Clinical usefulness of intravenous constant rate infusion of fentanyl and medetomidine under sevoflurane anesthesia in Thoroughbred racehorses undergoing internal fixation surgery

A total of 20 racehorses with longitudinal fractures underwent internal fixation surgery under sevoflurane anesthesia combined with infusion of medetomidine (3 µg/kg/hr) alone (10 horses, group M) or medetomidine and fentanyl (7 µg/kg/hr) (10 horses, group FM). In group FM, the end-tidal sevoflurane concentration during surgery was maintained significantly lower than in group M (2.8-2.9% for group M vs. 2.2-2.6% for group FM, P<0.01). The mean arterial blood pressure was maintained over 70 mmHg using dobutamine infusion (group M, 0.36-0.54 µg/kg/min; group FM, 0.27-0.65 µg/kg/min), and the recovery qualities were clinically acceptable in both groups. In conclusion, co-administration of fentanyl and medetomidine by constant rate infusion may be a clinically useful intraoperative anesthetic adjunct for horses to reduce the requirement of sevoflurane when they undergo orthopedic surgery.