anesthesia of pet birds

Anesthesia of Pet Birds Thomas G. Curro, DVM, MS

It is common for veterinarians who specialize in small animals and species to be presented with pet bird patients for procedures that require general anesthesia, including complete physical examination, venipuncture, diagnostic workup, or medical and surgical treat- ments. The basic principles of anesthetic management that govern mammalian anesthesia also apply to birds, although specific anatomic and physiological differences must be considered. The goal of this article is to present a, clinically applicable approach to pet bird anesthesia, including preanesthetic considerations, physical restraint, induction agents, anesthetic maintenance, supportive measures, anesthetic recovery, and management of anesthetic complications. Copyright�9 1998by W. B. Saunders Company.

Key words: Avian, anesthesia, Psittaciforme, Passeri- forme, restraint.

V eterinarians who specialize in small animals and avian species are commonly presented

with pet bird patients for procedures that require general anesthesia. The most common pet birds belong to two major orders. Passeriformes include perching birds, such as canaries and finches. Psittaciformes include a wide variety of hooked-bill or parrot species, such as budgerigars (mistakenly called parakeets), cockatiels, conures, Amazons, cockatoos, African Greys, ma- caws, and others. Less frequently, doves, pigeons, quail, toucans, and poultry may be presented as pet birds. Although this article will concentrate on the Passeriforme and Psittaciforme species, the basics of avian anesthesia apply to most bird species.

Overview of Anesthesia in Birds Anesthesia of birds may be required to per-

form a complete physical examination, venipuncture, diagnostic workup, and medical or surgical therapy. In fact, many procedures may be less

From the Westside Family Pet Clinic, Dane County Humane Society, Madison, WI.

Address reprint requests to Thomas G. C'u*~'o, D~CWI, MS, 4326 DeVolis Pky, Madison, WI53711.

Copyright �9 1998 by W. B. Saunders Company. 1055-937X/98/0701-0002508. 00/0

stressful if pe r fo rmed unde r general anesthesia, ra ther than with physical restraint alone. As anesthetic agents have evolved, so have avian anesthetic techniques. Before the introduction of safe inhalant anesthetics, injectable anesthetic techniques were used for immobilization. Once methoxyflurane and halothane were shown to be safe and effective inhalant agents, their use in veterinary medicine evolved to include birds. Isoflurane has replaced injectable techniques, as well as methoxyflurane and halothane, as the most commonly used and safest general anesthetic agent for birds. The basic principles of anesthetic management are the same for birds and mammals. Similarities and differences, dependen t on anatomy and physiology, will become apparent as they are presented.

Preanesthetic Preparation Before any anesthetic procedure in birds, a

complete history should include signalment, environment, diet, immediate health problem, past health problems, and past reactions to handling and anesthesia. A complete physical examination is desirable, but this may be the reason that the bird is being anesthetized.

Fasting

Fasting recommendat ions for birds, even small passerines, have varied f rom no preanesthetic fasting to an overnight fast. 1-~ Because of the high metabolic rate of birds, an extended fast may be detrimental, as hepatic glycogen stores can be quickly depleted. Birds larger than 300 g are less prone to hypoglycemia that could develop during a pro longed fast. A blood glucose of <200 m g / d L is considered hypoglycemic in most bird species.

Fasting is r ecommended to decrease the likeli- hood of regurgitation and aspiration of food. Ideally, a fast should be long enough to empty the crop of its contents. 4 It is especially important to empty the crop of fluid contents, which are easily refluxed during general anesthesia or recovery. Frugivorous birds, such as iorikeets,

10 Seminars in Avian and Exotic Pet Medicine, Vol 7, No I (January), 1998:pp 10-21

Anesthesia of Pet Birds 11

which normally have a diet high in fluid content , are especially susceptible to regurgitat ion and potential aspiration. 5 I f an emergency situation precludes waiting for the crop to empty, a small feeding tube may be passed into the esophagus a n d / o r crop to wick fluid by capillary action or to directly aspirate fluid contents. I f passing a tube would be an unacceptable stress before anesthesia, a feeding tube may be passed after a bird is induced and intubated to remove fluid before diagnostic or surgical manipulat ion. If crop surgery is to be pe r fo rmed , fasting should be long enough (4 to 6 hours) to empty the crop.

It is a generally accepted practice to fast healthy l~irds 1 to 3 hours before anesthesia. 5-s Generally, the smaller the bird, the shorter the fasting period. 4,9,1~ Small passerines, such as finches or canaries, or immature birds of larger species, should not need more than a 1 hou r fast. Larger birds (>300 g) will tolerate an overnight fast (8 to 10 hours) , but anesthesia should be p lanned for early in the day. An overnight fast is reasonable, because most birds do not eat dur ing this time. I n d e p e n d e n t of the food fast, water should be available until approximately 1 hou r before anesthesia in most species.

