How brain loses consciousness during anaesthesia decoded

Scientists have decoded how a common anaesthesia drug creates loss of consciousness in patients undergoing surgery.

In a new study that tracked brain activity in human volunteers over a two-hour period as they lost and regained consciousness, researchers from MIT and Massachusetts General Hospital (MGH) have identified distinctive brain patterns associated with different stages of general anaesthesia.

The findings shed light on how one commonly used anaesthesia drug exerts its effects, and could help doctors better monitor patients during surgery and prevent rare cases of patients waking up during operations.

“When anaesthesiologists are taking care of someone in the operating room, they can use the information in this article to make sure that someone is unconscious, and they can have a specific idea of when the person may be regaining consciousness,” said senior author Emery Brown.

The researchers studied healthy volunteers, measuring their brain activity with an array of 64 electrodes attached to the scalp.

Not only did they find patterns that appeared to correspond to what they saw in last year’s study, they were also able to discern much more detail, because they gave the dose of propofol over a longer period of time and followed subjects until they came out of anaesthesia.

While the subjects received propofol, the researchers monitored their responsiveness to sounds. Every four seconds, the subjects heard either a mechanical tone or a word, such as their name.

The researchers measured EEG activity throughout the process, as the subjects pressed a button to indicate whether they heard the sound.

As the subjects became less responsive, distinct brain patterns appeared. Early on, when the subjects were just beginning to lose consciousness, the researchers detected an oscillation of brain activity in the low frequency (0.1 to 1 hertz) and alpha frequency (8 to 12 hertz) bands, in the frontal cortex.

They also found a specific relationship between the oscillations in those two frequency bands: Alpha oscillations peaked as the low-frequency waves were at their lowest point.

When the brain reached a slightly deeper level of anaesthesia, a marked transition occurred: The alpha oscillations flipped so their highest points occurred when the low frequency waves were also peaking.

The researchers believe that these alpha and low-frequency oscillations, which they also detected in earlier study, produce unconsciousness by disrupting normal communication between different brain regions.