Consciousness Signature: Awareness Warnings during surgery

Anaesthesia wearing off mid-operation is not just the stuff of nightmares – it occurs in around 0.2 per cent of surgeries globally. Keeping tabs on the brain during surgery could stop this.

Indirect signs of consciousness such as changes in heart rate, blood pressure and muscle tone can be tracked during surgery but more reliable indicators are not currently available.

"It's slightly frightening that millions are given anaesthesia every year and the anaesthetist has no way of knowing with certainty that the patient is unconscious," says Tony Absalom at the University of Groningen in the Netherlands.

To look for a more concrete signature that could be monitored during surgery, Emery Brown at the Massachusetts Institute of Technology and his team attached an EEG cap with 64 electrodes to the heads of 10 adults.

They used this to measure changes in brain activity across multiple brain regions as unconsciousness was induced using a general anaesthetic.

As the volunteers lost and regained consciousness they were asked to press a button whenever they heard a click or a spoken word, allowing brain activity to be matched to different stages of wakefulness.

By constructing a montage of the activity from different parts of the brain, the team identified recognisable patterns that corresponded to different levels of consciousness, allowing them to tell when someone was waking up.

Mysterious sleep
EEG monitoring during surgery is carried out in about 2 per cent of hospitals in the UK, but only three or four electrodes are used, in a strip across the forehead.

This looks at just one brain region, however, and so can only give you a probability of unconsciousness, not a conclusive answer, says Absalom.

"You don't want something that says a patient is probably asleep. You want to know 'Are they or aren't they?'"

Using more electrodes, as Brown has done, should shore this up. Although applying more electrodes means the patient's head must be shaved and a conductive gel applied.

"The mechanisms underlying anaesthesia remain a mystery," says Ram Adapa at the University of Cambridge. But these results support the idea that anaesthesia affects the communication and synchronisation between brain regions, he says. "It is yet another piece of the jigsaw puzzle."

However, Adapa notes that it may be a while before the technique can be used in a clinical setting. EEG measurements are very sensitive to mechanical and electrical interference, he says, "and the operating room environment is unfortunately burdened by both."

Journal reference: 
Electroencephalogram signatures of loss and recovery of consciousness from propofol PNAS, 10.1073/pnas.1221180110