Physical Restraint

Before the administrat ion of preanesthet ic or anesthetic agents, the bird will need physical restraint. In the case of Psittaciformes, restraint of the head is impor tan t to prevent biting injury to the handler. The most c o m m o n restraint technique is to capture the bird with a hand- covered towel, sized appropriately for the species. Lea ther gloves should be avoided because many pet birds are hand-tame, and captur ing with a gloved hand may cause a bird to become hand-shy. Gloves also decrease tactile senses needed to assess the extent of the restraint.

For restraint, one hand restrains the head while the other hand controls the body (Fig 1). Restraint of the head begins by creating a ring a round the bird's neck with one hand. Gentle, but firm, pressure is used against the base of the skull and lower mandible to extend the head away f rom the body (Fig 2). This technique will work with the largest macaw, when the size of the head may preclude small hands f rom restraining the head directly. Passeriformes are commonly restrained with the bird cradled in the pa lm of

Figure 1. Preferred towel restraint technique for Psit- taciformes.

the hand, and the head restrained between the index and middle fingers.

It is not impor tan t to capture the bird in the ideal position, but ra ther to capture the bird quickly to avoid the stress of chasing the bird a round the cage. Dimming the lights in the room may facilitate capture. Once the bird is captured, it can be manipula ted to a prefer red position. Birds brea the via movemen t of the thoracic Cage, and restriction of the thorax by the handler must be avoided.

Preanesthetic Medications

Preanesthetic medicat ions are used to provide card iopulmonary and central nervous system (CNS) stabilization, physical restraint, sedation, muscle relaxation, and analgesia, a n d / o r to decrease the doses of concurrent ly used anesthetic agents. O f the numerous medicat ions that can be used during the preanesthet ic period, fluids are the most important . Crystalloid and colloid fluids aid in cardiovascular stabilization. Preoxygenation may enhance homeostasis in the respira tory-compromised patient. Steroids may be used in the event of shock or CNS injury. It is beyond the scope of this article to address all the nonanes the t i c p r emed ica t i ons in detail, although fluids will be discussed in a later section.

Preanesthetic medicat ions are used infre- quently in pe t birds. Although the use of anticho- linergics as premedicants is rare, a t ropine and glycopyrrolate are effective for the t rea tment of vagally induced bradycardia. 11,12 Diazepam and

12 Thomas G. Carro

Figure 2. Preferred restraint technique for Psittacifor- rues, demonstrating hand positioning without towel.

midazolam may produce sedation,5,7,1!-lB muscle relaxation during ketamine anesthesia, ~2,~4 and an anesthetic-sparing effect 15 in birds. Phenothi- azine tranquilizers have been noted to be ineffec- tive in birds, 11,16 although acepromazine has been used in combination with ketamine. 2,5,17 Opioid agents, such as butorphanol, have demonstrated potential benefi t in pet birds as sedatives and analgesics, la,19 and their use will be discussed unde r supportive therapy.

Induction

Intramuscular/Intravenous Agents

Historically, many injectable anesthetics have been used for immobilization and anesthesia in birds, including barbiturates, chloral hydrate, alpha chloralose, phenothiazines, dissociatives,

az-agonists, a lphaxalone/alphadolone acetate 2~ and, more recently, propofol. 22,23 Stable, uncom- plicated levels of anesthesia are difficult to produce with these agents. The advantages of using injectable anesthetics include ease of use in the field, minimal need for technical equipment, availability, ease of administration, rapid induction, and relatively low cost. Disadvantages in- elude elimination that is dependen t on bio- transformation and excretion, dose-dependent cardiopulmonary depression, potentially difficult reversal of drug effects in an emergency situation, potentially pro longed and violent recoveries, and lack of adequate muscle relaxation with some drugs. Propofol use for induction and maintenance in chickens is associated with significant adverse cardiopulmonary effects, and cannot be r ecommended for use in pet birds. 23 Injectable agents may be administered intramus- cularly (IM) (pectorals), intravenously (IV) (jugu- lar, cutaneous ulnar, medial metatarsal), or in- traosseously. 24 Catheterization is r ecommended for IV or intraosseous administration, and accu- rate body weight is essential for calculation of an injectable anesthetic dose.

