Anaesthetising avian patients is an uncommon procedure in general practice unless clinicians have a particular interest in exotic practice. For many clinicians, avian anaesthesia is limited to the evaluation of wild birds that present to the clinic for assessment. However, the need may arise for veterinary staff to perform a general anaesthetic on an avian patient. Common reasons for this to occur in general practice may be for radiographs to assess trauma, the removal of a metal identification ring from the leg, beak trimming or blood sampling in the case of an unwell bird.
While the prospect of anaesthetising an avian patient may be daunting, it is a common and relatively safe procedure that can be achieved with the equipment found in any veterinary practice.
Avian anatomy: what’s different?
There are several anatomical differences between birds and companion mammals that must be considered during general anaesthesia.
Air sacs
Perhaps the most well-known difference is the presence of air sacs. Birds have fixed, non-expansile lungs, which are involved in gas exchange. Air is pushed through the lungs by thin-walled sacs, which act as bellows moving air through the respiratory system (Lawton, 2016). This is an important consideration for positioning under general anaesthesia as the coelomic viscera can compress air sacs when an avian patient is recumbent, preventing full expansion and therefore reducing airflow to the lungs.
In addition, respiration is an active process in which the sternum moves downwards and the ribs move outwards (Lawton, 2016) to draw air into the air sacs. If a patient is deeply anaesthetised or placed in a way that restricts this movement, hypoventilation may occur. An assessment of air sac and lung volume under general anaesthesia by use of computed tomography (CT) showed that the greatest air sac volume and lowest lung density were achieved by placing patients in sternal recumbency (Figure 1) (Malka et al., 2009). Previous studies have suggested that placing patients in dorsal recumbency results in hypoventilation, but this conclusion has been successfully challenged (Hawkins et al., 2013).
The benefit of air sacs as a way to push air around the respiratory system is birds’ ability to develop a one-way system within the respiratory tract. With each inspiration and expiration, air is moved further through the one-way system, creating an efficient gas exchange system. Because of the efficiency of this system, avian patients respond to changes in inhalational anaesthesia faster than their mammalian counterparts.
Because of the efficiency of this system, avian patients respond to changes in inhalational anaesthesia faster than their mammalian counterparts
The crop, larynx and trachea
Most birds possess a food storage organ in the oesophagus called the crop. The crop has no sphincter, so fasting patients must be weighed up against the potential for hypoglycaemia during general anaesthesia (Heard, 2016). Fasting intervals before general anaesthesia are species-specific, with larger birds requiring longer periods of fasting and smaller psittacines and passerines requiring a far shorter fasting interval.
Birds also possess laryngeal anatomy that differs from that of companion mammals but is more similar to reptile patients. Birds lack an epiglottis and, instead, have a glottis positioned at the base of the tongue (Figure 2). This makes intubation relatively straightforward but results in a risk of aspiration if the patient is not intubated during general anaesthesia.
Birds have complete tracheal rings and wider tracheas than similarly sized mammalian counterparts (Heard, 2016). This is important for endotracheal tube selection as complete tracheal rings mean cuffed tubes should not be used due to the risk of compression of the tracheal mucosa and subsequent tracheal stenosis from trauma. Cases of post-intubation trauma often occur one to three weeks after the intubation event (Heard, 2016) and should be high on the differential list for a patient presenting with dyspnoea within this time frame.
It’s all in the planning
One of the most important aspects of performing anaesthesia on any avian patient is planning. Both patient and equipment factors must be considered before any general anaesthesia is attempted, and the author recommends that all the equipment needed for the procedure is set aside and tested before anaesthetic induction (Figure 3).
Prior to any general anaesthesia, a thorough clinical examination, including observing the patient from a distance in a quiet environment, should be performed (Sabater González and Adami, 2022).
While age, sex and patient weight appear to have no effect on mortality associated with general anaesthesia, the health status of patients before general anaesthesia has a significant effect on mortality outcomes, with patients assessed as “unhealthy” before general anaesthesia having an 11.5 to 185.2 times higher risk of death associated with anaesthesia (Hollwarth et al., 2022).
A similar study found an increased mortality rate with an increased length of general anaesthesia, reinforcing the need to have all equipment prepared prior to general anaesthesia to ensure the patient is anaesthetised for the shortest time achievable.
