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Brain Has Distinct Activity Pattern When Losing Consciousness During Anesthesia

A new study from the US reveals for the first time, that the brain has a distinct pattern of electrical activity as patients lose consciousness during anesthesia. The pattern shows very slow oscillations, reflecting a breakdown of communication between the different regions of the brain, each of which shows shorts bursts of activity alternating with longer silences.

The researchers write about their findings in a paper published online first on 5 November in the Proceedings of the National Academy of Sciences.

They hope that by improving understanding of what happens in the brain as it loses consciousness, the study will help anesthesiologists better maintain the right balance between too little and too much anesthetic.

Senior author Patrick Purdon, an instructor of anesthesia at Massachusetts General Hospital (MGH) and Harvard Medical School, says in a statement, clinicians will now know what to look for on the electroencephalograph (EEG) when putting a patient under anesthesia:

"We now finally have an objective physiological signal for measuring when someone's unconscious under anesthesia."

EEG Patterns in Epileptic Patients An EEG is a machine that records electrical activity of the brain through electrodes on the scalp. It measures changes in voltage resulting from the various currents flowing between neurons or brain cells.

For their study, Purdon and colleagues studied epileptic patients who had electrodes implanted in their brains to monitor seizures and were having an operation to remove them.

The patients received a common anesthetic known as propofol and had their brain activity monitored by EEG.

Propofol activates receptors on neurons, in a way that makes the brain cells less active, although exactly how this happens is not clear.

The researchers noticed the EEG showed a distinct pattern at the point where consciousness was lost. This was about 40 seconds after receiving the anesthetic, and was defined by the moment when patients stopped responding to sounds played to them every four seconds. Distinct Pattern of Overall and Local Brain Activity To record brain activity, Purdon and colleagues used two different sized of electrode, each size taking a different reading of brain activity. The larger electrodes, about the size of a large coin, were placed about 1 cm apart and recorded the overall EEG or brain wave pattern.

The smaller, more localized, electrodes were concentrated in a group of rows about 4 mm wide. Between 50 and 100 of these were implanted in each patient, in different brain regions.

These smaller electrodes recorded activity from individual neurons, and this study is thought to be the first to record neuron activity in patients as they lose consciousness.

The large electrodes showed that within one or two seconds of patients losing consciousness, the EEG pattern suddenly turned to low frequency oscillations, at about 1 cycle per second (about 1 Hz).

This coincided with the small electrodes showing a "flickering" pattern at individual neuron level. Individual neurons within localized brain regions were active for a few hundred milliseconds, then became quiet for a few hundred milliseconds. This created the oscillating pattern seen on the EEG, say the researchers.

"We show that propofol-induced unconsciousness occurs within seconds of the abrupt onset of a slow (

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