Brain Patterns Produced by General Anesthesia Revealed

A new study has identified the distinctive brain patterns associated with different stages of general anesthesia (GA), which could help monitor patients during surgery.

Researchers from MIT (Cambridge, MA, USA) and Massachusetts General Hospital (MGH; Boston, USA) recorded high-density electroencephalograms (EEGs) in human volunteers to develop EEG signatures that track consciousness during gradual induction of and emergence from GA under the influence of propofol. The subjects executed an auditory task at 4-second intervals, consisting of interleaved verbal and click stimuli to identify loss and recovery of consciousness. Measurement of the median frequency and bandwidth of the frontal EEG tracked the probability of response to the verbal stimuli during the transitions in consciousness.

The researchers found that during GA induction, subjects lost responsiveness to the less salient clicks before losing responsiveness to the more salient verbal stimuli; during emergence, they recovered responsiveness to the verbal stimuli before recovering responsiveness to the clicks. The researchers also found that loss of consciousness was marked simultaneously by an increase in low-frequency EEG power (<1 Hz), the loss of spatially coherent occipital alpha oscillations (8–12 Hz), and the appearance of spatially coherent frontal alpha oscillations. These dynamics reversed with recovery of consciousness.

When the brain reached a slightly deeper level of anesthesia, a marked transition occurred; the alpha oscillations flipped so that their highest points occurred when the low frequency waves were also peaking. The researchers believe that these alpha and low-frequency oscillations produce unconsciousness by disrupting the amount of information that can pass between the frontal cortex and the thalamus, which normally communicate with each other across a very broad frequency band to relay sensory information and control attention. The study was published on March 4, 2013, in the Proceedings of the National Academy of Sciences of the United States of America (PNAS).

“We found specific behavioral and electrophysiological changes that mark the transition between consciousness and unconsciousness induced by propofol, one of the most commonly used anesthetic drugs,” concluded lead author Patrick Purdon, of the MIT department of brain and cognitive sciences, and colleagues. “Our results provide insights into the mechanisms of propofol-induced unconsciousness and establish EEG signatures of this brain state that could be used to monitor the brain activity of patients receiving general anesthesia.”

Anesthesiologists reversibly manipulate the brain function of nearly 60,000 patients each day, but brain-state monitoring is not an accepted practice in anesthesia care because markers that reliably track changes in level of consciousness under GA have yet to be identified.

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