Clinical comparison of preanaesthetic intramuscular medetomidine and dexmedetomidine in domestic sheep

Medetomidine and its active d-enantiomer, dexmedetomidine, are highly selec tive alpha-2 agonists with potent sedative, anaesthetic-sparing and analges ic effects. These properties make them an ideal pre-anaesthetic medication for noxious surgical procedures. However, sheep can develop adverse hypoxae mic effects after intravenous alpha-2 agonists. Objective of the present st udy was to compare intramuscular injection of medetomidine or dexmedetomidi ne at equipotent doses as preanaesthetic medication prior to isoflurane ana esthesia in sheep. Nineteen healthy, adult, non-pregnant, female sheep of v arious breeds were used. The study was carried out as a randomised, blind t rial. Group A received 15 mug/kg bwt dexmedetomidine and group B received 3 0 mug/kg bwt medetomidine intramuscularly (i.m.) 30 minutes prior to induct ion of anaesthesia. Anaesthesia was induced with ketamine (2.0 mg/kg bwt i. v.) and maintained with isoflurane in 100% oxygen. End expired anaesthetic concentration (FEiso), respiratory frequency (f(R)), direct arterial blood pressures and heart rates (HR) were measured. Arterial blood samples were t aken at intervals. Data were averaged over time (sum of measurements/number of measurements) and tested for differences between groups by independent t-tests, and ANOVA for repeated measures followed by Bonferroni corrected t -tests. There were no differences in demographic data between the groups. D uration of anaesthesia [A: 170 (42) minutes, B: 144 (33) minutes] and durat ion of surgery [A:92 (32) minutes, B: 85 (31) minutes] were similar in both groups. Average FEiso concentrations required to maintain a surgical plane of anaesthesia were not significantly different between groups [A: 0.82 (0 .14) %; B: 1.00 (0.25)%]. Mean average f(R), did not differ between groups [A: 31 (14), B: 37 (15)]. Heart rates were significantly lower in group B o ver the course of the anaesthesia. Mean arterial blood pressures (MAP) were not significantly different between the groups. The PaO2 was less variable in group A than in group B. Individual minimum values were 19.1 kPa and 7. 9 kPa in group A and B, respectively. There were no significant differences in PaCO2 and p(a)H between the groups and over time. In conclusion, intram uscular application of dexmedetomidine at 15 mug/kg bwt and medetomidine at 30 mug/kg bwt prior to isoflurane anaesthesia induced similar changes in c linically monitored cardiorespiratory parameters. The observed differences (heart rates, PaO2) between dexmedetomidine and medetomidine at the chosen dose relationship can be considered clinically not significant. At the chos en dose rates individual animals responded with a transient drop in blood o xygenation, therefore careful monitoring is required. In addition, in compr omised sheep medetomidine and dexmedetomidine should be used carefully.