Bispectral index (BIS) monitor, a widely used device that employs brainwaves to help doctors prevent patients from waking up during surgery is no more effective than an older, far less costly technique, a new study reveals.

Michael Avidan of Washington University School of Medicine in St. Louis and colleagues discovered that the BIS device, made by Aspect Medical Systems Inc., and used in about 60 percent of U.S. operating rooms, did not help doctors prevent any more patients from waking up while under inhaled anesthesia.

“Our findings do not support routine BIS monitoring as part of standard practice. Reliance on BIS technology may provide patients and health care practitioners with a false sense od security about the reduction in the risk of anesthesia awareness,” Prof. Avidan wrote in the report.

Anesthesia awareness or unintended intraoperative awareness happens rarely – to about one or two out of 1,000 surgery patients - but it does happen. During surgery, patients become aware of things that are happening to them. Patients may feel pain, discomfort, and fear, but they cannot tell anyone what is happening to them as they are paralyzed by anesthesia. The experience is usually painless, and usually is brief, but occasionally it leaves patients deeply traumatized, says Prof. Avidan.

“Anesthesia awareness can be understood as people undergoing general anesthesia and having some sensory stimuli during the procedure – something visual or auditory or painful – which they remember afterward. It is often the case that it is not disturbing, that it is a brief experience, and patient will experience negative psychological consequences and even have posttraumatic stress disorder,” Prof. Avidan told WebMD.

This study is the first major scientific challenge to the monitors, which cost at least $3,000 and require new $16.50 sensors for each procedure. Prof. Avidan enrolled 1,941 patients considered to be at high risk for anesthesia in their randomized study. Risk factors include chronic use of certain drugs and a variety of cardiac and pulmonary problems.

Anesthesia was guided either by bispectral index monitoring (967 patients), targeted to the 40-60 range, or by monitoring end-tidal anesthetic gas concentrations in exhaled breath (974 patients). Based on patients’ answers on whether they had awareness or not, the researchers found that two patients in each group were determined to have had definite anesthesia awareness. The brain monitor group had four additional cases described as “possible” while the second group had one possible case. There was no statistically significant difference between the groups, said lead author Michael S. Avidan.

“The addition of the BIS-guided protocol did not reduce the frequency of definite or possible awarness,” Dr. Beverly Orser of the University of Toronto wrote in a commentary.

Aspect’s medical director, Dr. Scott Kelley did not welcome the study’s findings saying it does not reflect real-world conditions because doctors do not usually measure exhales gases. He also added that Avidan study fails to assess other important benefits of BIS technology, such as comfort and better recovery from anesthesia.

“It is important to emphasize that the results of this trial should not be extrapolated to patients receiving total intravenous anesthesia, which is considered to be a risk factor for anesthesia awareness. Indeed, BIS monitoring may be useful during total intravenous anesthesia, since it is not presently possible to monitor the blood concentrations of anesthetic agents continuously,” Prof. Avidan wrote in the study.

The findings of the study appeared in the March 13 issue of the New England Journal of Medicine.