Researchers Identify States of Unconsciousness with EEG Data

Brain wave patterns show a clear transition to a coma-like state in patients under general anesthesia.

Researchers have identified patterns of brain activity that correlate to the loss and recovery of consciousness while a patient undergoes general anesthesia. Their findings, reported yesterday in the Proceedings of the National Academy of Sciences online, could one day help anesthesiologists better monitor consciousness in their patients.

"Despite the fact that the primary purpose of anesthesiology is to render patients temporarily unconscious and [unable to remember], the brain is not actually monitored directly," says Robert Thiele, an anesthesiologist at the University of Virginia School of Medicine, who was not involved in the study. Instead, indirect signs such as blood pressure, heart rate, and reflexive movements are watched to gauge the depth of anesthesia, he says. "There is a great need for a more accurate depth of anesthesia monitor," says Thiele.

Some hospitals and researchers use electroencephalographic (EEG) monitoring, which measures electrical activity in the brain's cortex through electrodes on the scalp, but this data is typically simplified into a single number or index to assess a patient's level of consciousness. "The real thorn in the side of the indices is they suppose the people are unconscious at the same indices, that it applies for all people across all drugs, and that just cannot be, says Emery Brown, an anesthesiologist at Massachusetts General Hospital and neuroscientist at MIT and senior author on the study. "People respond to drugs differently, and different drugs have different signatures, he says.

So Brown and Patrick Purdon, a neuroscientist at Massachusetts General Hospital and MIT, and their colleagues set out to characterize specific EEG rhythms that associate with anesthesia-induced unconsciousness. The team recorded EEG patterns in 10 healthy volunteers while giving them increasing and then decreasing levels of a widely used anesthetic called propofol. The participants listened to verbal cues such as their name and auditory clicks and were told to press a button in response to each, which the researchers used as a sign of loss or recovery of consciousness.

The team found unique patterns in the brain waves of the volunteers as they lost and then regained consciousness. Changes in the relationship between brain waves of different frequencies predicted the transitions into and out of consciousness. The team also identified a pattern in the participants' brain waves that was associated with the deepest state of unconsciousness. The patterns, or "signatures," could one day be used to monitor and control sedation and unconsciousness in patients given propofol, say the authors.

The real beauty of the study is the promise of being able to differentiate between normal drug-induced and injury-induced sleep, says Dietrich Gravenstein, an anesthesiologist at the University of Florida College of Medicine. This could help researchers better understand these different brain states and could also be useful in the operating room, he says.

Anesthesiologists try to tune the amount of drugs given to a patient based on moment-to-moment needs, says Gravenstein. Immediately before a surgery, some anesthetic is given to make a patient sleepy and comfortable, but "once the surgery starts, you have a dramatic increase in need," says Gravenstein. "The problem is, we couldn't know if they are comfortable and naturally sleeping with a little bit of drug or basically unconscious from the medication. We don't want them to wake up as soon as the knife touches the skin," he says. Clear EEG signatures for each state could help guide anesthesiologists, although the timing of the analysis would have to be very fast for it to be useful during procedures, says Gravenstein.

The researchers hope their findings could be used in clinical settings and are currently training anesthesiologists how to use this. But most operating rooms and anesthesia procedures are not currently set up for this kind of monitoring, says Christoph Seubert, chief of neuroanesthesia at the University of Florida College of Medicine, who points out that the study used 64 EEG-monitoring electrodes. "The current depth of anesthesia monitoring happens with one to two channels of EEG placed over the frontal brain, not because that area is the most important as far as consciousness is concerned, but because it is an easy place to stick to the head," says Seubert. Finding out how little EEG data is needed to provide information about the transitions will be a necessary step toward making the signatures clinically useful, he says.

Beyond potentially helping patients, this kind of discovery could help uncover important brain processes, says Brown, who sees his medical specialty as a tool for studying fundamental aspects of consciousness (see "The Mystery Behind Anesthesia"). "If we are able to make very clean statements about how drug "x" operates in these circuits to produce these effects, then we will be able to provide robust information for people who want to solve the problem of how consciousness is generated."

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