Abstract
Ketamine-based total intravenous anaesthesia techniques are commonly used in equine practice for ponies requiring short procedures such as castration in field conditions. When a longer duration of recumbency than provided by the initial dose of anaesthetic agents is required, administration of supplementary ‘top-up’ doses of anaesthetic agents is required. This can be challenging in field conditions where available personnel and equipment are typically limited. Ideally, a single dose of anaesthetic agents would reliably achieve a longer duration of action whilst maintaining adequate anaesthetic, surgical and recovery qualities. Acceptable qualities are important for the safety of personnel and the ponies undergoing anaesthesia. This prospective, randomised and blinded study aimed to compare the UK-licensed induction dose of ketamine with an increased dose in ponies undergoing castration in field conditions. The hypothesis was that an increased dose would produce a longer duration of action without negatively affecting qualities of anaesthesia, surgical conditions and recovery. Ponies were randomly allocated to receive ketamine at either 2.2 mg kg-1 (K2.2) or 3 mg kg-1 (K3) combined with diazepam 20 μg kg-1 following pre-anaesthetic medication with romifidine and butorphanol. Quality of anaesthesia, surgery and recovery were scored using simple descriptive scales (SDS) and timings of key events recorded. Top-up doses of ketamine 0.5 mg kg-1 were administered if anaesthesia was inadequate during surgery. Time of top-up doses and total ketamine doses were recorded. Data were analysed using Student t-tests or the Mann-Whitney U test (p < 0.05). Thirty-six ponies completed the study. Six ponies enrolled were excluded due to cryptorchidism or surgical complications that required deviation from the anaesthetic protocol. There were no differences in timing of events recorded, number of ponies requiring top-up ketamine, timing and frequency of top-ups or total ketamine dose. Scores for anaesthetic and recovery qualities, and surgical conditions were similar between groups. Both induction doses of ketamine provided a similar duration of action and provided conditions suitable to anaesthetise ponies undergoing castration.

Key words: castration, equine anaesthesia, ketamine