Farm Animal Anesthesia. Группа авторов
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animals to prevent a malignant hyperthermia episode. In recent years, the development of a DNA test for malignant hyperthermia has enabled the accurate identification of the malignant hyperthermia mutation genotypes from carriers to noncarriers, resulting in the elimination of the malignant hyperthermia mutation from breeding stock, which is a significant economic gain for the pork industry [40–42].
1.6 Differences in Sensitivity to Anesthetics
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].
1.7 Preanesthetic Preparation
When possible, adult cattle should be fasted for 24–48 hours and water withheld for 24 hours before induction of anesthesia. Small ruminants, camelids, and swine should be fasted for 12–24 hours and water withheld for 8–12 hours before induction of anesthesia. Preanesthetic fasting may not completely prevent regurgitation, but it will decrease the amount of solid matter in the rumen content. Fasting also does not prevent bloating during anesthesia, but it reduces the rate of fermentation, thus reducing the amount of gas formation, the severity of bloating, and its effect on ventilation. In pigs, removal of alfalfa or other types of hay from their routine diet should be instituted 2–3 days prior to anesthesia to avoid prolonged gastric emptying time caused by this type of diet [11]. A shorter fasting period of 6–8 hours is sufficient for pigs undergoing most elective surgeries due to rapid intestinal transport times in the upper GI tract and less time required to empty the stomach [60]. Ruminants are born without a developed forestomach system and thus can be treated as monogastrics until 3 weeks of age [61]. Fasting of young ruminants less than 4 months old is not recommended because of the potential for hypoglycemia and prolonged recovery. Fasting may not be possible under emergency situations, and precautions should be taken to avoid aspiration of gastric fluid and ingesta. Prevention of regurgitation and aspiration of ruminal content can be achieved effectively by placing the animal in sternal recumbency and endotracheal intubation instituted immediately following induction. However, some practices may induce anesthesia with adult cattle already strapped to the table and in lateral recumbency. In this case, it is even more important to ensure animals are under an adequate plane of anesthesia to prevent stimulation of active regurgitation and allow immediate intubation. Regurgitation does not occur in pigs as commonly as in ruminants. However, vomiting can result from nonfasting prior to induction of anesthesia and following administration of xylazine. In general, removal of hay or alfalfa and withholding food for 12 hours and water for 6–8 hours the night before anesthesia should be sufficient for most elective surgeries [23].
In adult cattle, a 14‐gauge and 2‐ to 3‐in. needle is placed in the jugular vein for administration of IV anesthetics for induction of anesthesia and for maintenance of fluid therapy. A 14‐gauge, 5¼‐in. indwelling catheter can be used if postoperative IV medication or fluid therapy is needed. Cutdown of the skin at the catheterization site may be helpful to facilitate insertion of the catheter. A 16‐ or 18‐gauge catheter is appropriate for younger animals. The technique for IV catheterization in sheep and goats is similar to that used in calves. Venipuncture can be difficult in camelids because they have thick fiber coats and neck skin, and a less‐apparent jugular groove. The jugular vein lies deep to the sternomandibularis and brachiocephalicus muscles, ventral to the cervical vertebral transverse processes, and superficial to the carotid artery and vagosympathetic trunk within the carotid sheath for most of its length [62–66]. The jugular vein of camelids is not always visible even after occlusion of the vessels, particularly in adult males. The right internal jugular vein is the best choice for catheterization in these animals. A 14‐ or 16‐gauge indwelling catheter is appropriate for adult camelids, and an 18‐gauge catheter is suitable for younger animals. Catheters should be secured with suture or bandage. Skin cut down with a #15‐scalpel blade or a sharp 14‐gauge needle is helpful in passing the catheter into the vein [67]. An ear vein can be an alternative site for IV injection using a 25‐gauge needle or butterfly catheter to deliver a small volume of chemical restraint drugs in camelids. Also, camelids have four or five jugular valves that prevent flow of venous blood into the head when they lower the head during grazing. These valves may occlude the IV catheter and prevent backflow of the blood into the catheter, giving a false impression that the catheter may not be correctly placed in the vessel.
In swine, IV injection poses a greater challenge than in other species because pigs resist restraint and they have very few superficial veins accessible for IV injection or catheterization for administration of drugs or fluid therapy. In Vietnamese potbellied pigs, IV catheterization has been even more difficult