recorded electronically at 1-minute intervals; manual records were kept of anesthesia; and photographs of trends on the monitors were taken and stored digitally.

There were no important differences between the two groups in doses of sedative, hypnotic, opioid, analgesic, or neuromuscular-blocking drugs given, nor in the amount of anesthetic given; the rate of adverse postoperative outcomes including mortality; the median length of stay in intensive care; or the median length of hospital stay.

Patients’ intraoperative awareness was assessed via a questionnaire within 72 hours after surgery and at 30 days after extubation. A total of 49 patients reported some degree of awareness at some time when they were undergoing surgery, Dr. Avidan and his associates said (N. Engl. J. Med. 2011;365:591-600).

All patients who reported that they had memories of the period between "going to sleep" and "waking up" from anesthesia were further evaluated in more detail and were offered referral to a psychologist for counseling.

Contrary to expectations, there were fewer cases of definite intraoperative awareness in the ETAC group (0.07%) than in the BIS group (0.24%). Similarly, there were fewer cases of possible or definite intraoperative awareness in the ETAC group (0.28%) than in the BIS group (0.66%).

Thus, BIS was not superior in preventing intraoperative awareness.

This study was supported by the Foundation for Anesthesia Education and Research, the American Society of Anesthesiologists, the Winnipeg Regional Health Authority, and the departments of anesthesia at the University of Manitoba, Washington University, and the University of Chicago. The researchers reported no financial conflicts of interest.