Haemodynamic responses are routinely used for monitoring anaesthesia, but they can be elicited at spinal or brainstem levels and therefore do not provide information on cerebrocortical activity. To test the possibility that electroencephalography (EEG) might provide an objective assessment of unconsciousness and depth of anaesthesia the haemodynamic and EEG data from 17 ewes anaesthetised with halothane were analysed. Data were recorded at least every five minutes. An increase of 10 per cent or more in heart rate, systolic arterial pressure (sAP), or mean arterial pressure (MAP) was defined as a positive haemodynamic response to surgical stimulation. Simultaneously recorded EEG variables included 8-, 0-, and f-frequency band power, power ratios (0/6, a/8, 0/8), 80 per cent spectral edge frequency (SEF 80), and median frequency. In 11 of the ewes, a positive haemodynamic response was accompanied by a significant decrease in 6 activity and significant increases in a/6 ratio, P/6 ratio, and SEF 80, and there were significant correlations between the changes in heart rate, and 6 activity and P/6 ratio. In the other six ewes, there were no significant EEG responses in association with haemodynamic changes of less than 10 per cent.
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