Injectable Anesthesia and Analgesia of Birds by J. Paul ... - Ufersa

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muscle mass is minimal, thus the thigh muscles are preferred. Subcutaneous injections are not advocated because uptake of anesthetic is slowed, but if selected the recommended site is the inguinal region. Intravenous sites for injection or catheterization include the right jugular vein, brachial vein, or the medial metatarsal vein. Intraosseous catheters are useful when venous access is difficult, as occurs with hypotensive birds or very small birds. Intraosseous catheters can be placed in the proximal ulna or the cranial tibiotarsus. Uptake of the drug is comparable to intravenous injection of the drug [3]. Local Anesthetics The toxicity of lidocaine is similar for birds as it is for mammals and it has been reported to cause seizures and cardiac arrest [4]. Toxicity can be prevented by using appropriate concentrations and volumes. Lidocaine (1 - 2 mg/kg) can be used as a local anesthetic or to treat ventricular arrhythmias [4], and the maximal dose is 4 mg/kg. For the small avian patient this often requires that the stock concentration of lidocaine (2%; 20 mg/ml) be diluted. Because of reluctance to use local anesthetics in birds, information regarding long-acting local anesthetics, such as bupivacaine, is sparse. Topical benzocaine has been used for local analgesia during repair of minor wounds in small birds [5]. A 1:1 mixture of bupivacaine and dimethyl sulfoxide (DMSO) was applied to amputated chicken beaks immediately after amputation and feed intake was improved [6]. Intraarticular bupivacaine, at a dosage of 3 mg in 0.3 ml saline, was reported to be effective for treating arthritic pain in chickens [7]. Benzodiazepines Diazepam and midazolam can reduce anxiety during anesthetic induction and recovery. These sedatives work best if given 10 - 20 min prior to further manipulations. The bird’s behavior does not often reflect pre-anesthetic sedation, but birds appear to struggle less during restraint. High doses of midazolam produced sufficient sedation in geese to facilitate restraint for diagnostic procedures [8]. This is highly advantageous with dangerous birds such as large raptors, and long-legged birds such as cranes and ratites. The sedative effect of these drugs is evident during recovery which is slow and smooth. No studies have been done to determine the duration of effect for diazepam or midazolam. The benzodiazepines provide muscle relaxation when used in conjunction with ketamine and reduce the level of isoflurane needed for anesthesia [9]. Flumazenil administered IV, helps to reverse benzodiazepine-induced sedation and restores alertness as long as enough time has passed so that additional anesthetic agents are no longer effective. Dissociatives Ketamine is rarely used alone because it is associated with poor muscle relaxation, muscle tremors, myotonic contractions, opisthotonus and rough recoveries [1, 10-12]. The dose of ketamine depends on body weight, and its dosing follows the principles of allometric scaling so that large birds (>1 Kg) respond to 10 - 20 mg/kg whereas small birds (< 50 grams) require much higher doses, e.g. 70 - 80 mg/kg. Additionally, there is inter-species variability in the response to ketamine. For example, ketamine causes salivation, excitation and convulsions when given to vultures but these signs are rare in other birds [12]. When effective, anesthesia occurs within 5 - 10 min of intramuscular injection and may last 5 - 20 min depending on the dose and size of the bird. Recovery from ketamine, until the bird can perch or stand, can take 40 - 100 min, depending on dose, body temperature, metabolic health, and size of the bird. It is recommended that ketamine not be used alone and be combined with benzodiazepines or alpha 2 -adrenergic agonists to improve relaxation and depth of anesthesia. Tiletamine and Zolazepam Telazol® combines the effects of a dissociative drug (tiletamine) and a benzodiazepine (zolazepam). This combination has similarities to ketamine plus midazolam, but the smaller volume of Telazol that is typically used for anesthesia can be an advantage. At doses of 5 or 10 mg/kg it was an effective and safe anesthetic for great horned owls and screech owls although decreased heart and respiratory rates were noted [13]. Telazol at the same dose was unsatisfactory for anesthesia of red tailed hawks as they responded with salivation, and elevated heart and respiratory rates [13]. Despite the lack of information on Telazol in a variety of avian species, the primary disadvantage seems to be that a high dose provides anesthesia of short duration followed by a long (2 - 4 hour) and sometimes difficult recovery [11]. Alpha 2 -adrenergic agonists Xylazine, detomidine and medetomidine are usually used in combination with ketamine. The alpha- 2 -adrenergic agonists provide muscle relaxation, analgesia and sedation which smoothes induction and recovery. The greatest advantage of this group of drugs is the availability of specific antagonists to reverse the effects, allowing for smooth and rapid recovery. Atipamezole is recommended to reverse medetomidine and detomidine, and will also reverse the effects of xylazine. Yohimbine has been used in raptors and psittacines to reverse the effects of xylazine, both alone or in combination with ketamine [14-16]. Similar results were noted when tolazoline was used in turkey vultures to shorten xylazine plus ketamine anesthesia [17]. When reversing an alpha 2 -adrenergic agonist used in combination with ketamine, reversal must be timed so
