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Study examines prevention of intraoperative awareness


A device approved by the U.S. Food and Drug Administration to reduce intraoperative awareness does not lower the risk of the problem any more than a less expensive method, according to a report in the New England Journal of Medicine.

As many as one in 500 patients experience awareness during anesthesia, according to background information in the article. Those with such memories may need psychological care, and some might even develop post-traumatic stress disorder.

The study involved more than 6,000 surgical patients at Washington University in St. Louis, the University of Chicago and the University of Manitoba in Winnipeg. It focused on a subset of patients at high risk for awareness, including patients undergoing cardiac surgery, those who regularly consume large amounts of alcohol or take sedatives or certain types of pain-relieving drugs. The rate of intraoperative awareness in high-risk patients is estimated to be as high as 1%, according to the researchers.

“We were testing whether a protocol based on a device called a bispectral index or BIS monitor, which measures brain activity to determine how deeply a patient is anesthetized, could more effectively prevent intraoperative awareness than an alternative approach that measures anesthetic levels in a patient’s breath,” said first author Michael S. Avidan, MB, BCh, professor of anesthesiology at Washington University School of Medicine. “This trial showed that BIS measurement does not appear to be superior.”

A 2004 study found that using BIS monitors could reduce awareness incidence by 82%. Then, in 2008, Avidan and his colleagues did a second study that followed almost 2,000 surgery patients at Barnes-Jewish Hospital in St. Louis. That study found that using a BIS monitor, which involves placing a disposable sensor on the patient’s forehead to monitor brain activity, was no better at preventing awareness than the less-expensive protocol based on the measurement of end-tidal anesthetic-agent concentration, the concentration of anesthetic agents in patients’ exhaled breath.

That trial found that both protocols were probably effective in decreasing the incidence of awareness, and the study did not rule out the possibility that the BIS protocol might confer a slight, but clinically meaningful, advantage.

The new study was designed to resolve whether the BIS protocol did, in fact, confer that clinically meaningful advantage. In the multi-center trial, investigators found that 19 of the 2,861 high-risk surgical patients in the BIS monitor group experienced either definite or possible intraoperative awareness. Among those randomized to the exhaled anesthetic technique, only eight of the 2,852 had intraoperative awareness.

To view the study data, visit


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