Evaluation of the actions of general anaesthetics in the human brain

1. General anaesthesia is a state of analgesia and impaired cognitive function that results from the depressant effects of anaesthetics. 2. Graded increases in anaesthetic concentration cause a progressive impairment of cognitive function, the extremes of which are commonly described as "light" and "deep" anaesthesia. 3. Surgical stimulation produces activation in the EEG and may arouse a patient from a deeper to a lighter state of cognitive function. 4. It may be very difficult, particularly in patients with neuromuscular blockade, to identify a change in state which results in conscious awareness. 5. A range of methods has been used for evaluating depth of anaesthesia. 6. The evidence reviewed in this paper suggests that the median frequency in the EEG and the auditory evoked potential appear to be the best techniques for monitoring the graded effects of anaesthetics on the brain. 7. The median frequency, which is 10 Hz in conscious subjects, should be kept below 5 Hz during anaesthesia to ensure that there is no response to verbal command. 8. The auditory evoked potential measures the resulting effects of anaesthetic depression and surgical stimulation which is "depth of anaesthesia". 9. However it is not yet certain whether a particular feature e.g. Nb latency, or a collection of features between 20 and 80 msec gives the most reliable index of conscious awareness.