Combinations of ketamine with diazepam, midazolam, or xylazine produce restraint to light surgical plane anesthesia, depending on dosage. Ketamine alone usually produces inadequate anesthesia, and recoveries are often violent. Com- bining ketamine with diazepam or midazolam provides muscle relaxation and sedation, and reduces the degree of struggling on recovery, a,l~ Benzodiazepines are r ecommended because they produce minimal adverse cardiopulmonary effects. A tiletamine (dissociative) and zolazepam (benzodiazepine) combinat ion (Telazol; Fort Dodge Laboratories, Fort Dodge, IA), provides effects similar to ketamine and diazepam, with a longer duration of effect.

Ketamine combined with xylazine is a commonly repor ted injectable regimen for birds. 6,12, 17,25-29 Xylazine also improves anesthetic recovery and provides sedation and analgesia when used in combination with ketamine. Unfortunately, xylazine has potent cardiopulmonary-depressive effects, which are not compensated by the effects ofketamine. Sun Conures have been noted to be less tolerant of an xylazine-ketamine combination compared with other pet birds. 2v A benzodiazepine-ketamine combination is the best choice when an injectable anesthetic technique must be


used in a cardiopulmonary-compromised patient.

The potential for severe cardiopulmonary depression may be minimized by administering injectable agents in multiple small boluses (or as an infusion), titrated to effect, rather than as a single large bolus, especially when the IV or intraosseous route is used. 7 Current recommen- dations are that injectable anesthetics be used in pet birds only when an inhalant anesthetic, particularly isoflurane, is unavailable. Ideally, isoflurane anesthesia should be used in all situations of debilitation, or when a prolonged duration of anesthesia is required.

If injectable anesthesia is unavoidable, appropriate agents include ketamine combined with e i ther xylazine, diazepam, midazolam, or acepromazine (Table 1). Medetomidine (Domi- tor; Pfizer Animal Health, Exton, PA) with ketamine has also been reported, but is less commonly used than the other combinations at this time. Ti letamine/zolazepam is an effective injectable anesthetic combination (7.7 to 26.0 m g / k g IM). When referr ing to Table 1 dosages, the high-end dosage is used for smaller birds (<250 g), or when a surgical plane of anesthesia is desired in larger birds. The low-end dosage is used in larger birds (>250 g) or when only sedation is desired in smaller birds. Induct ion occurs in 5~to 10 minutes and complete recovery may take 2 to 4 hours or longer.

Antagonists

Antagonism of anesthetic agents is beneficial, especially when anesthetic emergencies develop or when duration of anesthesia is prolonged. Yohimbine and tolazoline have been used to reverse the effects of xylazine. 12,26 The sedative and respiratory-depressant effects of xylazine can be successfully reversed with yohimbine (1 m g / k g IM). Opioids, of which butorphanol has

Table 1. Common Ketamine Combinations and Dosages Used for Pet Bird Injectable Anesthesia

Drug Combined Ketamine With Ketamine

Drug Combination (mg/kg) (mg/kg)

Ketamine/xylazine 10-50 1.0-10.0 Ketamine/diazepam 10-50 0.5-2.0 Ketamine/midazolam 1040 0.5-1.5 Ketamine/acepromazine 10-25 0.5-1.0

demonstrated potential analgesic and sedative effects in pet birds, can be reversed with the pure opioid antagonist, naloxone (0.3 mg/kg) . Gener- ally, reversal of opioids should only be done as an emergency treatment of severely depressed birds, because reversal will also reverse analgesia. Diaz- epam and midazolam are antagonized with fluma- zenil, a pure benzodiazepine antagonist.

Inhalation Agents

Historically, ether, 3~ methoxyf lurane, 3234 halothane, 9,3~ and isoflurane have been used for inhalant anesthesia in birds. Nitrous oxide has also been used in combinat ion with other inhal- ants. Ideally, an inhalant should provide rapid anesthetic induction and recovery without pro- ducing profound cardiopulmonary depression or organ toxicity.

Isoflurane is currently the prefer red agent for general anesthesia in pet birds. This inhalant is a clinically proven safe, and effective anesthetic agent. Its potency has been evaluated based on minimum anesthetic concentrat ion (EDs0) studies in waterfowel, cranes, pigeons, and Psittacifor- rues. 15,19,~6,37 Because of isoflurane's low blood solubility and minimal metabolism (<0.2%) and the efficiency of avian respiratory gas exchange, induction, changes in anesthetic depth, and recovery are easily and quickly controlled. At sedative and light surgical planes of anesthesia with isoflurane, adverse cardiopulmonary effects are minimal, al though certain species seem particularly sensitive to the respiratory-depressant effects.

