Incidence of and risk factors for awareness during anaesthesia.

Explicit recall of events during general anaesthesia is detected by direct questioning, as patients may not report awareness spontaneously or if they are questioned non-specifically. More than one interview is needed and credibility of reports should always be verified. The overall incidence of awareness has decreased over the last 40 years and is now 0.1-0.2%. Prospective study of patients who undergo general anaesthesia is the only valid method for determining the incidence of awareness. Studies of patients recruited through referrals by colleagues or advertisements, studies of compensation claims and those carried out through quality improvement systems are inadequate. Several factors increase the risk of awareness, including light anaesthesia, some types of surgery, a history of awareness, chronic use of central nervous system depressants, younger age, obesity, inadequate or misused anaesthesia delivery systems, insufficient knowledge about awareness, and ignoring the use of electroencephalographic monitors when the risk is otherwise increased.