Methadone or Butorphanol as Pre-Anaesthetic Agents Combined with Romifidine in Horses Undergoing Elective Surgery: Qualitative Assessment of Sedation and Induction

Abstract

Simple Summary When considering sedation or general anaesthesia in horses, a multimodal strategy is commonly preferred over a single drug. This includes the association of alpha-2 adrenoceptor agonists, phenothiazines or opioids, to improve the overall sedative and analgesic effects accordingly. However, the use of opioids alone is limited in horses due to the risk of sympathetic stimulation, central nervous system stimulation, excitement and head jerking. In some countries, butorphanol is currently the only licensed and most used opioid in equine medicine. We aimed to evaluate the pre-anaesthetic association of romifidine with either butorphanol or methadone. The two combinations were administered before induction of general anaesthesia, which included diazepam and ketamine. Evaluations involved the degree of sedation and ataxia, effects on physiological parameters, such as heart and respiratory rates, rectal temperature, quality of induction, ease of intubation and the need for top-up agents before transition to the operating theatre and the institution of a maintenance regimen. Abstract While butorphanol is the most commonly used opioid in horses, methadone is not licensed in most countries. Our aim was to compare the effects of both drugs, combined with romifidine, regarding the quality of sedation and induction in horses undergoing elective surgery. Results indicate the suitability of both methadone and butorphanol in this patient population. Animals were scored 10 min after intravenous injection of sedatives. Despite lower overall sedation (OS) score in horses receiving methadone (p = 0.002), the quality and time of induction and intubation remained unchanged. None of the horses had the lowest OS score (no sedation), nor the highest score for ataxia (horse falling). Methadone induced a tendency for minor noise reaction yet minor head lowering scores, the latter being probably the most influencing parameter when scoring OS. Measured physiological parameters decreased in both groups, with greater bradycardia recorded after methadone (p = 0.017), including a higher incidence of atrioventricular blocks that resolved during general anaesthesia. The quality of induction was good–excellent in most of the animals. While comparisons between the degree of antinociception were beyond the scope of this study, analgesic potency might influence the choice when considering opioids as pre-anaesthetic drugs in combination with romifidine before surgery in equines.