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ACCORDING to the National Equestrian Survey of the British Equestrian Trade Association, there are around 1 million horses in Great Britain. Castration is probably the most commonly performed surgery in this large population of animals. Although the procedure can be performed in the standing sedated horse, many veterinary surgeons prefer general anaesthesia, not only because of personal safety and/or ethical reasons, but also because it provides better surgical control. A total intravenous protocol is often chosen because of the short duration of the intervention and because a large proportion of horses are castrated at home, where equipment for inhalation anaesthesia is not available. Furthermore, the Confidential Enquiry into Perioperative Equine Fatalities showed that, for short procedures, total intravenous anaesthesia carries a lower risk of mortality than maintenance of anaesthesia with an inhalant (Johnston and others 2002).
Traditionally, field castrations in horses were often performed after premedication with an α2 agonist, followed by induction of anaesthesia with a combination of thiopental and a muscle relaxant (for example, guaifenesin). However, thiopental is no longer licensed for veterinary use in several European countries, including the UK. Many veterinary surgeons therefore now use a dissociative agent to induce anaesthesia for castration of horses in the …