Anaesthesia is now common in avian medicine, for either diagnostic or surgical procedures. The period of anaesthesia may be brief - for example, to facilitate positioning for radiography - or prolonged for a complicated surgical operation. With improved understanding of avian physiology, the safety of anaesthesia in avian species has evolved significantly over recent years. This article aims to describe current best practice of avian anaesthesia for the commonly seen companion species in the UK.
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Aidan Raftery graduated from University College Dublin in 1982. He obtained the RCVS certificate in zoological medicine in 2000 and has been a chartered biologist since 2001. He worked in small animal practice until 1996, when he founded the Avian and Exotic Animal Clinic in Manchester, which is now a three-vet clinic, concentrating and equipped solely for exotic species. He has contributed many book chapters to texts on exotic animal medicine, including four current BSAVA manuals.
WHEN a general anaesthetic is considered necessary for a bird, the aims when preparing the patient are to minimise the risks and maximise the chances of a successful outcome. The anaesthesia and the procedure to be facilitated by it should be carefully planned in advance so that there is no unnecessary extension of the period under the anaesthetic.
A satisfactory outcome for the patient relies on the anaesthetist having the necessary knowledge and practical skills, and being able to integrate this knowledge effectively in the planning of the procedure, team work, communication and decision-making in the operating theatre environment.
Reasons for general anaesthesia
The decision to anaesthetise a bird must be based on the history, a thorough examination and a rational diagnostic/treatment plan. The potential risks versus the benefits need to be evaluated and discussed with the owner.
Assessment before general anaesthesia
The history should include signalment, environment, diet, presenting signs and any medical history. The reader is referred to textbooks of avian medicine for detailed discussions of history-taking in avian medicine (see References and Further Reading).
A thorough clinical examination, including assessment of body condition and weight (comparing with historical weights if available), is essential (Fig 1). Again, the reader is referred to textbooks of avian medicine for detailed information on the clinical examination of the avian patient.
It is important to remember that birds have a natural instinct to hide signs of illness and are often very ill by the time the owner notices that there is a problem.
Diagnostic testing depends on the patient, presenting clinical signs and any underlying disease. The history of the individual bird and of the flock, and/or geographical location should also be considered. The initial results may indicate that further diagnostic testing is required. The minimum clinical database recommended is shown in Box 1.
Box 1: Minimum clinical database recommended before anaesthesia
Packed cell volume
Blood smear evaluation
In many cases, initiating treatment before a general anaesthetic may reduce the anaesthetic risk and in some cases make the procedure unnecessary.
Preparation, equipment and necessary skills
The anaesthetic must be monitored by an assistant who has appropriate training and experience. The anaesthetist should not be distracted by having to procure equipment that may be needed and should not have to assist with the surgery/procedure.
Drug doses in the advent of emergencies should be calculated in advance (Box 2).
Box 2: Drugs for anaesthetic emergencies in birds*
Adrenaline 0.1 mg/ml intravenously or intraosseously or double dose down the endotracheal tube
Doxapram 2 mg/kg intramuscularly, intravenously or intraosseously
Fluids: immediate bolus of 10 ml/kg crystalloids and 5 ml/kg colloids intravenously or intraosseously
* From Lichtenberger 2007
All the equipment that might be needed for the procedure should be prepared beforehand. In addition, it is important to plan for the postanaesthetic care (eg, nutritional support, appropriate analgesia) and have a suitable recovery area ready.
Preparation of the patient
The patient must be stable before anaesthesia is induced. Additionally, the crop should not be full of fluid. In some patients, this will require withholding food for a short period, depending on the size of the bird. If the crop is found to be full of fluid then the induction should be delayed to allow time for it to empty. However, caution needs to be exercised as some patients will become rapidly weaker without adequate nutritional support.
A balanced analgesia plan has a major impact on recovery and this starts before or just after the induction of anaesthesia (Box 3). Some patients will also benefit from antianxiety drugs, with midazolam being the most widely used.
