Comparison of the effects of halothane, isoflurane and methoxyflurane on the electroencephalogram of the horse.

The study examines the effects of various concentrations of three anaesthetic agents – halothane, isoflurane and methoxyflurane – on the electroencephalogram (EEG) frequencies and evoked potential of horses, suggesting potential differences in their mechanisms of action.

Study Design and Parameters

• Eight ponies were examined under the effects of three different end-tidal concentrations of halothane, isoflurane and methoxyflurane. The ‘end-tidal concentration’ refers to the concentration of a gaseous substance (in this case, the anaesthetics) at the end of an exhalation.
• The three concentrations used for each agent were intended to match the minimum alveolar concentration (MAC), 1.25 times the MAC and 1.5 times the MAC, for each agent. The MAC is the concentration of an anaesthetic in the lungs that is needed to prevent a response to a painful stimuli in 50% of patients.
• The main parameters of the EEG measured were the median (F50) and the 95% spectral edge frequencies (F95). Additionally, researchers also studied the second differential (DD) of the middle latency auditory evoked potential (MLAEP), which is a measure of how neural pathways respond to sound stimuli.

Findings

• During halothane anaesthesia, the F95 value decreased progressively as the concentration of halothane increased, while the DD was lower at higher concentrations of the anaesthetic. There were no significant changes in the F50 value.
• During isoflurane anaesthesia, there was a small but significant increase in the F95 value between the intermediate and highest concentrations, with no changes detected in the F50 and DD values. The F95, F50 and DD values at all isoflurane concentrations were similar to those of halothane at the highest concentration.
• During methoxyflurane anaesthesia, both F95 and F50 values decreased as the concentration of methoxyflurane increased, while DD was lower at higher concentrations. F95, F50 and DD at all methoxyflurane concentrations were higher than those seen with halothane at the lowest concentration.

Implications

• The findings suggest differences in the mechanisms of action of the three anaesthetic agents, indicated by the different relative positions of the dose-response curves for EEG and MLAEP changes compared with antinociception (MAC) changes.
• This divergence may explain the incomplete adherence to the Meyer-Overton rule, which states that the potency of a gaseous anaesthetic is directly proportional to its lipid solubility.