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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
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WebMD Health News
Reviewed by Louise Chang, MD

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.

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