title: Clinical comparison of preanaesthetic intramuscular medetomidine and dexmedetomidine in domestic sheep.
abstract: 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 pre-anaesthetic medication for noxious surgical procedures. However, sheep can develop adverse hypoxaemic effects after intravenous alpha-2 agonists. 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, non-pregnant, female sheep of various breeds were used. The study was carried out as a randomised, blind trial. Group A received 15 micrograms/kg bwt dexmedetomidine and group B received 30 micrograms/kg bwt medetomidine intramuscularly (i.m.) 30 minutes prior to induction 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 (fR), direct arterial blood pressures and heart rates (HR) 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) minutes, B: 144 (33) minutes] and duration 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 fR, did not differ between groups [A: 31 (14), B: 37 (15)]. Heart rates were significantly lower in group B over 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 paH between the groups and over time. In conclusion, intramuscular application of dexmedetomidine at 15 micrograms/kg bwt and medetomidine at 30 micrograms/kg bwt prior to isoflurane anaesthesia induced 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 clinically not significant. At the chosen dose rates individual animals responded with a transient drop in blood oxygenation, therefore careful monitoring is required. In addition, in compromised sheep medetomidine and dexmedetomidine should be used carefully.
authors.affiliation: Veterinär-Chirurgische Klinik der Universität Zürich, Universität Zürich, Winterthurerstrasse 260, CH-8057 Zürich, Schweiz. skaest@vetchir.unizh.ch
authors.forename: S B
authors.initials: SB
authors.lastname: Kastner
authors.forename: M
authors.initials: M
authors.lastname: Boller
authors.forename: A
authors.initials: A
authors.lastname: Kutter
authors.forename: M K
authors.initials: MK
authors.lastname: Akens
authors.forename: R
authors.initials: R
authors.lastname: Bettschart-Wolfensberger
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage; D018685:Anesthetics, Inhalation; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D020927:Dexmedetomidine / Q000008:administration & dosage; D005260:Female; D006339:Heart Rate / Q000187:drug effects; D006993:Hypnotics and Sedatives / Q000494:pharmacology; D007273:Injections, Intramuscular / Q000662:veterinary; D007530:Isoflurane; D020926:Medetomidine / Q000008:administration & dosage; D011229:Preanesthetic Medication / Q000662:veterinary*; D012756:Sheep / Q000502:physiology*
chemical_list: D000316:Adrenergic alpha-Agonists; D018685:Anesthetics, Inhalation; D006993:Hypnotics and Sedatives; D020927:Dexmedetomidine; D007530:Isoflurane; D020926:Medetomidine