Clinical comparison of preanaesthetic intramuscular medetomidine and dexmedetomidine in domestic sheep
Kastner, S.B.; Boller, M.; Kutter, A.; Akens, M.K.; Bettschart-Wolfensberger, R.
Deutsche Tierärztliche Wochenschrift 108(10): 409-413
2001
ISSN/ISBN: 0341-6593 PMID: 11721587 Document Number: 526976
Medetomidine and its active d-enantiomer, dexmedetomidine, are highly selective alpha-2 agonists with potent sedative, anaesthetic-sparing and analgesic effects. These properties make them an ideal preanaesthetic medication for noxious surgical procedures. However, sheep can develop adverse hypoxaemic effects after intravenous administration of alpha-2 agonists. The objective of the present study was to compare intramuscular injection of medetomidine or dexmedetomidine at equipotent doses as preanaesthetic medication prior to isoflurane anaesthesia in sheep. Nineteen healthy, adult, nonpregnant, female sheep of various breeds were used. The study was carried out as a randomized, blind trial. Group A received 15 micro g/kg body weight dexmedetomidine and group B received 30 micro g/kg body weight medetomidine (Domitor, IM) 30 min prior to induction of anaesthesia. Anaesthesia was induced with ketamine (Narketan, 2.0 mg/kg body weight, IV) and maintained with isoflurane (Forene) in 100% oxygen. End expired anaesthetic concentration (FEISO), respiratory frequency (fR), direct arterial blood pressures and heart rates were measured. Arterial blood samples were taken 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. Duration of anaesthesia (A: 170 (42) min, B: 144 (33) min) and duration of surgery (A: 92 (32) min, B: 85 (31) min) were similar for both groups. Average FEISO concentrations required to maintain a surgical plane of anaesthesia did not differ significantly between groups (A: 0.82 (0.14) %, B: 1.00 (0.25) %). Mean average fR did not differ between groups (31 (14) for A, and 37 (15) for B). Heart rates were significantly lower in group B over the course of 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 and 7.9 kPa in groups A and B, respectively. There were no significant differences in PaCO2 and paH between the groups and over time. In conclusion, intramuscular administration of dexmedetomidine at 15 micro g/kg body weight and medetomidine at 30 micro g/kg body weight prior to isoflurane anaesthesia induces similar changes in clinically monitored cardiorespiratory parameters. The observed differences (heart rates, PaO2) between dexmedetomidine and medetomidine at the chosen dose relationship can be considered to be clinically insignificant. At the chosen dose rates, individual animals responded with a transient drop in blood oxygenation, therefore careful monitoring is required. Medetomidine and dexmedetomidine should be used carefully in compromised sheep.