In a state-of-the-art operating theatre in a modern British teaching hospital a patient lies on a table surrounded by masked and gowned figures.
He is breathing anaesthetic through a tube that has been put down his windpipe. The anaesthetist sits at his head monitoring a large array of shiny instruments. The surgeon selects a scalpel and the operation begins.
'Here we are 150 years after the introduction of anaesthetics gazing at a very impressive display of monitors,' says the anaesthetist, Professor Gareth Jones, of the Department of Anaesthesia at Addenbrooke's Hospital, Cambridge. 'And there is not a single dial here that tells us if the patient is unconscious. It is like flying an aeroplane without knowing how high off the ground you are.'
Waking up while on the operating table without being able to communicate the fact is a nightmare that comes true for the occasional patient: about one in 5,000 becomes conscious, able to feel the pain of the surgeon's knife, during surgery. 'These people are effectively awake during surgery. How much pain they suffer depends on the individual's pain threshold, everyone is different,' Professor Jones says.
A further one in 3,000 becomes aware during an operation without recalling any feeling of pain.
Happily, doctors at Addenbrooke's may have solved the problem. They have developed and built a machine, based on a personal computer, which can tell the anaesthetist exactly how unconscious the patient is throughout an operation.
Electrodes are placed on the patient's head and he or she wears a pair of earphones through which a series of clicks is sent to stimulate the brain.
At irregular intervals the tone of the click changes and the computer, via the attached electrodes, measures the brain's response to this event, digitises the signal and presents it as a changing wave pattern on a VDU screen in front of the anaesthetist.
From the position of the wave peak, the anaesthetist can tell whether sufficient anaesthetic is being given to keep the patient unconscious during surgery.
Of the 2.5 million anaesthetics now given in the UK every year, only 500 patients will be awake and in pain, he points out. About 800 cases may have some hazy recollection, perhaps of something that was said in the operating theatre, but they never complain of feeling pain.
Patients who have surgery following major trauma, such as road accidents, are at higher risk of waking up than those undergoing routine operations. In these cases light or even no anaesthesia is given because of the risk of killing an already very sick and weak patient. In such cases the incidence of awareness, and of being in pain, is about 11 per cent.
In addition, for many operations patients are given a muscle paralysing drug as well as an anaesthetic and are attached to a ventilator to keep them breathing.
This makes operating easier and is essential for some chest and stomach procedures. But it also means that should the patient wake up, he or she is unable to move or to call attention to his or her plight.
'The problem is that body-function signs such as blood pressure and heart rate are fairly unreliable indicators under anaesthetic,' says Professor Jones. 'They do not always rise if the patient is awake and/or experiencing pain.
'A totally paralysed patient can be wide awake. He hears the operating room conversation, finds out all about the surgeon's golf game and feels his innards being wrenched out. Pretty nasty.'
The anaesthesia monitoring machine has been developed by two members of Professor Jones's department, Dr Rajesh Munglani and Dr David Sapsford, senior lecturers in anaesthesia.
In order to calibrate the machine, they compared the changes in brain waves with psychological performance at various concentrations of anaesthetic drugs. In the best traditions of scientific discovery, they roped in some of their colleagues, wired them up to the computer, gave them carefully stepped-up amounts of isoflurane, a commonly- used anaesthetic, and put them through standard cognition and memory tests.
They found that painful stimulation - in this case an electric shock - often improved the drugged subject's test performances and that this change in awareness was successfully recorded by the computer.
Dr Munglani says the results confirm that the level of a patient's awareness is determined by the balance between the depressant effects of the anaesthetic and the stimulation he or she receives.
He says: 'The implications for anaesthetic practice are obvious. A previously unconscious patient may wake up when the surgeon begins to cut. Our machine will help to prevent that happening.'
Anaesthetics render people unconscious because they are poisons. At present, the problem for anaesthetists is that patients vary in their reaction to anaesthetic drugs - some need more than others. It is a delicate balance: too little and you could be in agony, too much and you could be dead.
This balance is particularly difficult to maintain in women undergoing caesareans, where the unborn child would be at risk if the mother received too much anaesthetic. Giving anaesthetic to the elderly is also risky because their heart rates and blood pressures may be already slow and low; further depressant effect of anaesthetic could be dangerous.
Mothers becoming conscious while undergoing caesarean section are among the best known horror stories. Carol Kewley woke up on the operating table after being admitted to the Victoria Hospital, Blackpool, to have her daughter, Emma, by caesarean section.
It was Mrs Kewley's second caesarean. The first, four years previously for her son, Michael, went without a hitch so she was not unduly worried.
But the second time, she recalls: 'I was suddenly aware of this terrible burning pain. It felt as if someone was ironing my stomach with a red hot iron.
'I did not know where I was. There was just this terrible pain and then something seemed to be tugging out my insides. It was agony. I remember fighting to breathe.
Mrs Kewley says she tried to scream but no sound came. 'When I tried to move there was a huge weight crushing me and squashing all air out of me. The pain and crushing just went on and on.
'Eventually I managed to get my eyes open. I could see a big round light above me and off to my right a figure in a mask and gown. After that I don't remember anything until I was outside on a trolley and my husband, David, was there.'
Professor Jones points out that however horrific this kind of case, it is easy to forget how much worse the figures were in the recent past.
'As recently as the 1960s three out of 100 of those having surgery under general anaesthesia had some sort of recollection of the operation,' he says. 'But nothing very much was said about it because
it was the norm. Figures are not available to tell us how many of those people were conscious and in pain.'
Clinical trials of the machine have now begun and it is currently being used with day case surgery. Initial results are promising, says Professor Jones.
'People always used to fear not waking up - dying - under anaesthetic. Now a substantial number are worried about waking up during surgery because they have heard or read about it happening.
'The presence in the operating theatre of equipment which tells the anaesthetist that they are unconscious will be a tremendous help towards reassuring them.'
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