Veterinary anaesthesia and analgesia2016; 27(2); 82-88; doi: 10.1046/j.1467-2995.2000.00022.x

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

Johnson, CB
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Bloomfield, M
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Taylor, PM

Abstract: To characterise the effects of thiopentone on the equine electroencephalogram during halothane anaesthesia. Methods: Prospective controlled study. Methods: 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%. EEG was recorded continuously and a binaural broad band click stimulus was provided throughout the experiment at 6.1224 Hz. An infusion of 500 mg thiopentone was given over 5 minutes. Samples were taken for blood gas analysis and plasma thiopentone assay (HPLC) 5 minutes prior to the start of the infusion and at 3, 5, 7, 10, 15, 20, 30, 45 and 60 minutes. 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 thiopentone 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 14.5 ± 2.4 μg mL (mean ± SD) occurring 5 minutes after the start of the infusion. The 95% spectral edge frequency of the EEG decreased by a maximum of 27.4 ± 18.4% 7 minutes after the start of the thiopentone infusion. No changes were seen in median frequency of the EEG or the second differential of the middle latency auditory evoked response. Conclusions: These results, coupled with the lack of antinociceptive action of thiopentone, support the hypothesis that median frequency of the EEG may be a useful indicator of nociception in anaesthetized animals. Conclusions: If the EEG is to become a useful monitoring technique then it is important to understand the relative contribution of changing plasma concentrations of the agents used in anaesthesia.

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Publication Date: 2016-11-16 PubMed ID: 28404058DOI: 10.1046/j.1467-2995.2000.00022.xGoogle Scholar: Lookup

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Summary

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This research explores the effects of the drug thiopentone on the brain activity (measured by an electroencephalogram or EEG) of horses while under halothane anaesthesia. It suggests that analysis of changes in EEG frequency could improve our understanding of pain perception in anesthetized animals.

Methodology

The researchers conducted a prospective controlled study involving eight healthy Welsh mountain pony geldings aged between 5 and 9 years, and weighing between 270 and 330 kg.
Anaesthesia was induced using thiopentone and maintained with halothane in oxygen. The end tidal halothane level was kept between 0.75% and 0.85%.
An EEG was used to continuously monitor the ponies’ brain activity. A broad band click stimulus was consistently provided during the experiment at a frequency of 6.1224Hz.
An infusion of 500mg thiopentone was administered over a period of 5 minutes.
Samples for gas analysis and plasma thiopentone assay (through high-performance liquid chromatography) were collected at various intervals, beginning 5 minutes before the infusion and concluding 60 minutes after.
The median and 95th percentile of the EEG were calculated using standard statistical techniques and mid-latency of auditory evoked response was generated.

Results

The peak plasma concentration of thiopentone was 14.5 ± 2.4 μg mL, occurring 5 minutes after the start of the infusion.
The 95% spectral edge frequency of the EEG decreased by a maximum of 27.4 ± 18.4% 7 minutes after the thiopentone infusion commenced.
No changes were detected in median frequency of the EEG or in the second differential of the middle latency auditory evoked response.

Conclusion

The observed effects of thiopentone on EEG readings, along with its lack of antinociceptive (pain-blocking) action, propose that median EEG frequency might be a valuable indictor of pain perception in animals under anaesthesia.
In order to utilise EEG as a viable monitoring technique, it’s crucial to understand the effects of changing plasma concentrations of the agents used in anaesthesia.

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APA

Johnson CB, Bloomfield M, Taylor PM. (2016). Effects of thiopentone on the equine electroencephalogram during anaesthesia with halothane in oxygen. Vet Anaesth Analg, 27(2), 82-88. https://doi.org/10.1046/j.1467-2995.2000.00022.x