Box 3: Analgesia in avian surgery
Butorphanol 1 to 2 mg/kg intramuscularly. (There are considerable species differences, eg, Amazon parrots need higher doses for significant analgesia [Curro 1993])
Incisional block: Lignocaine <2 mg/kg mixed with bupivacaine <2 mg/kg. Incision size determines the dose required (above are the maximum doses). Dilute for smaller patients for accurate dosing
Meloxicam 0.5 to 1 mg/kg intramuscularly
Ketoprofen 2 mg/kg intramuscularly
Midazolam 0.2 to 0.5 mg/kg
Mask induction (Fig 2) with the bird restrained by the Elizabethan hold (Fig 3) in a towel is the most common method. The capture and restraint needs to be executed smoothly, with a calm approach that generates minimum stress (Figs 4, 5 and 6); undue stress at this stage may predispose towards cardiac arrhythmias caused by the combination of the anaesthetic drugs with resultant endogenous catecholamine release (Hartsfield and McGrath 1986). The reader should refer to avian textbooks for detailed information on safe capture and restraint techniques.
The Bain anaesthetic non-rebreathing circuit is most commonly used. This is attached to a clear mask with a flexible non-porous diaphragm and placed over the head with an isoflurane concentration of 4 to 5 per cent and a flow rate of approximately 1 litre/minute. Sedation is usually achieved in less than a minute, quickly followed by surgical anaesthesia. At this stage the isoflurane concentration is reduced to a maintenance of between 2 and 4 per cent and the bird is ready for intubation.
Birds weighing more than 200 g should always be intubated. If the bird is below 100 g, the small tubes required are more likely to become blocked with secretions and the resulting higher resistance may start to impede ventilation.
The glottis is located behind the tongue (Fig 7). It can be viewed by gently pulling the tongue forward or alternatively pressing on the ventral aspect of the throat with one finger while holding the beak open to push the glottis anteriorly. The endotracheal tube (sparingly lubricated) is then inserted into the trachea (Fig 8). Care should be taken not to push the tube too far down as in many species the trachea narrows distally. The tracheal lining is easily traumatised resulting in postintubation tracheal stenosis, which usually becomes apparent two to three months after anaesthesia. This risk can be minimised by choosing the appropriate sized tube for the bird (Fig 9), careful placement of the tube, never using cuffed tubes (even a deflated cuff can cause damage), and keeping the trachea straight during the period of anaesthesia. The tube is secured to the mandible with tape (Fig 10).
A surgical plane of anaesthesia is maintained by adjusting the concentration of anaesthetic gas to a level where a toe pinch does not elicit a response and there is complete muscle relaxation of limbs and neck. Breathing should be steady and heart rate should be at or close to the allometrically determined level for that bird (Pokras and other 1993). The allometric heart rates of birds weighing 100 g, 500 g and 1000 g are 265, 184 and 156 beats per minute, respectively.
The anaesthetist must also monitor mucous membrane colour through either direct visualisation of the everted vent or the oral mucosa, and capillary refill time should be determined as an indirect measurement of peripheral tissue perfusion.
Anaesthetics for procedures lasting longer than five minutes should have monitoring enhanced by the addition of one or more of the following: Doppler blood pressure measurement every five minutes, capnometry, constant body core temperature display, electrocardiography, and pulse oximetry (in decreasing order of usefulness in avian anaesthesia) (Figs 11 and 12).
Vascular access via either an intravenous or intraosseous catheter (Boxes 4 and 5) will be essential to treat hypotension during the anaesthetic. If systolic blood pressure falls below 90 mmHg then the first step is to correct the cause, if possible. The most common causes are hypovolaemia due to bleeding or when the concentration of the inhalant anaesthetic is too high. Then immediately administer a bolus of intravenous or intraosseous fluids of 10 ml/kg crystalloids and 5 ml/kg colloids until the blood pressure is above 90 mm/Hg.
Box 4: Intravenous catheter placement
Peripheral venous catheters 21 to 25 gauge are used, depending on the size of the patient; hypodermic needles 23 to 25 gauge can be used in smaller birds. Intravenous catheterisation is an aseptic technique
The right jugular is approached over the featherless area of skin overlying it. A catheter of a length that will reach the thoracic inlet is placed. It is secured by either suturing to the skin or a bandage. Catheters are more difficult to maintain at this site.
The basilic vein on the medial aspect of the elbow is an alternative site. After the catheter is secured by skin suture, the wing is immobilised with a figure of eight bandage. Smaller sized catheters are used at this site.
