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

Authors

  • Joanna C Murrell BVSc, PhD, CVA,

    1. Department of Clinical Sciences of Companion Animals, University Utrecht, Faculty of Veterinary Medicine, PO Box 80154, NL-3508 TD, Utrecht
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  • Craig B Johnson BVSc, PhD, DVA, MRCA, Diplomate ECVA,

    1. Institute of Veterinary, Animal Biomedical Sciences; College of Sciences, Massey University, Private Bag 11-222, Palmerston North, New Zealand
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  • Kate L White MA, VetMB, DVA, Diplomate ECVA,

    1. Veterinary Anaesthesia Services, 57 Rectory Lane, Chelmsford, Essex, CM1 1RE, UK
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  • Polly M Taylor MA, VetMB, PhD, DVA, Diplomate ECVA,

    1. Taylor Monroe, Gravel Head Farm, Downham Common, Little Downham, Nr Ely, Cambridge, UK
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  • Zainal L Haberham MSc, PhD,

    1. Department of Clinical Sciences of Companion Animals, University Utrecht, Faculty of Veterinary Medicine, PO Box 80154, NL-3508 TD, Utrecht
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  • Avril E Waterman-Pearson BVSc, PhD, FRCVS, DVA, MRCA, Diplomate ECVA

    1. Department of Clinical Veterinary Science, University of Bristol, Langford House, Langford, Bristol, UK
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Joanna C. Murrell, Department of Clinical Sciences of Companion Animals, University Utrecht, Faculty of Veterinary Medicine, PO Box 80154, NL-3508 TD, Utrecht. E-mail: J.C.Murrell@vet.uu.nl

Abstract

Objective  To identify changes in the amplitude spectrum of the electroencephalogram (EEG) during a standardized surgical model of nociception in horses.

Animals  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.

Materials and methods  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.

Results  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.

Conclusions  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.

Clinical relevance  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.

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