Isoflurane concentrations of 4 to 5 vol% are used for mask induction and must be reduced when signs of sedation and anesthesia become apparent. Minimum anesthetic concentrations for maintenance in the intubated bird will average 1.45%. If maintained with a mask, concentrations may be 25% to 30% higher, unless the mask fits snugly. Surgical plane anesthesia will generally require isoflurane concentrat ions of 1.80% to 2.20% and potentially higher. Because of the dose-dependent respiratory depression of isoflurane, ventilation should be assisted or controlled, especially during prolonged procedures.

Inhalation Agent Delivery

Nonrebreathing, semiopen anesthesia circuits are r ecommended for inhalant delivery. The

14 Thomas G. Curro

advantages of these systems are low resistance to breathing and immediate changes in the delivered anesthetic concentrat ion when the vaporizer setting is changed. Disadvantages are that the required high oxygen flow rate wastes oxygen and anesthetic, produces excessive environ- mental pollution, and also causes significant pat ient cooling due to inspiration of cool dry gas.

Various configurations of nonrebrea th ing systems have been used. The Bain anesthesia circuit is commonly used because o f its low cost and weight. Small rebreathing bags (�89 L) are commercially available, or may be fashioned from bal- loonsY An effective anesthetic gas scavenger should always be used with inhalant anesthetic systems to minimize human exposure to waste gas pollution:

Anesthesia is induced with the head physically restrained and placed in an anesthetic mask attached to the nonrebreath ing circuit. Masks may be purchased commercially, or constructed from various sized plastic tubing or syringe casings and a latex diaphragm (exam glove). Equilibrating the circuit with inhalant before pat ient exposure (priming) has been advo- cated, 27 but is unnecessary because the high oxygen flow rate (>1.0 L /min ) quickly delivers the desired induction concentration. Priming also contributes to waste gas pollution. Once the bird is restrained in the mask, the desired concentration of anesthetic is delivered by one of two protocols. The first method begins at low inhalant concentrat ions and proceeds to higher concentrations as anesthetic effects deve!op, allow- ing rapid reversal of anesthesia if complications arise. This protocol has been suggested for debilitated birds because of less l ikelihood of overdosing. Unfortunately, the low-to-high protocol requires longer physical restraint and stress time, which can be especially detrimental to a debilitated bird. The benefit must be compared with this risk, with the goal being to avoid both overdose a n d / t h e stress associated with slow- onset anesthesia. The prefer red protocol begins with a high (4% to 5% isoflurane) concentrat ion of inhalant, which is decreased as clinical signs of anesthesia become apparent. The key to this me thod is close monitoring of clinical signs and an appropriate and timely decrease in inhalant concentrat ion delivered. Sedation usually occurs within seconds, and a light surgical plane of

anesthesia within minutes. This protocol has been shown to work safely on healthy, as well as debilitated, birds.

Once a light surgical plane of anesthesia has been induced, the mask should be removed, the bird intubated, and the endotracheal tube quickly connected to the delivery circuit. Continuous delivery of inhalant anesthetic is necessary to prevent wakening, especially during the induction phase. The vaporizer setting should then be adjusted to an appropriate concentrat ion to maintain a stable plane of anesthesia.

Maintenance

Intubation

Endotracheal intubation provides airway access for the delivery of oxygen and anesthetic gases and an effective route for delivery of manual or mechanical ventilation. The endotracheal tube also protects the airway from aspiration of secretions and ref luxed gastrointestinal contents.

For an endotracheal tube to per form these functions properly, a sealed airway is required, which cannot be achieved with either an uncuffed or an uninflated cuffed tube. Endotra- cheal intubation with cuffed tubes is recommended for use in birds despite the presence of complete cartilaginous tracheal rings in all species. The cuff must be carefully inflated just enough to prevent leakage when 10 to 15 cm H20 pressure is applied to the airway.

The size of the bird will dictate the size of endotracheal tube used. The smallest available cuffed tubes have an internal diameter (ID) of 3.0 mm. Psittaciformes as small as 350 g have been intubated with a 3.0 ID tube. Birds smaller than this will require the use of uncuffed tubes or large gauge IV catheters. Birds as small as 100 g may be intubated with these smaller tubes. Care should be taken when catheter-sized tubes are used, because they will not provide a sealed airway and may easily become plugged with secretions, mucus, or blood. Also, the resistance to gas flow through small catheters is high, which may significantly impede both spontaneous and manual ventilation. Use of Murphy tubes, which have a side opening as well as an end opening, decrease the chance of mucus occlusion. Airway patency should be checked regularly during general anesthesia. Birds smaller than 100 g are


best mainta ined with a mask. I f a mask is used to maintain anesthesia, mouth gags fashioned f rom pape r clip wire have been used to keep the fleshy tongues of certain avian species f rom obstruct ing the glottis.