The dorsal metatarsal vein is an alternative choice for birds over 300 g. At this site, the catheter can be effectively secured just with tape.
Box 5: Intraosseous catheter placement
For larger birds, short spinal needles 19 to 21 gauge are used, while hypodermic needles 23 to 25 gauge can be used in smaller birds. The site is prepared aseptically (a trocar is often needed to remove a bone plug from the needle and can be improvised from orthopaedic wire).
Distal ulna: the dorsal tubercle is identified and the needle inserted through this into the medullary cavity Proximal tibiotarsus: the cnemial crest is palpated with the stifle flexed and the needle is inserted slightly on the craniomedial aspect through the insertion of the patellar ligament into the medullary cavity.
Avoid other sites as the pneumatisation of medullary cavities varies with species.
Routine fluids administered during the anaesthetic are isotonic crystalloids, such as warm lactated ringers. These are given intravenously or intraosseously by continuous infusion or by slow bolus administration. Maintenance rates are calculated at 5 to 10 ml/kg per hour. There is considerable species variation but, in general, the smaller species require higher rates of administration.
Positioning for the procedure
Positioning of the patient while under general anaesthesia has a major bearing on respiratory efficiency. In dorsal recumbency, the viscera are pushing anteriorly, compressing the abdominal and thoracic air sacs and so compromising respiration. However, this position is commonly required to facilitate many procedures. Lateral recumbency is the most beneficial position for normal respiration, assuming that there is no unilateral respiratory disease, in which case it is important not to lay the bird on the normal side.
When the procedure is complete, the bird should be switched to 100 per cent oxygen and held vertically in a light grip so that its sternal excursions can be monitored. Heart rate should also be monitored during recovery. With recovery, the rate and depth of respiration and heart rate should increase. If recovery is slow, moving the wings in and out can help stimulate respiration. As recovery progresses normally, the anaesthetic monitoring devices can be progressively removed. This includes the temperature probe, blood pressure cuff and electrocardiogram leads. The endotracheal tube is removed once the bird begins to move its head. Before removal, it is important to check for any accumulated secretions, blood or regurgitated fluids and remove these if necessary. The intravenous or intraosseous catheter can be removed at this stage unless it will be required postoperatively for continued support. Most birds experience recovery delirium, which involves vigorous flapping of wings during which they can injure themselves. The bird is lightly controlled in a towel during this period. Monitoring must also continue. Once the patient can stand, it can be placed in the recovery area.
The recovery area should ideally provide thermal support at approximately 30°C for psittacines and most passarines. Appropriate food should be offered within 15 to 30 minutes. This is more important for the smaller species. Raptors occasionally regurgitate on recovery. Should this happen, the bird's head should be restrained downwards and the oral cavity cleaned out with long cotton swabs or equivalent.
Stop anaesthetic administration and check if a pulse or heart rate are present;
Place endotracheal tube if not already present;
Position bird in ventral recumbency with the head held up;
Start positive pressure ventilation;
Give intramuscular injection of doxapram hydrochloride; and
Monitor heart rate.
Start chest compressions at a rate of 60 to 80 per minute;
There will be respiratory arrest which needs to be treated concurrently; and
Give adrenaline intravenously, intraosseously or intratracheally.
The surgical team should practice resuscitation techniques in advance and always have a simple crash box present (Fig 13), with quick reference dose charts for the emergency drugs doses. At the Avian and Exotic Animal Clinic a plastic box is used, with two extendable shelves which have labelled compartments so that drugs and other emergency equipment can be found quickly. The crash box should contain laminated charts of: drug doses (in millilitres) for a range of bodyweights to facilitate speed in an emergency; list of drugs, disposables and other equipment which are to be kept in the box and their position; a cardiorespiratory resuscitation flow chart; and a record of stock checks of the crash box. The box should be checked at regular intervals to ensure that it is fully stocked, that everything is in date and in the correct position, and that unnecessary items are not being added.
