Farm Animal Anesthesia. Группа авторов

effects by their actions on both the central and peripheral α₂ adrenoceptors. Stimulation of central (presynaptic) α₂ adrenoceptors inhibits the release of catecholamines, thus reducing the response to excitatory input, and as a result sedation occurs. Peripheral (postsynaptic) α₂ receptors are found in the vasculature, pancreatic islet cells, and uterine muscles. As a result, transient hypertension, hypoinsulinemia, hyperglycemia, and oxytocin‐like effect are often associated with the administration of an α₂ agonist [33]. Other side effects associated with the administration of α₂ agonists include direct myocardial depression and augmentation of parasympathetic stimulation resulting in a decrease in cardiac output, bradycardia, and hypotension. Up to a sixfold increase in urine output subsequent to a decrease in secretion of antidiuretic hormone is a common side effect of α₂ agonists. The central muscle‐relaxing effect produced by α₂ agonists is believed to be mediated through the inhibition of nerve impulse transmission at the internuncial neurons of the spinal cord, brain stem, and subcortical level of the brain [34]. Because of this, α₂ agonists are often given in combination with anesthetics that do not provide adequate muscle relaxation for surgical procedures. For example, when ketamine is administered alone, it is often associated with muscle tremors, jerking activity, and rigidity; xylazine is administered concurrently to improve the muscle relaxation during ketamine anesthesia. All α₂ agonists, though considered as pure α₂ agonists, also have affinity for α₁ receptors. The α₂ : α₁ selectivity ratios for xylazine, detomidine, romifidine, and medetomidine/dexmedetomidine are 160 : 1, 260 : 1, 340 : 1, and 1620 : 1, respectively [35, 36].

      2.3.1 Cattle

      2.3.1.1 Xylazine

Xylazine is the most popular sedative in large animal practice today. Cattle are much more sensitive to xylazine than horses and require only one‐tenth of the dose needed for horses to produce the same degree of sedation [33]. The degree of sensitivity to xylazine varies within breeds, and Brahmans appear to be the most sensitive, Herefords intermediate, and Holsteins are the least sensitive [37]. Xylazine produces potent sedation, profound analgesia, and good muscle relaxation. It is frequently used for chemical restraint or anesthetic adjunct in ruminants. Xylazine alone produces dose‐dependent CNS depression from standing sedation (0.015–0.025 mg/kg IV or IM) [21, 38] to recumbency and immobilization (0.1 mg/kg IV or 0.2 mg/kg IM) [39]. Administration of xylazine to ruminants in the final trimester of pregnancy may cause premature parturition and retention of fetal membranes [40, 41]. In pregnant dairy cows during late gestation, administration of xylazine (0.04 mg/kg IV) resulted in a significant increase in uterine vascular resistance (118–156%) and a decrease in uterine blood flow (25–59%) accompanied by a drastic decrease in fetal O₂ delivery (59%) [42]. Due to these detrimental effects on the fetus, the use of xylazine during late gestation in pregnant cows is not recommended. Fayed et al. [43] observed pronounced and prolonged drug effects when xylazine was administered to cattle under high ambient temperature. Xylazine should be used with extreme caution in animals with preexisting cardiopulmonary disease or urinary tract obstruction due to its adverse effects on the myocardium and urine output [33]. Higher dose of xylazine (single average dose, 0.55 ± 0.18 mg/kg) delivered by tranquilizer gun has been used to produce complete immobilization to capture free‐ranging cattle [44]. Xylazine is often used with butorphanol to produce neuroleptanalgesia. Enhanced sedation and analgesia develop when these two drugs are administered concurrently. Administration of high doses of butorphanol alone to nonpainful cattle may induce slight CNS stimulation and behavioral changes. Thus, when used in combination with xylazine, it is recommended the dose of butorphanol be maintained below 0.05 mg/kg to avoid butorphanol‐induced CNS excitation offsetting the sedative effect of xylazine [45]. Detailed discussion of chemical restraint techniques using xylazine combinations is described in Chapter 3.

