It may be a patient's worst nightmare -- to lie on the table conscious and aware of the surgeon's knife despite general anesthesia.
"Unintended intraoperative awareness" is rare but worrisome, and anesthesiologists have been seeking ways to prevent it. One recent innovation is the so-called bispectral index, a monitoring system which measures electrical brain activity to attempt to detect awareness.
But in an international randomized trial, the bispectral index was no better in detecting a patient's awareness during surgery than the standard approach of measuring the concentration of anesthetic gases in a patient's breath, according to Michael Avidan of Washington University School of Medicine in St. Louis, Mo., and colleagues.
In fact, contrary to expectations, there were fewer cases of intraoperative awareness among patients monitored by measuring gas concentrations than among those whose brain activity was watched, Avidan and colleagues reported in the Aug. 18 issue of the New England Journal of Medicine.
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In principle, the monitoring would permit better detection of intraoperative awareness during surgery -- allowing physicians to compensate by increasing anesthesia.
But experts outside the trial said that neither approach is likely to prevent all cases of intraoperative awareness.
"There are multiple factors that go into the equation," said Dr. Howard Nearman of Case Western Reserve University School of Medicine in Cleveland, "that make it virtually impossible to prevent intraoperative awareness all the time while avoiding the consequences of anesthetic overdose."
Those other factors, he said in an email to ABC News/MedPage Today, include the patient's level of anxiety, what other drugs have been given, the intensity of the surgical stimulus, and the patient's overall health condition."
The "nature of consciousness" remains obscure and difficult to measure, argued Dr. Gregory Crosby of Brigham and Women's Hospital in Boston in an accompanying editorial.
"Monitors are meant to supplement, not supplant, clinical decision making," Crosby wrote. "It is unreasonable to expect any such monitor to unfailingly detect conscious awareness -- a specific and still mysterious property of the brain and mind."
The "real take-away (for both patients and the medical community)" is that the bispectral index monitor is useful but not the last word, according to Dr. Eugene Viscusi of Philadelphia's Thomas Jefferson University.
Instead, he said in an email to ABC News/MedPage Today, it's the anesthesiologist's management of the patient's care that makes the difference.
Like most anesthesiologists reached by ABC News/MedPage Today, Viscusi said cases of intraoperative awareness are exceedingly rare. "I have practiced anesthesiology since 1985 and have never had a case of intraoperative awareness to my knowledge," he said.
Dr. Michael Roizen of the Cleveland Clinic said his institution has two or three patients a year who spontaneously report intraoperative awareness out of the 160,000 who have surgery.
In high-risk patients, Roizen said in an email, the proportion is higher -- one in 200 to one in 400 -- and the New England Journal study found essentially that "the two most common techniques to combat that do not differ in efficacy of preventing it."