Adequate nutrition is very important for recovery. The goal of nutritional support is to provide the substrate for healing and repair, minimising loss of lean body mass and preventing the development of malnutrition. Initially assess the nutritional status of the patient. Body condition is the important factor rather than bodyweight. Many birds will eat on their own within a short time of recovery if a suitable familiar food is available and they are in a warm, quiet, stress-free environment. Those cases that are not eating will need tube feeding. This is especially critical in the most rapidly growing juveniles which, without adequate nutrition, will become progressively weaker.
Cases requiring short-term nutritional support can be given products such as Critical Care Formula (Vetark Professional). However, for longer-term nutritional support, a more complete elemental diet is required such as the Emeraid system (Lafeber International). The reader is referred to avian medicine textbooks and Chan and Freeman (2006) for more detailed information on nutritional support.
A cuff with a width that is 40 per cent the circumference of the limb is placed on the distal humerus or femur; for birds under 300 g, the results are more accurate if the cuff is placed on the distal humerus. In general, blood pressure measurement becomes progressively more difficult in smaller patients. The Doppler probe is placed over the proximal ulnar, distal ulnar or metatarsal arteries. The cuff is inflated until the blood flow sounds are cut off, then the cuff is deflated slowly until the first sound is heard, signifying systolic blood pressure.
If the systolic blood pressure is below 90 mmHg then immediately administer intravenous or intraosseous crystalloids at 10 mg/kg and colloids at 5 mg/kg as a bolus. At the same time, attend to any identified causes and continue to monitor blood pressure, giving further boluses until the blood pressure is above 90 mmHg.
Capnometry is the measurement of carbon dioxide in exhaled air. There is near equilibrium between arterial and alveolar carbon dioxide. Levels of carbon dioxide in exhaled air above 45 mmHg indicate inadequate respiration. Causes can be an increased depth of anaesthesia, the position of the patient compromising respiration, or some other factor, such as pathology of the respiratory system. It may be necessary to decrease the anaesthetic depth or reposition the patient.
Electrocardiography is useful for detecting cardiac arrhythmias. Early bradycardia and ST segment depression are often the first signs of anaesthetic overdose. However, the electrocardiogram will often be normal in the face of severe cardiopulmonary compromise. Ensure the electrocardiogram recorder used can measure the rapid heart rates of birds.
Hypothermia is a major concern, especially in smaller birds, and is often the cause of prolonged recoveries. A temperature probe placed either in the oesophagus or cloaca will allow the necessary adjustments to be made in time. Supplemental heat can be provided by a combination of heated operating tables, forced air warming blankets or circulation water blankets.
A pulse oximeter is less useful due to its reduced accuracy in avian patients. It may be used to monitor trends but is best applied in conjunction with capnometry.
With smaller species (eg, most commonly seen passarines), there is a higher risk of hypothermia and there are higher fluid requirements (up to 100 ml/kg per day). There is also a greater risk of administering drug overdoses, which can be reduced by obtaining accurate weights and diluting drugs for accurate dosing.
During mask induction of these species, periods of apnoea and bradycardia lasting up to five minutes can occur. This has been called the dive response and is a stress reaction caused by stimulation of trigeminal receptors in the beak and nares. Although potentially dangerous, this response can be reduced by the use of antianxiety drugs such as midazolam as a premedication, thus facilitating a safer mask induction.
There is a risk of regurgitation on recovery with raptors. With these birds, it is important to use an appropriate diet for nutritional support.
Ratites weighing less than 20 kg can be safely mask induced with isoflurane, as detailed above. Once intubated, oxygen flow rates should be in the region of 1 to 2 litres per minute. Recovery should be rapid and the bird should be restrained until conscious to prevent injuries in the recovery period. Larger birds, due to their size and potential danger (Fig 14), present a unique challenge to the avian anaesthetist. The reader is referred to West and others (2007) and advised to seek the help of an experienced colleague. The anaesthetic regimen must offer a quick, safe and minimally stressful induction, consistent maintenance and a rapid recovery, while at the same time minimising the exposure of the operator to risk of injury from the patient.
Keeping environmental stressors to a minimum is very important to the avian patient. Avoid the sound, smell and sight of dogs, cats, ferrets or other predators by hospitalising avian patients in a separate area. Gentle human contact and efficient low stress capture and handling for medication is beneficial and is facilitated by appropriate hospital cages (Fig 15) and staff handling expertise.
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