Читать книгу Farm Animal Anesthesia - Группа авторов - Страница 19

Xylazine is a potent sedative, analgesic, and muscle relaxant that is frequently used as a preanesthetic or anesthetic adjunct in farm animal species. Cattle are more sensitive to xylazine than horses, and they require only one‐tenth of the dose needed in horses to produce equipotent sedation [43]. Apparently, there are differences in the level of sensitivity to xylazine among breeds and species of these animals. It appears that Brahmans are the most sensitive, Herefords intermediate, and Holsteins are the least sensitive [44, 45]. Small ruminants are more sensitive to xylazine than camelids, whereas goats tend to be more sensitive than sheep and llamas are more sensitive than alpacas. Administration of xylazine to pregnant ruminants in the final trimester may cause premature parturition and retention of fetal membranes [46, 47]. In pregnant dairy cows during late gestation, intravenous (IV) administration of xylazine (0.04 mg/kg) resulted in a significant increase in uterine vascular resistance (118–156%) and a decrease in uterine blood flow (25–59%), which were accompanied by a drastic decrease in O₂ delivery to the fetus (59%) [48]. Therefore, the use of xylazine during late gestation in pregnant ruminants is not recommended to avoid detrimental effects to the fetus. Fayed et al. [49] observed pronounced and prolonged response when xylazine was administered to cattle under high ambient temperature. Interestingly, camelids are less sensitive to xylazine than ruminants, thus higher doses are required to produce a similar degree of sedation as in ruminants. In addition, the dose requirement is higher for alpacas than llamas. Compared to other farm animal species, pigs are the least sensitive to xylazine and other α₂ agonists. These drugs when used alone in pigs are not effective in producing adequate sedation. Vomiting has been observed following the administration of xylazine to pigs with digestive disturbances [11]. In addition to α₂ agonists, pigs are also less sensitive to the pharmacologic effects of opioids [50, 51]. Benzodiazepines, for example diazepam and midazolam, seem to produce reliable sedation in pigs even at doses that do not produce effective sedation in other species [52].

In regard to α₂ antagonists, ruminants and camelids are more sensitive to tolazoline than other species [53, 54]. When administered intravenously alone at 1.5 mg/kg to nonsedated Holstein calves, tolazoline caused coughing, increased frequency of defecation, and a mild increase in breathing effort. At higher IV doses from 2 to 10 mg/kg, adverse effects including bright red conjunctival mucous membrane, coughing, nasal discharge, profuse salivation, labored breathing, central nervous system depression, signs of abdominal pain, straining, head pressing, restlessness, and severe diarrhea were observed. However, there were no long‐lasting adverse effects observed in those calves [54]. Currently, lower doses of tolazoline at 0.5–1.5 mg/kg IV are recommended for use in all ruminants, including camelids. Others have suggested that IV administration of tolazoline should be avoided, except in emergency situations, to prevent adverse effects such as cardiac asystole [55].

There are concerns from potbellied pig owners and breeders regarding the statement that “injectable anesthetics should not be used in young pigs” and that “ketamine in particular should not be used in potbellied pigs of any age” [56]. These statements have never been proven or supported by controlled, scientific studies. Furthermore, the clinical experiences of this author and of most practicing veterinarians indicate otherwise.

Ruminants recover gradually but smoothly from Telazol anesthesia as a result of the slower metabolism and longer‐lasting effect of zolazepam [57, 58]. Pigs, on the other hand, often experience prolonged and stormy recovery characterized by swimming and paddling with repeated attempts to right themselves when recovering from Telazol anesthesia, similar to that observed when ketamine was used alone [52, 59]. Studies have shown that tiletamine and zolazepam are both eliminated more slowly in pigs than in other species [59] and tiletamine apparently outlasted zolazepam in pigs [57].