An estimated one in 15,000 patients undergoing surgery in the United Kingdom experiences intraoperative awareness, researchers in that country have found.

The figures come from the Fifth National Audit Project of the Royal College of Anaesthetists and the Association of Anaesthetists of Great Britain and Ireland. Previous audits looked at airway complications, morbidity and mortality reviews, complications of central neuraxial blocks and the supervision of consultant anesthetists.

For the survey, researchers queried anesthetists at all 329 hospitals in the U.K.’s National Health System in 2011; 7,125 (82%) responded.

Anesthetists reported 153 cases of “accidental” awareness, for an incidence of one per 15,414 surgeries. That rate was substantially lower than a previous estimate of one to two cases per 1,000 surgeries reported by patients themselves.

“If both sets of data are valid, then it means that for approximately every 15,000 general anaesthetics administered, the anaesthetist may learn of just one case of [accidental awareness] while up to around 30 other patients will experience [awareness] but not report it,” the researchers wrote.

“We believe that our baseline information on more than 150 cases of [awareness] in the UK exceeds that of any previous publication in this area, and may represent the most complete survey of medical practitioner experience of any disease or complication, across an entire nation,” they added.

Nearly half (47%) of the cases of awareness occurred on or after induction of anesthesia but before the start of surgery, according to the researchers. Of the rest, 30% occurred during surgery and 23% occurred after the procedure but before full emergence from anesthesia, they said.

About one-third of patients reported pain associated with awareness, the researchers said, while “only a fraction” took legal action as a result of the incident—although they noted that such cases can have long delays after the episode.

The researchers also found that approximately one in 1,000 patients given neuromuscular blocking agents reported intraoperative awareness.

Although roughly six in 10 hospitals in the survey reported having monitors for depth of anesthesia, clinicians used the devices in less than 2% of surgeries, the study found. Only 12 hospitals (4.5%) said they had some policy in place to prevent or manage awareness.

The findings appear in both the British Journal of Anaesthesia and Anaesthesia—the first time the two journals have simultaneously published the same paper, their editors wrote.