At present it is very difficult to tell whether a patient is awake or not, they could be paralysed but still feel pain

Professor Gavin Kenny, Glasgow University

Two in every thousand patients are not properly put to sleep before surgery in the UK.

On occasions patients are paralysed on the operating theatre, but are fully aware of what is happening.

Some require treatment for post-traumatic stress disorder.

Now scientists at the University of Glasgow have developed a computer system which can monitor brain waves to ensure anaesthetic has worked properly during operations.

The Auditory Evoked Potential System can also measure how much anaesthetic a patient needs and administer a correct dose, then top it up later if measurements indicate more is required.

Professor Gavin Kenny, head of the University of Glasgow's department of anaesthesia, described the new system at the Edinburgh Science Festival on Thursday.

Professor Kenny quoted a patient who had not been properly anaesthetised.

Screaming inside

I just started screaming inside my head and I prayed I would not die

Woman conscious during surgery

She afterwards said: "I just started screaming inside my head and I prayed I would not die."

He said: "This can lead to major trauma causing sleep disorders and similar problems, although some patients may be aware of what is happening without feeling pain.

"At present it is very difficult to tell whether a patient is awake or not, they could be paralysed but still feel pain.

"With this system it will be possible to tell whether the patient is awake or not."

Electrodes are attached to the patient's skull and electronic "clicks" are administered to the patient's ear while brain waves are measured to determine the response to that stimulus.

Professor Kenny believes the system could save the NHS a lot of money as it would cut down on litigation costs from affected patients suing hospitals.

He said: "This possibly happens a lot more than we realise.

"Some patients may suffer during operations and not remember afterwards, but still suffer traumatic disorders like fear of the dark or sleeplessness."

Professor Leo Strunin, president of the Royal College of Anaesthetists, told BBC News Online that other techniques had tried and failed to monitor patients' awareness with absolute accuracy.

He said: "A non-invasive system which was cheap and easy to use would be a major step forward.

"But a lot of things interfere with auditory potential and mask whether a patient is awake or asleep - it is not an easy technology to use."

The Glasgow team is currently working on a commercial prototype.

They hope the machine will be in use in hospitals in about 18 months.