Most birds are easy to intubate. They lack an epiglottis and the glottis is located on the mid- line at the base of the tongue (Fig 3). To visualize the glottis, the tongue is gently grasped and pulled forward with a forceps, or the tongue is pressed against the mandible with a cotton- t ipped applicator. An endotracheal tube, spar- ingly lubricated, is then gently inserted directly into the trachea, and secured to the maxilla with tape. Endotracheal tube length should only extend caudally beyond the thoracic inlet and rostrally past the end of the beak no more than the length of the endotracheal tube adapter. The latter will minimize the dead space attr ibutable to the endotracheal tube.

In the event of airway obstruction, or to p e r f o r m procedures of the head or oral cavity, inhalant anesthetics may be adminis tered into the air sacs. Percutaneous catheters for inhalant administrat ion may be placed in the clavicular, caudal thoracic, or abdominal air sacs, and the p rocedure for catheter p lacement has been de- s c r i b e d 9 ,4~ Briefly, a skin incision is made over

the desired p lacement site, the air sac m e m b r a n e is exposed, a sterile catheter is passed into the air sac, and the skin incision is closed to secure the catheter in place. Effective ventilation and delivery of gases can be adminis tered through air sac catheters. Caudal thoracic or abdominal air sac catheters are prefer red because gas del ivery at these sites bet ter mimic the general flow of gases through the avian respiratory system. Oxygen and anesthetic gases flow f rom the cannulated air sac and exit th rough the trachea. Unfortu- nately, scavenging of waste gases is not practical when this technique is used.

Supportive Therapy Positioning/Thermal Support

Positioning of the avian pat ient during anesthesia depends on the p rocedure being per- formed. Birds are usually posi t ioned in ei ther lateral or dorsal recumbency. When posit ioning an anesthetized bird, it is critical that unre- stricted movemen t of the thoracic cage be maintained. Because birds lack a diaphragm, ventilation is achieved by the expansion and contraction of the thoracic cage. I f this movemen t is im- peded, hypoventilation will be significant. A1-

Figure 3. Demonstration of the bird glottal opening before endotracheal intubation.

16 Thomas G. Curro

though one repor t indicated that birds in dorsal recumbency hypoventilate, 41 it evaluated heavy- bodied poultry that, because of their body con- figuration and thoracic musculature, could not ventilate adequately. Anesthetized, spontane- ously breathing cockatoos have been studied in dorsal recumbency for up to 7 hours without development of significant hypoventilation.~9

Special padding is not needed for pet birds; in most cases, a soft towel is adequate. If more rigid, sustained positioning is needed, a conforming body pad may be used.

Maintenance of normal body tempera ture during anesthesia is extremely important in pet birds. Hypothermia reduces anesthetic requirements, increases the potential for cardiac instabil- ity, and prolongs recoveryY The normal body temperature for birds ranges from 40 to 44~ Because of their small size and high body surface- to-volume ratio and the use of high oxygen flow rates, birds can become hypothermic very quickly during anesthesia.

To minimize heat loss, all anesthetized birds should be provided with thermal support. Meth- ods to prevent heat loss include insulating the patient with clear plastic surgical drapes or wrapping nonsurgical field regions in plastic. Plucking of feathers and surgical preparat ion with alcohol should be minimized. The use of a chlorhexidine solution and warm water is a better choice for aseptic skin preparation. Supple- mental heat can be provided by increasing the ambient temperature, using a circulating warm water blanket (40.5~ administering warmed fluids, positioning a warming lamp over the bird, and placing heated water containers near the patient. Latex gloves or empty fluid bags can be filled with water, sealed, and warmed in a microwave oven. Heated water containers should never be placed directly next to the bird. They may initially be too hot and, as they cool, may actually convect heat away from the patient. Body temperature should be moni tored with a cloacal t he rmomete r or temperature probe to evaluate the effectiveness of heat retent ion measures.

Fluid Administration

As with other animals, the goal of fluid administration is to provide daily fluid requirements, and to correct preexisting dehydration, electrolyte imbalances, and losses of intravascular volume due to hemorrhage. Daily fluid require-

ment in resting birds is estimated to be 40 to 60 mL/kg/day . The estimated fluid replacement to correct dehydration is calculated as:

Deficit (mL) = body weight (g) x % dehydration

Debilitated birds should be assumed to be 5% to 7% dehydrated. A packed cell volume (PCV) >55% to 60% is an indication of dehydration. A serum uric acid of >30 m g / d L may indicate ei ther dehydration or renal disease. Dehydration should be corrected with �88 to �89 of the calculated fluid deficit in the first 4 to 6 hours, with the remaining volume administered over the follow- ing24 hours. 42 If a bird must be anesthetized on an emergency basis before volume stabilization, the volume deficit should be incorporated into the anesthetic maintenance fluids.

