ABSTRACT: In this second article on anaesthesia in exotics, anaesthesia of the avian patient will be discussed, concentrating on the birds most commonly presented in practice – the Psittaciformes (parrots), Passeriformes (small perching birds), Falconiformes (diurnal birds of prey) and Strigiformes (owls).

Relevant anatomy and physiology

Each group of birds has vastly different anatomical and physiological variations, but there are a few key points relevant to anaesthesia that they all share.

•   Birds have a much faster metabolism than most other animals, so as a result they naturally maintain a much higher temperature (varying between 39-42°C depending on species), need to eat more frequently, and metabolise drugs more rapidly

•   They also share a unique respiratory system, composed of a series of airsacs which act as bellows to pump air through the small, relatively immobile, lungs. Air passes around the respiratory system and through the lungs in two cycles, to allow for extremely efficient gaseous exchange

•   In addition, like reptiles, birds do not have a diaphragm. Breathing is therefore, controlled by small movements of their intercostals and abdominal muscles. As a result it is important not to restrict these movements under anaesthesia.

Pre-anaesthetic considerations

A full pre-anaesthetic examination, including obtaining an accurate weight, should be attempted with every patient. Birds can, however, easily be stressed, especially if not accustomed to regular handling, and this may lead to the sudden release of adrenalin and other catecholamines which can occasionally be fatal. In highly stressed birds, therefore, conscious examination is best kept brief, and a full examination under general anaesthesia is preferred.

Once the patient has been assessed initially, it should be stabilised. If stressed or dyspnoeic, the bird should be placed in a warm (30°C) incubator supplemented with oxygen. Pre-oxygenation via a chamber is preferable to using a mask, as it decreases stress for the bird.

Fluid therapy – and even nutritional support – may also be required. Fluids may be administered by the subcutaneous or oral route (Figure 1), or even via an intravenous or intraosseous catheter, although these are best placed under general anaesthesia, unless the bird is very collapsed. Nutritional support is normally given by crop tube if the patient is not eating enough to maintain its weight voluntarily.

Figure 1: Subcutaneous fluids may be easily administered in the inguinal region

Pre-anaesthetic starvation is generally required in order to ensure that the crop has been emptied prior to anaesthesia.

In most parrots, a starvation period of two to three hours should ensure crop emptying and prevent regurgitation, but it is always best to palpate the crop to check this prior to anaesthesia.

It is not advisable to starve birds weighing <100g, as their rapid metabolism predisposes them to hypoglycaemia. Raptors, in contrast, need starving significantly longer (<12 hours), and an anaesthetic should generally be delayed until a raptor has ‘cast’ – regurgitated the indigestible parts of its last meal, such as fur or feathers.

Pre-anaesthetic bloods may be taken in some cases, but in most birds safely obtaining a blood sample will require sedation or a general anaesthetic, so these are not commonly carried out unless there are particular concerns with an individual.

Induction

Isoflurane administered by mask is still the most commonly used anaesthetic in avian medicine (Figure 2), and generally induction occurs within 30 seconds to two minutes using 4% isoflurane, with much less excitement than in mammals. Sevoflurane is also beginning to be increasingly used, but is not yet licensed in these species.

Figure 2: Birds should be held firmly in a towel for mask induction to prevent struggling

Pre-medication with benzodiazepines or butorphanol may be of benefit to reduce the required anaesthetic gas concentration and stress at induction, but this should be weighed up against the stress of increased handling to premedicate the bird. Injectable induction agents may be preferred in some of the larger birds such as swans, but are not generally used for most patients in practice (Figure 3).

Figure 3: Induction via the medial metatarsal vein may be easily performed in swans

Masks may be the purpose-made variety, although bottles or even syringe cases may be adapted for particularly small patients or those with a long beak. For any procedure that is going to last longer than 10 minutes or any high-risk cases, the patient should ideally be intubated. This is relatively easy in a bird weighing >100g, although birds smaller than this can be technically challenging, and there is an increased risk of mucus obstructing the endotracheal tube in an airway this small.

Uncuffed endotracheal tubes are preferred as the avian trachea is made up of complete cartilage rings. Specialised small endotracheal tubes are commercially available, or alternatively urinary or intravenous catheters may be adapted for the purpose.

The glottis may be easily visualised by pulling the tongue forward with plastic forceps (Figure 4). The tube should then be tied or taped securely in place in order to prevent movement which may lead to the formation of tracheal strictures following anaesthesia (Figure 5).

