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.

ABSTRACT

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 (i.v.) medetomidine (7 µg kg(-1)); anaesthesia was induced with i.v. ketamine (2.2 mg kg(-1)) and diazepam (0.02 mg kg(-1)) and maintained with isoflurane and a CRI of medetomidine (3.5 µg kg(-1) hour(-1)). Group MB (n = 31) received butorphanol CRI (25 µg kg(-1) i.v. bolus then 25 µg 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 µg 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.