Healthy, anesthetized birds should receive replacement fluids at a rate of 10 m L / k g / h r for the first 2 hours, and then 5 to 8 m L / k g / h r to prevent overhydration. Fluid replacement should be considered on an individual animal basis. If hemorrhage occurs, the administered volume of a crystalloid replacement fluid should be three times the blood loss volume. If blood volume loss is >30% of the estimated normal blood volume, a blood transfusion is indicated.

Oral, subcutaneous (SC), IV, or intraosseous (intramedullary) routes are appropriate for fluid administration. Alert birds without gastrointestinal disease benefit f rom oral fluid administration. Debilitated birds and birds with altered mentat ion should receive parenteral fluids. In noncritical situations, SC fluids may be administered. Preferred sites for SC fluid delivery include the propatagium (wing web), intrascapu- lar region, and inguinal region. Care should be taken to avoid inadvertent administration of SC fluids into an air sac.

Birds requiring emergency therapy should receive IV or intraosseous fluids. These two routes offer the fastest and most effective meth- ods for large fluid volume administration. Jugu- lar, cutaneous ulnar, and medial metatarsal veins are common sites of IV access, depending on the size of the bird. Avian veins are fragile and have little SC supportive tissue. Therefore , care must be taken to prevent vein laceration and perivaseu- lar hemorrhage. Although butterfly catheters can be used for venipuncture, they should only be used in immobilized birds. Over-the-needle type catheters are r e co m m en d ed and well toler-


ated by debilitated, conscious birds, if mobility is restricted. Catheters should be sutured or glued with cyanoacrylate tissue glue to the underlying skin. Restrictive collars (Elizabethan, tube) may be used to prevent dislodgment of the catheter.

When venous access is unavailable because of the size of the bird or hydration status, fluids may be administered via an intraosseous catheter. The distal ulna and the proximal tibiotarsus provide access sites for intraosseous catheterization. Procedures for catheter placement have been described. 43 Aseptic preparat ion of the site is essential. Spinal needles (22 or 20 G) approximately one-third the length of the bone are r ecommended in larger birds, whereas hypodermic needles (27 to 25 G) have been used in smaller birds. 43 Catheters are sutured or taped in place. If ulnar access is used, the wing should be immobilized for the duration of catheter placement. Intraosseous catheters should not be placed in pneumatic bones (humerus or femur).

During anesthesia, balanced electrolyte solutions may be given by SC boluses, IV boluses, or continuous infusion. Lactated Ringer's solution (LRS) is the most commonly used crystalloid. As a replacement fluid, LRS is useful for rehydra- tion, intravascular volume support, and fluid maintenance. Dextrose solutions may be used for the prevention or t reatment of hypoglycemia. Parenteral administration of dextrose solutions should be conservative because they may induce compartmental shifts in water and electro- lytes, leading to hypovolemia. 42 Dextrose solutions >2.5% concentrat ion are contraindicated for SC use. Half-strength dextrose with half- strength LRS is useful during pro longed anesthesia.

There are few reports describing the use of synthetic colloids (Dextran, starch, oxypolygela- tin) in birds. 44 These solutions are useful when osmotic support is needed for intravascular volume expansion, Synthetic colloids should be considered during hypovolemic, hypotensive, or hypoproteinemic crises when blood products are unavailable. If a hemorrhagic event produces a >30% total body blood volume loss, crystalloid therapy needs to be supplemented with a synthetic or natural colloid product.

Blood transfusions are indicated in birds with a total plasma protein of <2.5 g/dL, a PCV of <15% to 25%, or an acute blood loss >30% of the total blood volume. Total blood volume

(mL) may be estimated as 10% of the body weight (g). Donor blood should be from a bird of close parental relation or of a similar phyloge- netic lineage. Because the availability of similar lineage blood donors may be limited, chicken or pigeon s blood may be transfused successfully to Psittaciformes. Single transfusions do not usually p roduce immunologic complications. Major a n d / o r minor crossmatching should be done for multiple transfusions. No commercial crossmatching test kits are available for birds. Disease- free donors, screened for appropriate hematoge- nously spread avian diseases, are essential.

All fluids should be warmed before administration to minimize their contr ibution to anesthetic- induced hypothermia. Crystalloid and synthetic colloid solutions may be easily warmed to a temperature of 38 ~ to 39~ in a microwave oven without affecting their composition. Natural blood products should be warmed in a warm water bath.

