The Effects of Dexmedetomidine Continuous Rate Infusion (CRI) on Isoflurane Anaesthesia in Healthy Horses

The concept of the modern anaesthesiological technique partial intravenous anaesthesia (PIVA) is by means of infusion of different pharmacological agents as a supplement to inhalation anaesthetics to reduce the concentration of volatile agents maintaining surgical anaesthesia and to decrease their noxious side effects mainly on cardiovascular and respiratory systems. Alpha-2 agonists are agents with frequent use in equine practice either as sedatives or in general anaesthesia PIVA protocols. The most selective amongst them, dexmedetomidine, is characterized by fewer side effects, lower doses, and fast elimination which make it appropriate for application as a continuous rate infusion (CRI). The purpose of this study was to trace out the effects of dexmedetomidine continuous rate infusion (CRI) as part of PIVA using isoflurane on volatile agent requirements, cardiovascular function, respiration and coagulation parameters, and recovery in healthy horses. Six healthy horses with average age 9.0 ± 5.1 year and mean body weight 247.7 ± 71.4 kg were subjected to either 3-hour lasting isoflurane or isoflurane-dexmedetomidine anaesthesia two weeks apart. The main clinical and anaesthesiological parameters were monitored in 10 minutes intervals. Electrolytes, acid-base, blood gases, and coagulation parameters were measured at the beginning and at the end of each anaesthesia. Recovery times and qualities were also recorded. The results showed that, the addition of dexmedetomidine by CRI at 1.75 μg·kg-1·hour-1 to isoflurane anaesthesia slightly reduced isoflurane requirement for maintenance of surgical anaesthesia but had negligible effects on the recovery time and quality. PIVA using dexmedetomidine and isoflurane produced respiratory acidosis similar to isoflurane anaesthesia alone but with significantly more pronounced hypoxaemia and hyperlactaemia. Both investigated anaesthesia protocols did not influence significantly haemocoagulation parameters.