      Epidural administration of xylazine to standing cattle produced effective perineal analgesia for 2.5–4 hours. Compared to epidural lidocaine, xylazine produced less disruption of hind limb motor function and provides a longer duration of perineal analgesia [46, 47]. Systemic effects like mild to moderate sedation and slight ataxia sometimes occur following caudal epidural administration of xylazine, which is a result of absorption of the drug into blood circulation from the injection site and/or diffusion of the drug into cerebrospinal fluid (CSF) with subsequent cranial migration of the drug into the CNS. Similarly, studies in humans [48] and dogs [49] showed that diffusion of epidurally administered morphine into the CSF and the subsequent migration of the drug up the spinal cord, rather than the total injected drug volume, were the primary factors responsible for the widespread analgesia of epidural morphine. IV administration of an α₂ antagonist such as tolazoline reversed the systemic effects (sedation and ataxia) but did not affect the caudal epidural analgesia of xylazine [50]. It is believed that the epidural analgesia of xylazine is the result of the binding of xylazine to the α₂ adrenoceptors located in the dorsal horn of the spinal cord, not the effect of xylazine on the central α₂ adrenoceptors in the CNS [51, 52]. Therefore, IV or IM administration of an α₂ antagonist does not affect the binding of an α₂ agonist to the receptors in the epidural space due to low concentration of the α₂ antagonist in the epidural space.

      2.3.1.2 Detomidine

Detomidine has pharmacologic effects that are very similar to those of xylazine. Interestingly, ruminants appear to be less sensitive to detomidine than they are to xylazine. The dose of detomidine required to produce standing sedation is similar to the dose required for horses. When administered at 0.05 mg/kg IV or IM to adult cattle, detomidine produced effective sedation, though the analgesic effect appeared to be more intense when the drug was administered intravenously [53]. Standing sedation of 30–60 minutes was evident with 0.025–0.01 mg/kg IV of detomidine [38, 54, 55]. Unlike xylazine, detomidine does not produce an oxytocin‐like effect on the uterus in gravid cattle at IV doses less than 0.04 mg/kg. However, doses greater than 0.04 mg/kg were observed to cause increased electrical activity of the uterine muscle without inducing synchronizing burst potentials characteristics of parturition [56, 57]. This indicates that detomidine is less likely to induce premature parturition in pregnant ruminants at recommended doses and this may make it safer for use in pregnant ruminants. Slight sedation without analgesic effect was observed when 0.01 mg/kg of IM detomidine was given to Lanka buffaloes. Moderate sedation with analgesia was observed at 0.02 mg/kg, while deep sedation with excellent analgesia occurred with 0.04 mg/kg. However, 50% of the animals receiving 0.04 mg/kg became recumbent. When increasing the dose to 0.08 mg/kg, detomidine induced recumbency and complete immobilization in these buffaloes. The author of the study concluded that 0.02 and 0.04 mg/kg of detomidine induced adequate sedation and analgesia that suffices most clinical and practical purposes [58]. At higher doses (0.08–0.1 mg/kg IM), detomidine induced complete immobilization and excellent muscle relaxation, but regurgitation and subsequent aspiration pneumonia could be a risk if the animal's airway is not protected [58, 59]. In dairy cattle, IV detomidine alone (0.1 mg/kg) induced moderate sedation with significant decreased heart rate and respiratory rate for 39–55 minutes. Similar to xylazine and butorphanol administered to Holstein cows as mentioned previously, CNS excitation produced by butorphanol (0.05 mg/kg IV) seemed to offset the sedative effect of detomidine (0.1 mg/kg IV) when the two drugs were administered concurrently [45].

      Caudal epidural administration of detomidine (0.04 mg/kg) induced perineal analgesia within 5 minutes following administration and lasted for 175 minutes [60]. In horses, caudal epidural detomidine has a slightly faster onset and shorter duration of perineal analgesia than xylazine, 12.5 ± 2.7 and 160 ± 8 minutes versus 13.1 ± 3.7 and greater than 165–180 minutes, respectively [61].

      Recently, sublingual detomidine gel with a concentration of 7.6 mg/ml in a 3‐ml syringe is available for horses to be used in the field or on the farm. When administered sublingually to horses, the oral bioavailability was 22% and the peak plasma concentration was approximately 40% that of the plasma concentration following IM administration. A mild to moderate degree of sedation occurred within 30–40 minutes and lasted 90–180 minutes [62, 63]. Minor procedures like grooming or examination