How general anaesthetics cause unconsciousness

Meansurements made from patient's brains while they were being anaesthetised has shed new light on how general anaesthetics cause unconsciousness.

The majority of studies done to date on anaesthesia and unconsciousness have involved recording brain activity once people are already unconscious, missing the all-important transition into oblivion that occurs as the anaesthetic agent kicks in.

Now scientists at MIT and Harvard, writing in PNAS, have made direct recordings of nerve activity from inside the brains of patients as they were rendered unconscious.

Laura Lewis and her co-worker Veronica Weiner recruited 3 patients with epilepsy. Electrode arrays had been implanted temporarily into the brains of these patients to help their doctors to investigate and manage their conditions. This done, the electrodes were scheduled to be removed surgically.

This offered the team the opportunity to record how the patients' brain activity altered, both at the level of individual nerve cells and networks of neurones, as the anaesthetic was administered.

To pinpoint the time of loss of consciousness, the patients were instructed to press a button whenever they heard their name spoken to them through a pair of headphones. At the same time the patients received a dose of the anaesthetic agent propofol.

The recordings made by Lewis and Weiner show that, at the time when the patients stopped responding and became unconscious, brain activity altered dramatically.

Each of the different brain regions adopted a slowly oscillating pattern of rising and falling levels of activity.

This had the effect of isolating each of the brain regions from one another, because when one region was more active, another region was inactive.

So although different brain areas still remain active during the anaesthetic, the lack of "sync", the researchers speculate, disrupts functional connectivity and processing between them, leaving each brain region functionally isolated.

"It's as if one brain region is in Boston and the other is in Singapore: they can't make phone calls to each other because one is asleep when the other is awake," says Lewis.

Admittedly the team did test just one single anaesthetic agent - propofol - so they cannot generalise with certainty that this mechanism will apply to all general anaesthetics, but the results are in strong agreement with previous work in including studies on animals.

Lewis and Weiner also acknowledge that their study subjects were epileptics, although the part of the brain tested in this experiment was remote from the subjects' epileptic regions.

The researchers hope that their discovery will lead to better ways to monitor patients under anesthesia by using the brain activity patterns they have spotted as hallmarks of unconsciousness, which have previously been lacking...