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The effect of anaesthetic agents on primary cortical evoked responses.

(1) The effect of increasing dosage of ethyl carbamate (urethane), trichloroethylene and pentobarbitone anaesthesia is to decrease die amplitude, increase the latency and produce a more stereotyped response, recorded from the short latency receiving area of the cerebral cortex to electrical or mechanical limb stimulation in both chronic and acute animals. (2) The extent of the short latency receiving area (defined as that from which responses with the shortest latency +12.5% of this latency could be obtained) increased in extent from the lightly to the deeply anaesthetized state. (3) The only consistent non-nervous change produced by these anaesthetic agents was a decrease in arterial Po2 as the anaesthetic depth was increased. (4) That cerebral hypoxia was not the cause of the changes seen in the evoked response was shown by the fact that the effect of the anaesthetic agents was the same in animals respiring 95% O2, 5% CO2 or in animals artificially respired. (5) It is concluded that the anaesthetic agents exert their effects on a central nervous structure. (6) Animals with cerebral oedema appear to be anaesthetic resistant. No change in the evoked cortical response was seen in any animal which showed the slightest signs of cerebral oedema.