Control of painful conditions is becoming an increasingly important part of veterinary medicine. Many of the available analgesics are considered effective for a variety of painful conditions in horses, but their use may be limited for various reasons. The development of unwanted side effects are a major factor that limits their use. Tramadol has only recently gained significant attention as an analgesic in animals despite its having been used in humans in Germany since 1977. Tramadol hydrochloride, (1RS,2RS)-2-[(dimethylamino)-methyl]-1-(3- methoxyphenyl)-cyclohexanol hydrocloride, is a centrally acting analgesic that has been used clinically for the two decades to treat acute and chronic pain in humans. Tramadol produces its antinociceptive effect in animals and analgesic effect in humans by both opioid and non-opioid mechanism of action. Tramadol is available as a racemic mixture composed of (+) and (-) enantiomers in equal proportions. The racemic mixture produces analgesia by synergistic action of two enantiomers and their main O-desmethylated metabolites (M1). In fact (+)M1 has been demonstrated to have an affinity to μ-opioid receptors that is 200 times greater than that of the parent compound. On the other hand, the monoaminergic component in tramadol analgesia is mediated by (-)tramadol, by means of an inhibition of the re-uptake of the neurotransmitters serotonina and norepinephrine. The therapeutic use of tramadol has not been associated with significant adverse effects, such as respiratory depression, constipation or sedation, and the drug has been used successfully in humans and many animals to produce analgesia in peri-operative and post-operative periods. The pharmacokinetics of tramadol have been investigated in several animal species and recently also in horse. After IV administration at a dose of 2.0 mg/kg in only two horses muscle twitching were observed and physical examination revealed decreased gastrointestinal sounds. Moreover, there are two studies that showed good analgesic effect after epidural administration of tramadol to horses without any significant influence on heart rate, respiratory rate, arterial blood pressure, body temperature and behaviour. Caudal epidural administration of tramadol was compared with morphine and both are potential drugs in management of perineal and lumbosacral pain in horse The purpose of this study was to investigate the effects and tolerability of systemically administered tramadol at two different dosages (1.0 mg/kg EV and 2.0 mg/kg EV) in the horse. Heart rate, respiratory rate, arterial blood pressure, body temperature, central nervous system excitement, head ptosis, arterial pH, PaO2 and PaCO2 were measured during 12-hour period and the different was statistically evaluated. Analgesia was not documented. Results of this study suggest that tramadol can be administered to horse systematically at a dose of 2.0 mg/kg.
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