Comparison of Detomidine or Romifidine in Combination with Morphine for Standing Magnetic Resonance Imaging in Horses

Simple Summary In equine patients, when possible, specific procedures are preferably performed under deep sedation to decrease the risk of mortality and morbidity associated with general anesthesia in these animals; however, the sedation protocols used must be effective and not dangerous, that is, they must not induce respiratory depression or ataxia, which are essential features of both diagnostic and surgical procedures. A constant rate infusion of different alpha-2 agonists and opioids is recommended to induce a balanced and stable level of sedation and analgesia during a surgical procedure and to avoid a possibly higher risk of over- or undersedation with a repeated single bolus. Recently, standing magnetic resonance imaging has been introduced as an advanced diagnostic equine imaging technique. During the procedure, the complete immobility of the horse is of crucial importance in order to avoid the acquisition of poor-quality and non-diagnostic images. The goal of this prospective, blind clinical trial was to compare the quality of sedation, the presence/absence of ataxia, and the effects on the respiratory and cardiac systems caused by the administration of an infusion of romifidine or detomidine with morphine at a constant rate in standing horses subjected to a low-field magnetic resonance imaging examination. This study also aimed to determine which of the two sedative protocols is more suitable for avoiding patient motion and the risk of motion artifacts.Abstract The aim of this study was to determine the most appropriate sedation protocol for a standing magnetic resonance imaging (MRI) examination in horses, comparing continuous rate infusions (CRIs) of detomidine and romifidine combined with a single bolus of morphine. Sixteen horses referred for standing low-field open-magnet MRI were randomly assigned to one of two sedation protocols. The horses were premedicated with 0.03 mg/kg of intramuscular acepromazine, and those animals belonging to Group D received an intravenous (IV) loading dose of detomidine (0.01 mg/kg) 30 min later, while those of Group R received romifidine (0.04 mg/kg). If the horses were inadequately sedated, an additional dose of IV detomidine (0.005 mg/kg) or romifidine (0.02 mg/kg) was administered, according to the animal's group. During the MRI, a single IV bolus of morphine (0.05 mg/kg) was administered, and according to which group it belonged to, the animal started the administration of detomidine (0.01 mg/kg/h) or romifidine (0.02 mg/kg/h). Heart rate (HR), respiratory rate (RR), rectal temperature (RT), depth of sedation, and degree of ataxia were evaluated every 10 min during MRI. Two horses belonging to Group D and four horses from Group R needed additional sedation before entering the MRI unit because they were unsatisfactorily sedated. No side effects were observed following morphine bolus administration. During the MRI procedure, five horses in Group R received an additional IV romifidine bolus (0.01 mg/kg) because the depth of sedation score was 1 and the ataxia score was 0. Any substantial differences were recorded between the two treatments in terms of HR, RR, and RT. In conclusion, at the doses used, a detomidine-morphine combination following a CRI of detomidine appears more suitable than a romifidine-morphine combination following a CRI of romifidine for maintaining an adequate depth of sedation and adequate immobility in horses undergoing standing MRI.