Introduction
Depth of anaesthesia is usually assessed from a combination of measures including the presence or absence of responses to painful stimuli, changes in the pattern and depth of respiration, changes in muscle tone, and changes in heart rate and blood pressure. More sophisticated techniques for assessment of depth of anaesthesia have been developed for use in people, for example, measurement of the electroencephalogram (EEG) and of sensory or somatic evoked potentials. Although these techniques are not yet widely applied in animals they may be of value in some circumstances, for example during prolonged periods of anaesthesia and when using neuromuscular blocking drugs.