Each patient can be assigned an American Society of Anesthesiologists (ASA) grade based on clinical history and patient examination before the procedure, which helps veterinary staff determine their general anaesthesia risk and any precautions that need to be taken prior to or during anaesthesia.
While age, sex and patient weight appear to have no effect on mortality associated with general anaesthesia, the health status of patients before general anaesthesia has a significant effect on mortality outcomes
Premedication
Once the procedure has been planned and discussed between the clinical team, premedication should be considered. Not all procedures require premedication (blood sampling and other non-invasive procedures, for example), but most anaesthetic agents do not provide anti-nociception, and analgesia is often required for anything that would be expected to be painful. Premedication is also useful to decrease patient anxiety, facilitate handling and reduce the requirement for inhalational anaesthesia (Heard, 2016). Administration of premedication can be provided by intravenous, intraosseous, intramuscular or intranasal routes.
Anaesthesia
Induction and securing an airway
Induction can occur by inhalational or injectable routes. Inhalational anaesthesia is still very commonly used as an induction method in avian patients because of their highly efficient respiratory systems, reducing handling time and stress (Lawton, 2016). Halogenated ethers such as isoflurane and sevoflurane can be provided via a face mask (Figure 4) and have been shown to provide rapid, reliable inductions and smooth recoveries from general anaesthesia in avian patients (Granone et al., 2012). Injectable anaesthetic agents can also be used, either at a set dose rate or to effect. Alfaxalone, ketamine and propofol have been successfully used to induce many species of avian patients with predictable outcomes (Heard, 2016).
Once the patient is anaesthetised, a suitably sized non-cuffed endotracheal tube can be used to secure an airway. If patient positioning prevents full expansion of the air sacs, intubation can facilitate intermittent positive pressure ventilation (IPPV), preventing hypoventilation (Lawton, 2016). IPPV can be performed manually by the anaesthetist or by use of a mechanical ventilator for small exotic patients.
Maintenance and monitoring
During the maintenance of anaesthesia, several monitoring techniques can be employed. The assessment of reflexes, including withdrawal, palpebral and corneal reflexes and cloacal muscle tone, can be used to assess the depth of anaesthesia (Sabater González and Adami, 2022). However, a sudden piloerection of feathers under general anaesthesia may indicate cardiac arrest, therefore patient status should be assessed immediately in case cardiopulmonary resuscitation is required (Heard, 2016).
A stethoscope can be the most useful tool for monitoring patients under general anaesthesia. Auscultation of the heart and air sacs can give valuable information that cannot be gained by machine monitoring. An oesophageal stethoscope can be of particular use for cardiac auscultation, although the patient must have been adequately fasted before use. Pulses can also be assessed by palpation or by placing a Doppler probe over a peripheral artery to assess blood flow (Sabater González and Adami, 2022). Blood pressure can be measured indirectly with a Doppler probe and blood pressure cuff, with normal ranges of 90 to 150mmHg reported in anaesthetised psittacines (Lichtenberger, 2005).
End tidal carbon dioxide can be monitored easily with a capnograph if the patient is intubated (Figure 5), ideally with microstream capnography that allows sampling of very small tidal volumes (Sabater González and Adami, 2022).
Temperatures should also be monitored for extended anaesthetic procedures to ensure the patient is not losing or gaining too much heat.
Recovery
During anaesthetic recovery, it is imperative that the patient is held until it can stand and support the weight of its head. Many patients will experience recovery delirium, which is usually associated with vigorous flapping of the wings (Raftery, 2013). Because of this, it is recommended that avian patients are placed in a loosely held towel during recovery to prevent self-trauma, or trauma to the handler when dealing with birds of prey (Figure 6). Once standing, patients should be placed in a warm, dark, quiet recovery kennel and frequently monitored.
A significant proportion of the mortalities associated with general anaesthesia occur during the recovery period. One study showed that more than 80 percent of recorded deaths associated with general anaesthesia occurred following anaesthetic, with most of these falling within zero to three hours post-anaesthesia (Hollwarth et al., 2022).
A significant proportion of the mortalities associated with general anaesthesia occur during the recovery period
Conclusion
Avian general anaesthesia can present a steep learning curve compared to general anaesthesia in companion mammals, but most concepts can be translated between species. If performed correctly with adequate prior planning, avian patients can have smooth and successful procedures under anaesthesia. Case selection and thorough patient examination before procedures are crucial to achieve successful outcomes when performing general anaesthesia on avian patients.
