Measurement of depth of anaesthesia.

It is a fitting place to begin an essay on "Measurement of depth of anaesthesia" with reference to the work of John Snow. He was the first qualified physician to devote his full time to anaesthesia and published his monograph On the Inhalation of the Vapour of Ether in 1847. He described the progression from conscious normality to respiratory paralysis with ether and quoted five recognizable stages. He deduced that the patient was completely ready for operation when all excitomotor activity had ceased, when muscular relaxation was present and when the breathing became automatic and regular, his fourth stage of etherization. In. patients who did not show much muscular rigidity in the third stage, he allowed operation to begin and anticipated signs such as moaning, flinching and movement which, although they were signs of pain, did not interfere with operation or reach consciousness. He was fully aware that the only reason for carrying anaesthesia deeper than the third stage was to control muscle rigidity and involuntary movement which might interfere with operation, and he often maintained anaesthesia in the amnesic stage where full analgesia was present.

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