The introduction of anaesthesia using ether and chloroform in the mid-nineteenth century meant that patients were largely spared the horror of surgery whilst conscious or merely sedated. Oblivion and survival were not assured, however, when those volatile agents were administered by less skilled practitioners. Now, in the twenty-first century, anaesthesia is reassuringly safe. Although estimates show that some 0.1% of patients die during anaesthesia, mortality during this period is confounded with both the effects of surgery and the patient's state of health which may compromise survival during a procedure. Deaths caused by the anaesthetic are therefore very rare. Where deaths can be attributed to the anaesthetic, they may arise from equipment malfunction or human error, rather than the anaesthetic itself (Arnstein, 1997).

Concern does attach, however, to the effects of anaesthesia upon cognitive function. The increasing trend towards day-case surgery means that patients are admitted to hospital, anaesthetized and subjected to a surgical procedure or investigation and then discharged a few hours later. The critical issue is whether patients' cognitive functioning has recovered sufficiently at the time of discharge for them to be regarded as ‘street-fit’. As many patients do not heed advice to be cautious in their post-anaesthetic activities, there is clearly a practical imperative to establish the degree and duration of impairment after anaesthesia.

The typical methodology of studies of recovery

Most clinical studies estimate the ‘average’ recovery profile of an anaesthetic agent (in other words, the group mean response to the anaesthetic) rather than whether an individual patient has recovered their normal level of functioning.