Changes in the equine EEG during surgery: The effect of an intravenous infusion of thiopentone

Spontaneous EEG changes during castration have been identified in horses anaesthetized with halothane ( Murrell et al. 1999 ). This study, using the same model, investigated the effect of thiopentone on the response of the equine EEG to surgical stimulation. Six yearling ponies, mean weight 210 ± 36 kg, were studied. Following pre‐anaesthetic medication with acetylpromazine, general anaesthesia was induced with guaiphenesin and thiopentone. Anaesthesia was maintained with halothane, F É _hal 1.2%, vaporized in oxygen and an infusion of thiopentone IV. The infusion was started 30 minutes after the induction of anaesthesia to achieve a target plasma concentration of 10 µg ml^?1. Ventilation was controlled to maintain normocapnia (PaCO₂ was measured by arterial blood gas analysis) and the EEG was recorded continuously. Baseline measurements were recorded over 5 minutes at least 10 minutes after the infusion began but before the start of surgery. Castration was defined as section of the spermatic cord. Six blood samples were taken during the baseline and castration time periods for analysis of serum thiopentone concentration by high performance liquid chromatography. The derived EEG variables median (F50) and spectral edge (F95) frequencies and total power (Atot) were examined. For each horse, the EEG data were averaged to produce a single value for F50, F95 and Atot every 30 seconds. These values, recorded during the five minutes baseline and two castration time periods were compared using repeated measures anova . Data are presented as mean ± SD The mean serum concentration of thiopentone during the infusion (23 ± 10.5 µg ml^?1) varied widely between individual animals. The F50 was significantly higher (p = 0.0001) during castration compared to the baseline period [104.4 ± 8.8% (testicle 1); 105.8 ± 13.4% (testicle 2)]. Atot decreased significantly (p < 0.0001) during castration [98.8 ± 4.4% (testicle 1); 93.7 ± 6.5% (testicle 2)]. The measured serum thiopentone concentrations were larger than the target concentration, which made the results more difficult to interpret. The ponies appeared to be divided into two groups. In four animals F50, F95 and Atot changed very little during castration compared to the baseline time period. Three of these animals had the largest serum thiopentone concentrations. In the two other animals F50 increased and Atot decreased, the changes were particularly marked in one animal. These animals had lower serum thiopentone concentrations than the first group. Compared to the previous study ( Murrell et al. 1999 ), in the two ponies which responded with EEG changes during castration, the decrease in Atot was smaller in magnitude, the increase in F50 was similar. Changes in Atot may indicate changes in the adequacy of anaesthesia ( Hall & Clarke 1991 ). An infusion of thiopentone IV did not obtund an increase in F50 but minimized changes in adequacy of anaesthesia during castration. These results support an anti‐analgesic action of thiopentone on the equine central nervous system ( Hall & Clarke 1991 ). Acknowledgements: JM is a Horserace Betting Levy Board Scholar.