Monitoring Sensitivity to anesthesia differs between indi-

viduals and species. Therefore , moni tor ing the effects and depth of anesthesia is essential, because birds are prone to develop anesthetic complications relatively acutely. Ideally, anesthetic depth should be just adequate to per form the necessary procedure, and respiratory, cardiovascular, and CNS status should be continuously monitored.

Respiration is usually rapid and irregular with a shallow tidal volume during initial stages of induction, but becomes slow, regular, and deep as a medium surgical plane is achieved. Respira- tory rate and tidal volume cont inue to decrease as anesthetic depth increases. Visual monitor ing of the bird and the r eb rea th ing bag will allow assessment of respiratory rate and provide crude evaluation of tidal volume. Clear, plastic surgical drapes should be used to assist visual monitoring. Electronic respiratory devices, which produce an auditory tidal flow signal, are effective for assessment of respiratory rate. Unfortunately, some do not function well with the small tidal volumes of pet birds or with high oxygen flow rates used in birds.

Hear t rate and rhythm should be monitored. Direct auscultation with a stethoscope is useful, but may be difficult in small birds under surgical drapes. An electrocardiograph (ECG) provides

18 Thomas G. Curro

monitor ing of the heart rate, rhythm, and electrical conduct ion activity. The avian ECG has been described, 45 and normal ECG values have been established for African Grey and Amazon Par- rots. 46 The mammalian hexagonal ECG lead system is applied to birds. Normally, the avian RS complex deflects downward in lead II. Attach- ment of most ECG leads involves an alligator clip that is potentially traumatic to the thin skin of birds. To prevent injury, alligator clips can be attached to stainless steel wire suture that is looped through the skin, or leads with stick-on attachments may be used. Likewise, small (27 to 25 G) hypodermic needles may be placed cutane- ously, to which clips may be attached. Another useful technique involves cutting small pieces of gauze to pad the skin under the clips. Moistening the gauze pads with alcohol provides adequate ECG lead contact. Leads are commonly attached to the propatagium and to loose skin in the stifle region. Although an ECG will reflect myocardial electrical activity, it does not indicate adequate cardiac output and tissue perfusion.

Hear t rate, rhythm, and tissue perfusion can be moni tored with a Doppler flow probe device, which is discussed in detail in Bailey's article. In larger birds, an inflatable cuff and sphygmoma- nomete r may be used in combination with the Doppler to moni tor systolic blood pressure.

Pulse oximeters are becoming a popular monitoring tool in veterinary medicine. These devices provide an indirect measure of hemoglobin saturation with oxygen. Pulse oximeters are useful evaluators of changes in pulse rate and percentage of oxygen saturation. Reflectance

p r o b e s have been successfully used orally and cloacally.

CNS monitor ing involves the evaluation of muscle tone and variOus reflexes. Anesthetic depth follows a relatively predictable progression in pet birds. During the initial stages of anesthesia, birds will be lethargic and have drooping eyelids, a lowered head, and ruffled feathers, but will be easily arousable. As the bird progresses to a light surgical plane, palpebral, corneal, cere, and pedal reflexes remain present and potentially brisk, 'but there is no voluntary movement at this stage. At a medium surgical plane, corneal and pedal reflexes are slow to intermit tent with loss of the palpebral reflex. During deep surgical anesthesia, respirations are slow and shallow, and reflexes are no longer present. 1~

In larger birds, ventilation, oxygenation, and acid-base status may be assessed with the measure- ment of arterial blood gases. However, arterial blood samples are technically difficult to obtain in most pet birds, and evaluation requires the immediate availability of a b lood gas analyzer.

Ventilation

Ventilation in birds results f rom active muscu- lar movement of the thoracic cage during inspiration and expiration. Any restriction of thoracic movement (excessive restraint, heavy surgical drapes) will exacerbate anesthetic-induced hypoventilation. Ventilation should be manually assisted or mechanically control led in all anesthetized birds because hypoventilation can result in cardiac depression and arrest. Emergency param- eters for ventilation are discussed under Compli- cations.

Inhalant anesthetics p roduce dose-dependent depression of ventilation, and certain avian species are more susceptible to this effect. In a study comparing three species of parrots, cockatoos and Blue-fronted Amazons ventilated normally, but African Grey Parrots hypoventilated at minimum anesthetic concentrat ions of isoflurane. 19 Additional reports support the finding that Afri- can Grey Parrots do hypoventilate during isoflurane anesthesia. 4s,49 Xylazine is a p rofound respiratory depressant and ventilation should be moni tored closely during injectable regimens that include this sedative.

