Brain & Nervous System Health Center

Awake-During-Surgery Device Worth a Nod?

Study Challenges Benefit of BIS, a Brain Monitor That Helps Doctors Detect When Patients Are Waking

From the WebMD Archives

March 12, 2008 --Doctors don't need a widely used brain monitor to cut patients' risk of waking during surgery, a new study suggests.

Doctors call it "unintended intraoperative awareness" or "anesthesia awareness." Patients call it downright scary. It 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.

They may feel pain, discomfort, and fear. Paralyzed by anesthesia, they are unable to tell anyone what is happening to them. The experience usually is painless, and usually is brief. But once in a while it leaves patients deeply traumatized, says anesthesiology professor Michael Avidan, MB, BCh, of Washington University School of Medicine, St. Louis.

"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," Avidan tells WebMD. "It is often the case that it is not disturbing, that it is a brief experience, and patients do not complain of any long-term effects. But a certain proportion of patients will experience negative psychological consequences and even have posttraumatic stress disorder."

Some patients are at 10 times higher risk of anesthesia awareness than other patients. One in 100 of these patients experience a "waking" event. These high-risk patients:

  • Undergo intensive surgeries such as open-heart surgery or lung surgery, or
  • Are very ill with underlying diseases, such as heart conditions, that make it difficult for doctors to induce deep anesthesia, or
  • Frequently use strong painkillers or alcohol.

(Have you been awake or aware during surgery? Share your experience on WebMD's Health Cafe message board.)

Does BIS Brain Monitor Cut Waking During Surgery?

The bispectral index (BIS) monitor is supposed to help doctors know when patients are waking. The device, made by Aspect Medical Systems, uses a simple array of electrodes attached to a patient's forehead to monitor brain waves.

The device then uses a secret, proprietary algorithm to calculate a consciousness-level score. On a 0 to 100 scale, where zero is no brain activity and 100 is full consciousness, patients with a score of 40 to 60 are not supposed to experience anesthesia awareness.


In 2004, a clinical trial suggested that the device cut the risk of anesthesia awareness among high-risk surgery patients. Driven by media reports of patients who experienced waking during surgery -- and by a frightening Hollywood movie, Awake -- Advent's device became widely used. It's now found in about 60% of all U.S. operating rooms and is used in about 17% of surgeries requiring general anesthesia.

But does the device work better than the standard technique tracking the amount of anesthetic gas a patient exhales? Avidan and colleagues tested this in a new clinical trial that enrolled patients with at least one factor that put them at high risk of anesthesia awareness.

The researchers randomly assigned about 1,000 surgical patients to the Aspect BIS monitor, and about 1,000 patients to standard monitoring (except that an alarm was set to go off when exhaled anesthesia levels dropped below optimal levels).

Four patients experienced anesthesia awareness. Two were in the standard monitoring group. Two were in the group that got BIS monitoring.

"I don't think, presently, the evidence would persuade me that use of technologies like BIS monitoring would reduce the likelihood of anesthesia awareness," Avidan says. "I would still use these monitors, but only in the context of clinical trials. We are doing a 6,000-patient study in high-risk patients. And the University of Michigan is doing a 30,000-patient study. It seems strange to me there should be such a widespread change in practice before we have more compelling evidence than only one trial demonstrating benefit."

An editorial by Beverley A. Orser, MD, PhD, professor of anesthesia and physiology at the University of Toronto, accompanies Avidan and colleagues' report in the March 13 issue of The New England Journal of Medicine. Orser says she is not surprised that the BIS monitor was no better than enhanced anesthesia monitoring at preventing waking during surgery.

"I question the ability of this device to prevent anesthesia awareness because I don't think it is targeting the areas of the brain responsible for this activity," Orser tells WebMD. "We had the device at our center and tried it, and our group did not feel it was something of sufficient value to justify the cost. Our decision as a group was this is not validated enough to use for clinical practice."


But Scott D. Kelley, MD, vice president and medical director for Aspect Medical Systems Inc., sees the Avidan study much differently.

Kelley notes that the rate of anesthesia awareness seen in the study was about two per 1,000 patients -- 10 times lower than would be expected for these high-risk patients. This, he says, shows that BIS monitoring does indeed reduce waking during surgery.

"What they used as a control protocol is not standard practice," Kelley tells WebMD. "I went through 40 operating rooms yesterday, and not a single one had one of these exhaled gas alarms turned on. So what the control group received in the Avidan study was not the kind of care the average patient can expect to get. I don't want patients to believe that starting tomorrow they will get same kind of care the control group got in the Avidan study. And are there other consequences to that kind of treatment care? We don't know. Their protocol has not been validated in clinical trials."

Kelley also brushes aside Orser's criticism of what the Aspect BIS monitor measures.

"There are clearly deep structures within the brain where some of these control centers for consciousness exist. But as clinicians trying to take care of a wide number of patients, what is important here is the surface reading of the electroencephalogram," he says. "We have broad clinical experience in more than 25 million patients. This technology being adopted by so many doctors works, and works well, and has patient impact. I believe I, as an anesthesiologist, and my patients benefit greatly from this technology."

What Anesthesiologists Tell Patients About Waking During Surgery

Orser worries that efforts to promote the BIS monitor are unduly alarming patients.

"This worry about anesthesia awareness is really frightening our patients, more than is in their interest," she says. "It is disappointing this device isn't the silver bullet for treating this. But I reassure patients that the incidence of anesthesia awareness is very low."

Avidan advises anesthesiologists to discuss the issue with patients prior to surgery and to urge patients to contact their doctors if they have uncomfortable memories after surgery.


WebMD asked Avidan what he says to patients who tell him they are worried about waking during surgery.

"I say, 'I guarantee I will do everything that has been clinically validated to reduce the risk. I will reduce the use of paralytic drugs -- because the scariest thing is being unable to move, and sometimes patients are given these more than is necessary. And I will give you a lot of pain medication so even if you do wake up, you will not experience any discomfort. Know that if you do wake up during surgery, it is usually brief and without discomfort. But if this does occur, and you feel you do remember things -- and I will ask you about this after surgery -- then I will refer you for professional counseling,'" Avidan says.

WebMD Health News Reviewed by Louise Chang, MD on March 12, 2008



Avidan, M.S. The New England Journal of Medicine, March 13, 2008; vol 358: pp 1097-1108.

Orser, B.A. The New England Journal of Medicine, March 13, 2008; vol 358: pp 1189-1191.

Michael Avidan, MB, BCh, associate professor of anesthesiology and surgery, Washington University School of Medicine, St. Louis.

Adam C. Searleman, student, medical science training program, Washington University School of Medicine, St. Louis.

Beverley A. Orser MD, PhD, FRCPC, professor of anesthesia and physiology, University of Toronto; staff anesthesiologist, Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada.

Scott D. Kelley, MD, vice president and medical director, Aspect Medical Systems Inc., Newton, Mass.; staff anesthesiologist, Brigham & Women's Hospital, Boston.

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