Effects of guaiphenesin on the equine electroencephalogram during anaesthesia with halothane in oxygen

Objective To identify and characterize the effects of guaiphenesin (GGE) on the electroencephalogram during halothane anaesthesia. Study design Prospective controlled study. Animals Eight healthy Welsh mountain pony geldings between 5 and 9 years old and weighing between 270 and 330 kg (mean 301 kg). Methods Anaesthesia was induced with thiopentone and maintained using halothane in oxygen. End tidal halothane was maintained above 0.75 and below 0.85%. The EEG was recorded continuously and a binaural broad band click stimulus was provided throughout the experiment at 6.1224 Hz. An infusion of 1500 mg GGE was given over 5 minutes. Samples were taken for blood gas analysis and plasma GGE assay (HPLC) 5 minutes prior to the start of the infusion and at 3, 5, 7, 10, 15, 20, 30, 45 and 60 minutes thereafter. The median and 95th percentile of the EEG were calculated using standard statistical techniques and the mid‐latency of the auditory evoked response was generated. The values of EEG variables at each time point were compared to the average value for the 15 minute period before the infusion was started. Arterial blood gas values and plasma GGE concentration were compared to the baseline sample taken prior to the start of the infusion. Comparisons were made using analysis of variance for repeated measures followed by Dunnett's test if a significant difference was detected. Results The peak serum plasma concentration was 49.6 ± 7.8 μg mL^?1 (mean ± SD) occurring five minutes after the start of the infusion. The 95% spectral edge frequency (F95) of the EEG decreased by a maximum of 5.2 ± 14.3% 5 minutes after the start of the GGE infusion. This change did not reach statistical significance (p= 0.07). When three nonresponders were excluded, the depression in F95 at 5 minutes in the remaining five animals became 13.0 ± 12.0% and was statistically significant (p= 0.02). No changes were seen in median frequency of the EEG or the second differential of the middle latency auditory evoked potential. Conclusions These results did not demonstrate any statistically significant GGE‐induced changes in the EEG. However, there was some visible depression of F95 in five of the animals studied even though the dose of GGE used was considerably less than that used in most clinical circumstances. Clinical relevance The EEG effects seen in this study concur with the commonly held view that while GGE has some sedative effects, it is not a reliable anaesthetic agent.