Analgesics

Analgesics, particularly opioids, are gaining popularity as a therapeutic adjunct in pet birds. Their use is largely based on clinical impressions. The published dose for butorphanol (3 to 4 mg/kg) resulted from a clinical trial involving budgerigars.~S Controlled research on the analgesic effects of butorphanol in cockatoos and African Grey Parrots has demonstra ted potential analgesic properties.19,36 Butorphanol does cause a decrease in both heart rate and minute ventilation, but nei ther effect results in significant physiologic alterations.

Nonsteroidal anti-inflammatory agents have also been used clinically in birds. Flunixin meglumine (1 to 10 m g /k g IM) and ace@salicylic acid (aspirin) (one 5 grain tablet /250 mL dr inking


water) have been recommended, al though nei- ther has been critically evaluated in birds.

Recovery Recovery should include delivery of 100%

oxygen after the inhalant has been discontinued. After swabbing the oral cavity of any accumu- lated secretions, detaching the endotracheal tube from the beak, and deflating the cuff, the tube is removed when the bird is light enough to object to its presence.

Regardless of the anesthetic protocol used, most birds will experience emergence delirium, which usually occurs at the time o f or shortly after extubation. Vigorous wing flapping and disorientation are common and usually more p ronounced during recovery from injectable anesthetics. A potential complication of anesthetic recovery is self-inflicted trauma, and restraint is essential to prevent postanesthedc injury. Birds should be lightly wrapped in a towel and manually restrained by available personnel until able to stand. Recovery from isoflurane anesthesia occurs within 5 to 15 minutes after anesthetic delivery is discontinued. If recovery personnel are not available, or if a pro longed recovery is anticipated, birds can be wrapped in a towel or paper and placed in a small, empty, padded enclosure. Restraint should be minimal so that once a bird recovers sufficiently, it can easily free itself f rom the restraint. As with induction, restraint must not interfere with the normal breathing movement.

The recovery area should be warm (25 to 30~ quiet, and dimly lighted or dark to minimize external stimulation during recovery. Food and water should be offered as soon as the bird is alert and able to perch.

Complications Preoperative

Like all o ther species, birds should be stabi- lized before general anesthesia, whenever pos- sible. Contraindicafions for general anesthesia include shock, respiratory distress, ascites, dehydration, anemia (PCV <15% to 17%), hypoproteinemia (total protein <2.5 to 3.0 g /dL) , hypoglycemia (<200 mg/dL) , metabolic acidosis, and: a fluid-filled crop. Unless corrected, these

pathophysiological changes will greatly increase the patient's anesthetic risk status.

Intraoperative Intraoperative hemorrhage must be immedi-

ately controlled. Estimated blood volume of birds, in milliliters, is 10% of the body weight in grams. The average volume of a drop of blood is 0.05 mL. Because the blood volume of a 30-g bird is 3 mL, the loss of 20 drops (or 1 mL, which is barely a spot on a gauze sponge) will result in a 30% blood loss in this bird. Although this is a dramatic example, it does demonstrate the critical importance of intraoperative hemostasis in birds. Delicate surgical technique and the application of electrocautery will aid in minimizing blood loss.

Hypoventilation (decreased tidal volume or rate) or apnea for longer than 20 seconds requires application of ventilatory support and assessment of airway patency and anesthetic depth. Decreasing the anesthetic depth is usually indicated, and should precede application of assisted or controlled positive-pressure ventilation. Assisted ventilation is delivered with a peak inspiratory pressure of 10 to 15 cm H20, 2 to 3 times per minute, in addition to the bird's rate. Controlled ventilation should deliver 10 to 12 brea ths /minute with a peak inspiratory pressure of 8 to 12 cm H2O pressure. If an endotracheal tube is not placed, and the tracheal airway is unobstructed, effective ventilation may be delivered by moving the s ternum ventrally and dor- sally, which produces inflation and deflation of the air sacs, respectively. This action should produce adequate movement of gas through the lungs because of the avxan unidirectional gas flow pattern. Cessation o f breathing in birds may be rapidly followed by cardiac arrest. Therefore, immediate recognit ion and intervention is required.

It is best to prevent cardiac arrest by maintain- ing normovolemia, adequate oxygenation and ventilation, appropriate anesthetic depth, minimal use of cardiodepressive drugs, and adequate monitoring. If cardiac arrest occurs, circulation may be assisted by manipulat ing the s ternum as described for ventilatory assistance, which uses the thoracic-pump mechanism of cardiopulmonary resuscitation. Direct cardiac massage is difficult in birds because of the structure of the thoracic cage. 1V or intracardiac epinephr ine

20 Thomas G. Curro

may be used in an attempt to stimulate the return o f cardiac function. 1~ Unfortunately, cardiac resuscitation is frequently and frustratingly unsuccessful in birds and prevention is o f critical importance.

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