as to avoid the bird recovering under the effects of ketamine alone as this can result in a rough recovery. Alpha 2 -adrenergic agonist drugs are not recommended as single anesthetic or immobilization agents for birds. In pigeons and Amazon parrots, high doses of medetomidine had a sedative effect but did not immobilize the birds [18]. Xylazine administered alone causes respiratory depression, excitation, convulsions and prolonged recovery [12]. All alpha 2 -adrenergic agonists have profound cardiopulmonary effects. Xylazine and medetomidine cause decreases in HR, RR, blood pH, hypoxemia, and hypercarbia [4,12,14,18]. The arrythmogenic effects of the alpha 2 -adrenergic agonists can lead to cardiovascular instability and, when coupled with hypoventilation and hypercarbia, can have an irreversible, fatal effect. Alpha 2 -adrenergic agonist drugs are a poor choice of anesthetic, alone or in combination, when a bird is highly stressed. General excitement can effectively over-ride the sedative effects of alpha 2 -adrenergic agonists, although the mechanism for this effect is not clear. Therefore, when using alpha 2 -adrenergic agonist drugs, approach the bird quietly, inject the drug and place the bird back into a familiar, quiet and dimly lit enclosure while waiting for the drug to take effect. The induction period is 5 - 10 min, depending on dose and size of the bird. Ratites, raptors and long-billed birds can have a hood placed over the head for calming when a dark cage is not available. Xylazine plus ketamine combinations have been evaluated in several avian species. Blood pressure becomes elevated, heart rate is decreased, and hypoxemia, hypoventilation, and hypercapnia occur [19-20]. An anesthetic combination consisting of medetomidine, midazolam and ketamine was evaluated and found to be unsafe for use in ducks [21] as it caused bradycardia, primarily attributed to the medetomidine [21,22]. Medetomidine also decreases respiratory rate. Apnea followed by a fatal decrease in heart rate and blood pressure was documented in four of twelve ducks receiving medetomidine [21]. Atipamezole and flumazenil were given intravenously to reverse medetomidine and midazolam, respectively, and the ducks rapidly regained consciousness and voluntary movement [21]. Propofol Propofol is an intravenously administered anesthetic with rapid onset, smooth induction, short duration of effect, and smooth, rapid recovery. Intravenous catheters are highly recommended for its administration because the drug must be given slowly for induction and often given repeatedly to maintain anesthesia. A maximum of 2 mg/kg bolus every 30 seconds is recommended for induction, after which 0.5 - 1.0 mg/kg/min is used to maintain surgical anesthesia [1,23]. In a study using ducks, propofol was given as an initial IV bolus and was constantly bolused at 1 - 4 mg/kg every 5 min to maintain a light plane of anesthesia [21]. In studies that monitored cardiopulmonary responses to propofol, mean arterial pressure (MAP) decreased significantly [1,23]. A short period of apnea following induction is a consistent finding [21,22,24] and respiratory depression can occur during induction and maintenance with propofol [23, 24]. Cardiac arrhythmias including ventricular premature contractions and ventricular tachycardia, were common in chickens and profound bradycardia was noted in ducks after the initial bolus of propofol [21,23]. Propofol has a narrow margin of safety in birds and supplemental oxygen and respiratory assistance must be provided to counteract apnea, hypoventilation and hypoxemia [21,23-25]. Anticholinergics The use of anticholinergics for birds is controversial. Indeed, atropine and glycopyrrolate are effective for the treatment of vagally induced bradycardia [26]. Some argue, however, that they cause respiratory secretions to become more viscous and thus more likely to plug narrow endotracheal tubes [27]. Others [28] recommend anticholinergics for their ability to reduce respiratory mucus production and prevent formation of mucus plugs in small endotracheal tubes [4]. The oculocardiac reflex has been reported in a cockatiel and suggests that treatment with an anticholinergic prior to or during ocular surgery may prevent this reflex which is thought to be caused by ocular manipulation resulting in cardiac dysrhythmias [29]. Ratites (Ostriches, Emus, Cassowaries and Rheas) In these large birds, injectable anesthetics are frequently used for short procedures and for induction of anesthesia prior to inhalation anesthesia. Several reports have been written on anesthetic protocols for ratites and a recent review compared the most common protocols [22,30-35]. These bird, when healthy, are too strong and unpredictable for simple mask induction with inhaled anesthetics. Intravenous injections can be given in the jugular vein or brachial vein, although the emu’s brachial vein is small and difficult to access. Placing a catheter in the jugular, brachial, or medial metatarsal vein will facilitate IV injection and induction. Injectable anesthetics most commonly used for ratites include combinations of Alpha 2 -adrenergic agonists followed by ketamine, or a benzodiazepine followed by ketamine, tiletamine-zolazepam, carfentanil or etorphine [28,34]. Benzodiazepines given prior to induction help produce smooth inductions and smooth but slow recoveries. Induction with tiletamine/zolazepam is excellent and rapid, although when given IV, violent recoveries have been reported [30,34]. Benzodiazepines given with tiletamine-zolazepam will smooth recovery [30]. Induction with xylazine-ketamine is adequate, but recovery can be difficult [34]. Carfentanil is not recommended due to an excitatory response even when used with xylazine [34]. When etorphine was combined with medetomidine, recumbency occurred rapidly, birds were sedate and muscle relaxation was adequate [33]. Other etorphine combinations, when given to free-ranging ostriches, caused initial
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