A clinical study on the effect in horses during medetomidine–isoflurane anaesthesia, of butorphanol constant rate infusion on isoflurane requirements, on cardiopulmonary function and on recovery characteristics

Objective To test if the addition of butorphanol by constant rate infusion (CRI) to medetomidine– isoflurane anaesthesia reduced isoflurane requirements, and influenced cardiopulmonary function and/or recovery characteristics. Study design Prospective blinded randomised clinical trial. Animals 61 horses undergoing elective surgery. Methods Horses were sedated with intravenous (IV) medetomidine (7 lg kg)1); anaesthesia was induced with IV ketamine (2.2 mg kg)1) and diazepam (0.02 mg kg)1) and maintained with isoflurane and a CRI of medetomidine (3.5 lg kg)1 hour)1). Group MB (n = 31) received butorphanol CRI (25 lg kg)1 IV bolus then 25 lg kg)1 hour)1); Group M (n = 30) an equal volume of saline. Artificial ventilation maintained end-tidal CO2 in the normal range. Horses received lactated Ringer’s solution 5 mL kg)1 hour)1, dobutamine <1.25 lg kg)1 minute)1 and colloids if required. Inspired and exhaled gases, heart rate and mean arterial blood pressure (MAP) were monitored continuously; pH and arterial blood gases were measured every 30 minutes. Recovery was timed and scored. Data were analyzed using two way repeated measures ANOVA, independent t-tests or Mann–Whitney Rank Sum test (p < 0.05). Results There was no difference between groups with respect to anaesthesia duration, end-tidal isoflurane (MB: mean 1.06 ± SD 0.11, M: 1.05 ± 0.1%), MAP (MB: 88 ± 9, M: 87 ± 7 mmHg), heart rate (MB: 33 ± 6, M: 35 ± 8 beats minute)1), pH, PaO2 (MB: 19.2 ± 6.6, M: 18.2 ± 6.6 kPa) or PaCO2. Recovery times and quality did not differ between groups, but the time to extubation was significantly longer in group MB (26.9 ± 10.9 minutes) than in group M (20.4 ± 9.4 minutes). Conclusion and clinical relevance Butorphanol CRI at the dose used does not decrease isoflurane requirements in horses anaesthetised with medetomidine– isoflurane and has no influence on cardiopulmonary function or recovery.