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."
Indeed, the study found that 27 of 5,713 patients -- all thought to be at high risk for intraoperative awareness -- had either definite or possible awareness during their surgery, based on interviews afterward.
Only nine had what experts considered to be definite awareness.
The patients had been randomly assigned to be monitored either by the bispectral index -- the BIS group -- or the standard method, known as end tidal anesthetic-agent concentration, or ETAC.
When the researchers looked at what happened in those groups, they found seven of the 2,861 patients in the BIS group had definite intraoperative awareness, compared with two of 2,852 in the ETAC group.
Also, there were 11 cases of possible intraoperative awareness in the BIS group and six in the ETAC group.
But the numbers were not considered to reflect a real difference in the risk of awareness during surgery, the researchers reported.
Patients who are aware during their surgery rarely feel pain, according to Dr. Mark Schlesinger of Hackensack University Medical Center in Hackensack, NJ -- although some patients have described unpleasant sensations.
"Usually, they are just aware without discomfort," he said in an email to ABC News/MedPage Today.
In the study, Avidan and colleagues reported, none of the patients thought have been definitely or possibly aware reported feeling pain.
The researchers cautioned that the results may not apply to all surgical patients; those in the study were at high risk for intraoperative awareness and were sedated with gases. Results might differ for patients at lower risk or sedated intravenously, they noted.