Some effects of anaesthesia on the electrical activity of the equine brain

• Main Author: Johnson, C. B.
• Published: University of Cambridge 1996
• Subjects: 636.089
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.605651

The morphology of the electroencephalogram (EEG), has long been known to change in response to anaesthetic agents. Recently, Fourier analysis has allowed these changes to be quantified in terms of the frequency composition of the EEG. This has allowed sophisticated pharmacokinetic/pharmacodynamic investigation of centrally acting agents as well as comparison of the effects of different classes of drugs on central nervous system function. Despite this, the application of the EEG as an aid to the monitoring of adequacy of anaesthesia has been disappointing. This may be due to the pharmacological complexity of modern clinical anaesthetic practice as well as to the many other variables which can affect the EEG in the clinical environment. The horse is unusual because of the high mortality rate associated with anaesthesia as well as the relative pharmacological simplicity of the anaesthetic techniques used in this species. There are few published reports of the effects of anaesthetic agents on the equine EEG, but those that do exist suggest that quantitative EEG analysis may be useful at clinical dose rates. The aims of the work submitted in this thesis were as follows:- 1) To develop and use the techniques of EEG power spectral analysis and auditory evoked potential analysis in the horse and to apply these techniques to the study of inhalational and intravenous anaesthetic agents. 2) To use the information gained from the study of individual agents to investigate the effects of combinations of intravenous anaesthetic agents on the EEG. 3) To investigate the potential of the EEG as a useful monitoring tool in the clinical situation. The thesis describes the investigation of the effects of various anaesthetic drugs on the median and spectral edge frequencies of the EEG together with the second differential of the middle latency auditory evoked potential (MLAEP) in a group of eight Welsh Mountain ponies.