Introduction

It is ironic that if a bird dies during a surgical procedure, it is undoubtedly the 'anaesthetist's fault', while a successful outcome is of course wholly due to the 'surgeon's skill'! While at first glance avian anaesthesia may seem simpler than in other animals (after all, you just use 'gas', don't you?), rapid simultaneous cardiorespiratory arrest and the difficulty of resuscitation highlight the fact that safe and reliable avian anaesthesia needs some basic knowledge of the differences in avian anatomy and physiology from mammals.

Pre-Anaesthetic Preparation

Birds should not be starved for longer than needed to empty the crop (2-3 hours) to prevent regurgitation and aspiration pneumonia, as they have a high metabolic rate and poor hepatic glycogen storage. It is often simple to palpate the crop, in species that have one, before anaesthesia, to ensure there is no remaining food or stasis. A good clinical examination before anaesthesia is essential for determining risk factors such as dehydration or respiratory compromise that will need addressing for safe anaesthesia. While birds are prone to hypothermia during anaesthesia, care must equally be taken not to inadvertently overheat birds, as this can rapidly prove fatal in small patients.

Preoperative Analgesia

It is well recognised that it is easier to prevent pain than to treat it after a noxious event. Preoperative analgesia will also result in a more stable anaesthesia with need for less anaesthetic agent, and often helps contribute to a smoother recovery. Local anaesthesia with lidocaine given intramuscularly or subcutaneously at 1-4 mg/kg (diluted 1:10) can be a useful adjunct to opioids or non-steroidal antiinflammatory drugs in surgical procedures.

Inhalant Anaesthesia

Mask induction is the most common technique. A latex glove can be placed over the mask opening with a smaller cut in the centre providing a seal, which can be used for intermittent positive pressure ventilation (IPPV) in a small patient that may not be intubated. Non-rebreathing systems such as modified Jackson-Rees and Ayre's T-piece, are most commonly used in avian anaesthesia and rely on high oxygen flow rates to remove carbon dioxide. They also allow immediate response to vaporiser setting changes and offer low resistance to breathing. Oxygen flow rates with these systems should be two to three times the minute ventilation (200 ml/kg/min), and if a mask is used in conjunction with these a total of five times minute ventilation should be used. This gives a handy flow rate of 1L/ kg/min for birds on a non-rebreathing system with a facemask.

The most commonly used inhalation agents are isoflurane and sevoflurane. Sevoflurane has a lower blood-gas partition coefficient and is much more soluble in blood, so leads to a faster recovery time and time to standing. It is however more expensive. It has also been reported to depress plasma ionised calcium levels, which may be a consideration in African grey parrots, which are prone to hypocalcaemia, as this could cause seizures.

IPPV by closing the valve and compression of the T-piece bag, or via a mechanical ventilator, is extremely useful as birds have a small functional residual capacity, which makes periods of apnoea life threatening. Without airflow there is no gaseous exchange in the avian lung. Manual ventilation should be performed at one breath every 4-5 seconds.

Maintenance of cardiac output and circulatory volume are essential to adequate tissue oxygenation. This can be addressed by fluid therapy and limitation of haemorrhage. Poor peripheral vascular perfusion may be evaluated by the ulnar vein filling time (after occlusion). A filling time greater than 0.5 seconds is indicative of poor peripheral vascular perfusion, usually accompanied by decreased turgor of the vein. Other signs may be tachycardia, tachypnoea and pale mucous membranes.

Monitoring Anaesthesia

When auscultating the cardiorespiratory system, rate and rhythm are all important (Figure 1). Changes in these will indicate a change in anaesthetic depth, and alert one to a bird that is either too deep, or reacting to the stimulation of surgery. In contrast to mammals, simultaneous cardio-respiratory arrest is common in birds so respiratory rate and rhythm are particularly important. A small-headed neonatal stethoscope is useful. Dysrhythmias are difficult to hear due to the rapid heart rate, but can be due to primary cardiac disease, or systemic disease such as septicaemia or shock. When auscultating the respiratory system, it is important to remember that the avian lung has a rigid almost sponge-like faveolar structure and no alveoli like mammals. Abnormal sounds require different interpretation to those in mammals. Not having alveoli, birds do not develop crackles, but an air sacculitis may result in a clicking sound on auscultation due to adherence or friction of the inflamed air sac surfaces.

Apart from encouraging retrospective rather than proactive monitoring, using a pulse oximeter can also distract one from monitoring vital signs properly, and high heart rates and poor contact can mean most of the time is spent adjusting the probe, rather than actually monitoring the patient. Cardiorespiratory arrest is most often simultaneous in birds, and by the time changes are noticed via the pulse oximeter, it may be too late. Electrocardiography (ECG) monitoring may be more useful in monitoring anaesthesia in more critical cases. It should be remembered that the mean electrical axis of ventricular depolarisation, the (Q)RS complex, is negative in birds due to depolarisation starting subepicardially, in contrast to mammals.

Just as in other animals reflexes can be a useful indication of the plane of anaesthesia. It is worth bearing mind, however, that loss of the third eyelid reflex normally indicates a bird to be in an excessively, even dangerously deep plane.

Figure 1. Approximate normal heart rates of birds (beats per minute).

Cardiorespiratory Resuscitation

The 'ABC' of triage and emergency stabilisation (airways, breathing and circulation) (Figure 2) is just as useful as in other animals, but in birds it is important to remember that simultaneous cardiorespiratory arrest is common. In the case of cardiorespiratory arrest, once an endotracheal tube or air sac tube is in place to establish an airway (A), the bird should be manually ventilated at one breath per 5 seconds (B). Doxapram can be administered intravenously or sublingually if intravenous access is not possible, at 10-20 mg/kg to stimulate respiration. Adrenaline can be administered either intravenously or, if not possible, sublingually (the avian tongue has a highly vascular structure), while cardiac massage or 'circulatory pumping' is performed (C). Even canaries can be intubated by using an intravenous catheter, attached to a cut down 2 ml syringe barrel, and a connector. If this is not possible, a mask and cardiac massage may be sufficient.

Figure 2. Triage priorities.

Recovery

Recovery from gaseous anaesthesia is rapid, but patients should still be kept warm to prevent hypothermia, and in a quiet, darkened environment to prevent the bird injuring itself with excessive movement. In more ill patients recovery in a warmed incubator with oxygen supplementation is recommended.