Figure 4: The glottis in a raptor may be easily visualised by pulling the tongue forward. Parrots have thicker more fleshy tongues and a smaller gape

Figure 5: Securing the endotracheal tube firmly in place is important to prevent the formation of tracheal strictures

In an emergency situation, when endotracheal intubation is not possible, airsac cannulation can be a useful technique. This is carried out by making a small incision between the last two ribs and a cannula placed here can be attached to a normal breathing circuit, and anaesthesia may be maintained by this route (Figure 6).

Figure 6: An African Grey being ventilated via an air sac

Maintenance

Once intubated, birds may be maintained on gaseous anaesthesia. Owing to their efficient respiratory cycle, however, birds respond to changes in anaesthetic gas concentration very rapidly, and a period of apnoea in a bird will lead much more rapidly to cardiac arrest than in a similarly sized mammal. It is therefore advisable to ventilate any bird which is going to undergo a procedure longer than 10 minutes.

Ventilation may be manual or, ideally, by a mechanical ventilator. Respiratory rates may be set at 10-15 breaths per minute. The appropriate pressure will depend on the size of the individual patient, but it is best to start with a low pressure and then to increase this slowly until small breathing movements are seen, resembling those of the conscious bird.

It is also important to conserve heat in these small patients as their rapid metabolism, and high surface area:volume ratio predisposes them to dangerous heat losses. Temperatures may be maintained by minimising plucking of surgical sites, the use of warm scrubbing agents, a warm background room temperature, and the use of ‘hot hands’, although care must be taken to ensure these are not in direct contact with the patient in order to avoid burns.

As with mammals, peri-anaesthetic fluid therapy is advised at similar rates.

Monitoring

Anaesthetic monitoring is critical in these smaller patients, and constant monitoring of respiratory rate and rhythm is very important. It is also useful to become familiar with palpating the brachial pulse (Figure 7). Various reflexes – including palpebral and pedal reflexes – can also be assessed to judge depth of anaesthesia, but the corneal reflex is most useful in birds (Figure 8).

Figure 7: The brachial pulse may be easily palpated in the axillary region

Figure 8: The corneal reflex may be checked with a damp cotton bud, and should remain present throughout anaesthesia

A damp cotton bud should be used to touch the corneal surface, and the nictitating membrane should move across the eye. If anaesthesia is too deep, the membrane will move more slowly, and concentration of gaseous anaesthetic should be reduced. Ocular lubricants may also be useful during a prolonged anaesthesia to prevent corneal damage when checking this reflex.

Additional monitoring aids may also be used in birds, just as in mammals. They include ECG, capnography and pulse oximetery, but these methods are beyond the scope of this article.

Post-anaesthesia

Recovery post-anaesthesia is generally thought to be the highest risk time for avian patients. Careful monitoring is, therefore, required throughout this period. Extubation should only occur when jaw movements are increasing and voluntary breathing is occurring. It is then advisable to transfer the patient to a face mask and supplement with oxygen.

The patient should be held upright, with the head supported and the body only restrained gently to prevent any restriction of breathing. The bird should be supported in this position until able to perch, at which point it can be placed in a pre-prepared warm incubator with additional oxygen if necessary.

Food should be offered as soon as the bird is no longer ataxic, and if not eating within two hours, crop tubing should be carried out to prevent hypoglycaemia (Figure 9).

Figure 9: Recovery should be in a warm incubator and food should be offered as soon as possible to prevent hypoglycaemia

Analgesia

Finally, it is important to remember that most anaesthetic agents we use in birds have little or no lasting effect in providing analgesia, so additional analgesics should be used for any potentially painful procedure. Butorphanol appears to be the most effective opioid in birds, and NSAIDS also appear effective and are routinely used.

Local anaesthetics should also be considered, especially to minimise self-trauma post surgery.

Summary

By understanding the relevant differences between birds and mammals, it should be possible to provide the same standard of anaesthetic care for birds as for our traditional companion animal patients.

Author

Joanna Hedley BVM&S MRCVS

Joanna Hedley has had a varied clinical background since graduation from Edinburgh vet school in 2003, having worked in mixed, small animal and exotic practice, and wildlife rehabilitation both in the UK and abroad. She is currently senior clinical training scholar in exotics and wildlife medicine at the Royal (Dick] School of Veterinary Studies and is working towards her RCVS Certificate in Zoological Medicine.

Bibliography and further reading

BSAVA, Gloucester Pigeons and Passerine Birds. HARCOURT-BROWN, N. and CHITTY, J. (2005). Manual of Psittacine Birds. BSAVA, Gloucester. CHITTY, J. and LIERZ, M. (2008). Manual of Raptors, GIRLING, S. J. (2003). Veterinary Nursing of Exotic Pets. Blackwell Publishing, Oxford LONGLEY, L. A. (2008). Anaesthesia of Exotic Pets. Saunders Elsevier

Veterinary Nursing Journal • VOL 25 • No10 • October 2010 •