Researchers have identified electroencephalogram (EEG) signatures of changes in consciousness caused by propofol.
"The EEG signatures we have identified for propofol can be computed in real-time, are easy to recognize, and can be interpreted in a way that relates directly to the mechanisms through which this anesthetic is postulated to induce unconsciousness," say lead researcher Patrick Purdon (Massachusetts General Hospital, Boston, USA) and co-workers.
The team detected these patterns in 10 healthy volunteers, who had to press a button in response to auditory stimuli (clicks or verbal) during anesthesia induction and emergence.
At baseline and during induction, activity in the alpha EEG frequency band was concentrated in the occipital area, which the researchers say is consistent with the "awake eyes-closed" state - the participants were asked to keep their eyes closed throughout to avoid eye-blink artifacts in the EEG. This activity disappeared at the point of loss of consciousness (LOC; no reaction to stimuli), and reappeared with return of consciousness (ROC).
Conversely, frontally focused activity in the alpha and beta channels appeared at LOC and disappeared at ROC. There was also about a 10-fold increase in low-frequency power at LOC, which was not centered on a particular area and disappeared at ROC.
"The standard definitions of the alpha, beta, and gamma EEG frequency bands provide a convenient shorthand for describing different oscillations," write Purdon et al in the Proceedings of the National Academy of Sciences of the United States of America.
However, they caution that "these propofol-induced oscillations are not confined neatly to single bands but instead move among these bands to varying degrees during both induction and emergence."
To that end, the researchers looked at how the phase of the low-frequency oscillations related to the amplitude of the alpha and beta oscillations. They found a distinct pattern that extended across the point of LOC and of ROC, where the alpha and beta amplitudes were largest at the troughs of the low-frequency oscillations (trough-max). During unconsciousness, the pattern reversed, with the alpha and beta amplitudes largest at the peaks of the low-frequency oscillations (peak-max).
Purdon et al say that further study of EEG patterns caused by other anesthetic drugs should "translate into more reliable approaches to monitoring the brain states of patients receiving [general anesthesia] and for tailoring drug dosing based on specific, real-time knowledge of these states."
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