Attenuation of the stress response could be defined only poorly because of limited knowledge regarding the nature of stress. By contrast, heart rate and blood pressure are clinically measurable components of stress. Unconsciousness, which consists of amnesia and hypnosis, is also a parameter that could not be well defined. The EEG can be considered a measure of depth of anaesthesia for several reasons. It represents cortical electrical activity derived from snmmated excitatory and inhibitory post synaptic activity, which are controlled and paced by subcortical thalamic nuclei. This electrical activity has direct physiologic correlates relevant to depth of anaesthesia.