To wake in fright.

Most episodes ofawareness under anaesthesia are due tofaulty apparatus or technique When ether was introduced in the 1840s as an anaesthetic many of the original patients were pain free but not unconscious. Further developments led to the recognition of a need for a concentration of anaesthetic in the brain that produced both analgesia and unconsciousness. Such a state (Guedel stage III) was invariably in use until neuromuscular blocking drugs were introduced 50 years ago. Since then it has become fashionable to reduce the inhaled concentration ofgaseous and volatile anaesthetics to minimise their other effects and to produce relaxation for surgery by neuromuscular blockade. Such "balanced anaesthesia" has led to the: problem of patients being aware of what is happening when they expected to be unconscious. The range of awareness varies from being pain free but able to recall snatches of conversation or other events during the operation to being fully awake, in intense pain and unable, because of paralysis, to communicate with anyone. 1-3 How common is it? Liu and his colleagues talked0to 1000 patients who had undergone general anaesthesia and found two patients who recalled something about their anaesthetic.4 In both, the dose and concentrations of anaesthetic agents were probably too low. From their review of some 3000 general anaesthetics for caesarean section Lyons and Mac-Donald calculated that the incidence of awareness was 1:75 when the dose of thiopentone was restricted to 4 mg/kg and the inhaled gas concentrations to 50% nitrous oxide and 0-5% halothane.' More recently, with the induction'-dose of thio-pentone increased to between 5 and 7 mg/kg and isoflurane (1%) replacing halothane-the incidence of awareness fell to-1:238. Increasing the concentration of inspired anaesthetic gas therefore reduces the risk of awareness. As Scott has pointed out the effects of volatile anaesthetics on the fetus are small6; if necessary they can be eliminated with normal methods of resuscitation. Rveviewing the records of the Medical Defence Union, Hargrove found that most episodes of awareness could be attributed either to faulty technique (70%) or to failure of apparatus (20%),7 which routine preoperative checks should detect. Awareness under general anaesthesia should not happen if sufficient brain concentrations are reached, although some patients will require much higher'than average doses of anaesthetic drugs to achieve this. Conventional clinical signs alone may not be enough to indicate that the depth of anaesthesia is too low especially inma paralysed patient. In the' past decade a …