British Journal of Anaesthesia, Vol 81, Issue 5 748-753, Copyright © 1998 by The Board of Management and Trustees of the British Journal of Anaesthesia
C. B. Johnson and P. M. Taylor
We have investigated in eight ponies the effects of three different end-
tidal concentrations of halothane, isoflurane and methoxyflurane on median
(F50) and 95% spectral edge (F95) frequencies of the EEG and the second
differential (DD) of the middle latency auditory evoked potential (MLAEP).
The three concentrations of each agent were chosen to represent
approximately the minimum alveolar concentration (MAC), 1.25 MAC and 1.5
MAC for each agent. During halothane anaesthesia, F95 decreased
progressively as halothane concentration increased, from mean 13.9 (SD 2.6)
at 0.8% to 11.9 (1.1) at 1.2%. DD was lower during anaesthesia with the
highest concentration (21 (6.5)) compared with the lowest (27.6 (11.4)).
There were no significant changes in F50. During isoflurane anaesthesia,
there was a small, but significant increase in F95 between the intermediate
and highest concentrations (10.2 (1.5) to 10.8 (1.6)). There were no
changes in F50 and DD. Values of F95, F50 and DD at all isoflurane
concentrations were similar to those of halothane at the highest
concentration. During methoxyflurane anaesthesia, F95 and F50 decreased
progressively as methoxyflurane concentration was increased, from 21.3
(0.7) and 6.5 (1), respectively, at 0.26%, to 20.1 (0.6) and 5.6 (0.8),
respectively, at 0.39%. DD was lower during anaesthesia with the highest
concentration of methoxyflurane (25.7 (7.8)) compared with the lowest (39.7
(20.6)). Values of F95, F50 and DD at all methoxyflurane concentrations
were higher than those seen with halothane at the lowest concentration. The
different relative positions of the dose-response curves for EEG and MLAEP
changes compared with antinociception (MAC) changes suggest differences in
the mechanisms of action of these three agents. These differences may
explain the incomplete adherence to the Meyer-Overton rule.
CLINICAL INVESTIGATIONS
Comparison of the effects of halothane, isoflurane and methoxyflurane on the electroencephalogram of the horse
Department of Clinical Veterinary Science, University of Bristol, Langford House, Langford, Bristol BS18 7DU; Department of Clinical Veterinary Medicine, University of Cambridge, Madingley Rd, Cambridge CB3 0ES
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