Consciousness during general anaesthesia 'is rare'
The risk of waking from a general anaesthetic while under the surgeon's knife is extremely small - about one in 15,000 - research reveals.
Investigators surveyed 82% of the UK's consultant anaesthetists - 7,125 in all - asking them to report any cases of accidental awareness in the past year.
Out of nearly 3 million operations in 2011 there were 153 reported cases.
Most of these patients either came round too soon from general anaesthetic or took too long to go under.
A third - 46 in total - were conscious throughout the operation.
According to the anaesthetists, very few patients suffered any pain or distress as a result of the experience.
The researchers from the Royal College of Anaesthetists and the Association of Anaesthetists of Great Britain and Ireland say their preliminary findings, published in the journal Anaesthesia, should be reassuring for patients.
Prof Tim Cook, co-author of the study and a consultant anaesthetist in Bath, said: "Most of the cases involved the patient being aware during induction when they are being sent off to sleep or after the operation itself has finished.
"They might feel some awareness of tugging or hear some noises.
"Few anaesthetists that we surveyed said their patients complained of pain or distress or made a formal complaint."
Prof Cook said it was a fine balance trying to ensure that a patient was suitably sedated.
"You want to make sure that they are under but you don't want to give them too much because that is dangerous in itself."
Some hospitals have specific monitors for measuring the depth of anaesthesia.
Prof Cook and his team found even when this kit was available, it was not often used - only 2% of the anaesthetists they surveyed said they routinely used brain monitoring technology.
Prof Cook says it is possible that accidental awareness is more common than his study findings suggest. Smaller surveys of patients have revealed higher rates - of as much as one in every 500 operations done under general anaesthetic.
He says anaesthetists want to learn more about accidental awareness so they can avoid more patients experiencing it.
"We are spending the next year studying as many of the cases as possible to learn more from patients' experiences. Then we will be able to offer new insights and guidance."
Theirs is thought to be the largest study of its kind.