Did you see that movie “Awake” with Jessica Alba and Hayden Christensen? The story focuses on a man who suffers “anesthetic awareness” and finds himself awake and aware, but paralyzed, during heart surgery. Well an FDA approved device called a bispectral index for anesthesiologists to use that is supposed to prevent this problem turns out to not work at all according to a new study.
Unintended intraoperative awareness, also known as anesthesia awareness, occurs when a patient becomes aware during surgery and remembers being in pain or feeling distress during the operation. Between one in 500 and one in 1,000 surgery patients experience awareness during anesthesia. Those with such memories often need psychological help, and some even develop post-traumatic stress disorder.
Michael S. Avidan, MB, BCh, professor of anesthesiology at Washington University School of Medicine explains:
“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. This trial showed that the BIS measurement does not appear to be superior.”
The 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 (ETAC), the concentration of anesthetic agents in patients’ exhaled breath.
Rather than keeping track of drug concentrations, BIS monitors use data from a single channel electroencephalogram (EEG) or brain-wave trace. When a person is fully awake, their BIS measurement approaches 100. No detectable brain activity would show up on the monitor as zero. The manufacturer recommends BIS readings should be between 40 and 60 during surgical anesthesia.
The study focused on a subset of patients at high risk for awareness. They include patients undergoing cardiac surgery, those who regularly consume large amounts of alcohol or take sedatives or certain types of pain-killing drugs. Patients who previously have had episodes of awareness also are at high risk. The rate of intraoperative awareness in high-risk patients is estimated to be as high as 1 percent.
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.
Alex S. Evers, MD, the Henry E. Mallinckrodt Professor and head of the Department of Anesthesiology at Washington University School of Medicine states:
“Our study does not show that the BIS monitor is ineffective. It’s just that it’s not superior to the protocol based on measuring the concentration of anesthetic a patient actually has received, which is a simpler and less expensive approach.”
Although neither technique prevented awareness in all patients, both methods seemed to work most of the time, even in the high-risk patients who were part of the study. Although intraoperative awareness was very rare, it did sometimes occur when monitoring techniques indicated patients should be unconscious.
Michael S. Avidan, MB, BCh, professor of anesthesiology at Washington University School of Medicine continues:
“It is quite striking that for many years anesthesiologists have monitored the cardio-respiratory system very vigilantly, but we haven’t rigorously tried to monitor the nervous system, which is the site of action for anesthetic agents. That’s what the bispectral index attempted to do. New technology has to prove its superiority over more cost-effective alternatives. Our trial showed, compellingly, that the protocol based on the bispectral index is not superior.”
Avidan says other monitoring techniques based on EEG measurements may be useful in monitoring patients during surgery and might avoid a problem with the BIS monitor, which runs EEG readings through a proprietary algorithm that takes some time to process. There can be lags as long as a minute before a patient’s bispectral index reading changes in response to an arousal. He says faster processing of EEG values might be useful in producing a better real-time measurement of how deeply a patient is anesthetized.
Written by Sy Kraft