Abstract

Anaesthesia of avian patients is frequently utilized to perform surgery, painful procedures, and certain examinations. Anaesthesia may also be beneficial when handling fractious or fearful patients to minimize stress. Inhalant anaesthesia is the safest protocol to use as injectable anaesthetics may lead to prolonged and rough recoveries and there is marked interspecies variation in sensitivity to anaesthetic drugs. There are multiple described anaesthetic protocols for use in birds and the regimen chosen should be adapted to the individual circumstances and chosen for ease of use and minimal side effects. The veterinarian should keep the unique respiratory physiology and anatomy of the bird in mind when working with avian patients.

Introduction

The avian respiratory system is unique in the animal kingdom and employs two distinct and separate systems for ventilation and gaseous exchange.¹

The avian glottis lacks an epiglottis and intubation is subsequently easier than in mammals. The trachea consists of complete tracheal rings and is proportionately larger than in mammals, creating increased dead space. Cuffed endotracheal tubes should not be used to prevent pressure necrosis of the trachea which is unable to expand due to the complete tracheal rings.

The sound-producing organ, the syrinx, is situated at the tracheal bifurcation.

Gaseous exchange occurs at the level of the parabronchi which are long narrow tubes that have numerous openings into the atria which have ducts that lead to air capillaries. As the air flow through the terminal parabronchi is bidirectional, gaseous exchange occurs in a crosscurrent manner.¹

Most birds have nine airsacs - paired abdominal, paired caudal and cranial thoracic, paired cervical, and a single clavicular sac. The airsacs are separated functionally into a cranial and a caudal group.¹ The airsacs function as bellows to the rigid avian lung during both inspiration and expiration. The airsacs are poorly vascularized and play no role in gaseous exchange.

Both inspiration and expiration require muscular activity. There is no diaphragm and thoracic muscles are used to drive ventilation.

Gaseous exchange occurs during both inspiration and expiration, making the avian respiratory system extremely efficient. During inspiration, air flows both through the lungs and into the caudal airsacs. During expiration, the air from the lungs moves into the cranial airsacs and out and the fresh air from the caudal airsacs flows through the lungs.

The four-chambered heart is proportionately larger with a larger stroke volume, lower heart rate, higher blood pressure and higher cardiac output than in similarly sized mammals.¹

Use of Inhalation Agents

Isoflurane and sevoflurane are the two recommended inhalation agents. Mask induction is used and the bird then intubated. Chamber induction is also effective. The ET tube is then securely attached to the lower beak. Isoflurane is used at 5% for induction and maintenance is normally achieved at 2.5%.

In waterfowl there is a diving response which causes episodes of apnoea and bradycardia during induction with inhalation agents. Should this occur, the inhalation agent should be turned off and the bird oxygenated for a few minutes until respiration returns to normal.

Proper restraint, usually with a towel should be used at all times to prevent flapping and flailing and subsequent injury.

Non-rebreathing circuits are typically used unless the bird is very large (> 10 kg).

Intermittent positive pressure ventilation (IPPV) by compression of the reservoir bag or with a ventilator is recommended during anaesthesia. The patient is typically placed in dorsal recumbency and in this position the pressure of the abdominal viscera causes compression of the airsacs and subsequent hypoventilation.

Injectable Anaesthetics

There is significant interspecies variation in sensitivity to injectable agents and prolonged and violent recoveries are common. Injectable agents are normally only used in field conditions where inhalation anaesthesia is impractical.

Ketamine hydrochloride in combination with diazepam or medetomidine is used but results are unpredictable due to interspecies variation. Carpenter's Exotic Animal Formulary lists a number of recommended regimens.

Propofol can be used successfully for intravenous induction in larger patients. The onset of anaesthesia and recovery are rapid with little residual or cumulative effect. It does, however, cause a dose-dependent cardiovascular and respiratory depression.¹

Analgesia

Opioids are commonly used for analgesia. In pigeons, it appears that the kappa opioid receptors account for the majority of the opioid receptor sites.¹ Thus, butorphanol, a kappa agonist is a better analgesic than mu opioid agonists. Use of butorphanol reduces the concentration of isoflurane needed to maintain anaesthesia.¹ Butorphanol is used at 0.2–1 mg/kg.²

Nonsteroidal anti-inflammatories are also recommended for analgesia. Meloxicam (0.2–0.5 mg/kg)² and carprofen (10 mg/kg)² have proved safe and effective.

Local anaesthetics should be used as an adjunct to anaesthesia and not for local anaesthesia in the awake bird. Lignocaine at 1–3 mg/kg² has been recommended.

Monitoring

The most critical aspect of the process. Apnoea, hypoventilation, hypothermia, and regurgitation are the most common perianaesthetic complications.¹

A Doppler flow probe is an effective means of monitoring pulse rate and rhythm. A pulse oximeter with a transflectance probe may be placed inside the cloaca or in the oesophagus. Without access to advanced monitoring equipment, respiratory rate and tidal volume as well as pulse rate and turgidity of the ulnar vein to assess blood pressure can easily be monitored.

Corneal reflex, jaw tone, cloacal reflex, pedal reflex, and muscle tone are used to assess anaesthetic depth.

Ideally, a surgical plane of anaesthesia is reached when the eyelids are closed with mydriatic pupils and a delayed pupillary light reflex. The nictitating membrane should move slowly over the entire cornea when stimulated and all pain reflexes should be absent.¹

The eyes should be lubricated at induction.

Hypothermia occurs quickly during anaesthesia. Patients should be covered with towels or drapes and supplementary heat using forced air blankets or heating pads should be provided. Care should be taken not to wet the entire bird when preparing the surgical site as this will speed up the rate of heat loss. Plucking of feathers should also be minimized.

Regurgitation can be prevented by starving the patient prior to anaesthetic. Due to their high metabolic rates the period of starvation should be as short as possible to prevent hypoglycaemia. Small finches may need as little as 30 minutes while vultures can safely be starved for 24 hours.

Recovery

The patient should be safely restrained until the period of unconscious excitement has passed, as flapping and flailing occur and injury can result. Care should be taken not to hold the patient too firmly, as pressure over the thorax will limit the expansion of the respiratory muscles and cause hypoxia.

Supplemental heat during the recovery period is critical. Incubators or brooders are ideal for recovery but infrared lights or even the use of a hair dryer can be used when facilities are limited.

Oxygen should be supplied from when the inhalant agent is turned off until the bird is alert.

Conclusion

Anaesthesia of birds is an extremely useful procedure and, with proper technique and monitoring, has an excellent success rate. Inhalation anaesthesia is the technique of choice, with the lowest rate of complications. Appropriate analgesia must be used with painful procedures and careful perianaesthetic monitoring must be performed to identify possible complications at the earliest possible time. Clinicians should ensure proper postanaesthetic care including restraint and supplemental heat to minimize complications on recovery and should starve patients for a species appropriate period of time before the anaesthetic to prevent regurgitation. Following the proper technique, avian anaesthesia can become a routine and successful procedure in avian practice.

References

1.  Harrison GJ, Lightfoot TL. Clinical Avian Medicine. Vol 2. Palm Beach, FL, USA: Spix Publishing, Inc. 2006: 747–760.

2.  Carpenter JW. Exotic Animal Formulary. 4th ed. St. Louis, MO, USA: Elsevier Saunders; 2013: 256–276.