Changes in the EEG during castration in horses and ponies anaesthetized with halothane

To identify changes in the amplitude spectrum of the electroencephalogram (EEG) during a standardized surgical model of nociception in horses. Thirteen entire male horses and ponies referred to Division of Clinical Veterinary Science, Bristol (n = 9) and Department of Clinical Veterinary Medicine (n = 4) for castration. Following pre–anaesthetic medication with acepromazine, anaesthesia was induced with guaiphenesin and thiopental and maintained with halothane in oxygen. The EEG was recorded continuously using subcutaneous needle electrodes. Additional monitoring comprised ECG, arterial blood pressure, blood gas analysis, airway gases, and body temperature. All animals were castrated using a closed technique. The raw EEG was analysed after completion of each investigation and the EEG variables median frequency (F50), spectral edge frequency (SEF) 95% and total amplitude were derived from the spectra using standard techniques. The mean values of EEG variables recorded during a baseline time period (recorded before the start of surgery) and castration of each testicle were compared using analysis of variance for repeated measures. Total amplitude (Atot) decreased and F50 increased during castration of each testicle compared to the baseline time period [(89.0 ± 7.8% testicle 1, 87.0 ± 7.8% testicle 2) and (110.0 ± 15.0% testicle 1, 109.0 ± 15.0% testicle 2), respectively]. Changes in SEF 95% were not significant. De–synchronization was identified in the EEG during the nociceptive stimulus of castration. The results suggest that an increase in F50 may be a specific marker for nociception in the horse. Studies investigating the efficacy of analgesic agents in horses are limited by difficulties in peri–operative pain assessment. This model, using EEG changes associated with nociceptive stimulation, can be used to investigate the anti–nociceptive efficacy of different anaesthetic agents in the horse.