Due to the easily and often highly stressed nature of many avian patients, the use of chemical restraint, whether full anesthesia or less commonly sedation, is being used more and more in avian practice.
Naturally all methods of chemical restraint require that the patient be first restrained manually, even though it may only be for a short period, before the medication can be administered. The advantage therefore does not lie so much in avoiding manual restraint, but in minimising the period of manual restraint in order to reduce stress.
Before any form of chemical immobilisation can be used, an assessment of the patient’s health status has to be made, i.e. is the procedure necessary for life-saving medication / treatment, and is the patient’s condition likely to be worsened by the drugs used? Another area which must be considered and appreciated is the avian patient’s natural respiratory physiology which differs from its better known mammalian counterpart.
Overview of avian respiratory anatomy and physiology
For an overview of the respiratory cycle of the avian patient see site.
The avian respiratory system differs in a number of ways from the mammalian system, namely (starting rostrally):
• Vestigial larynx: The bird has a glottis, but there are no vocal cords or epiglottis. All of the sounds a bird makes come from an area known as the syrinx that lies at the bifurcation of the trachea deep within the ‘chest’.
• Complete cartilaginous rings to the trachea: This is important for two reasons. Firstly, it is relatively difficult (although not impossible!) to throttle a bird by compressing the neck. Secondly, there is no ‘give’ to the trachea and so inflatable cuffs on endotracheal tubes should not be over-inflated as this will cause pressure necrosis to the lining of the trachea. Instead it may be better to use a snug fitting uncuffed endotracheal tube. For some procedures, such as flushing the crop of a bird, it may be necessary to use and inflate the cuff on the endotracheal tube to prevent inhalation pneumonia. Extreme care should then be used.
• The avian respiratory cycle: This is such that the bird may gain oxygen from air on inspiration and expiration. A series of balloon-like air sacs exist inside the body of the bird, which in combination with the movement of the avian ribcage and sternum act to push air backwards and forwards through a rigid lung structure. This means that birds may extract oxygen from the air on both inspiration and expiration. This makes them much more efficient at gaseous exchange than mammals and allows the clinician to use this to his / her advantage should the upper airway / trachea become blocked. In this instance the caudally located air sacs may be cannulated with a cut-down endotracheal tube and the patient ventilated via this route.
It is useful to run biochemical and haemocytological tests on avian patients prior to administering anesthetics. Blood can be taken from the right jugular vein in all species, from the brachial vein running cranially on the ventral aspect of the humerus in the larger species and from the medial metatarsal vein in many waterfowl. Minimal tests include an assessment of the haematocrit, the total blood proteins, blood calcium, uric acid levels for kidney function and if possible both aspartate transaminase (AST) and bile acid levels to assess liver function. More comprehensive tests in the case of seriously ill patients should of course be performed if required.
Because of the high metabolic rate of avian patients, extended fasting may be detrimental to health, as hepatic glycogen stores can be quickly depleted. Birds larger than 300 g body weight are slightly less likely to become hypoglycaemic.
Fasting ensures emptying of the crop – the sac-like structure which acts as a storage chamber for food and sits at the inlet of the bird’s neck – and so stops passive reflux of fluid / food material during the anesthetic. Some species do not have a crop (such as most ducks and geese, penguins, seagulls and toucans), but they simply have a more distensible oesophagus which acts as its own storage chamber.
Most birds are fasted for 1-3 hours prior to their anesthetic depending on body size – the smallest having the shortest period of fasting (e.g. canaries, budgerigars). Birds >300g may be able to tolerate an overnight fast of 8-10 hours, assuming good health and body condition, and this may be necessary if surgery on the gastrointestinal system or crop is intended.
Whatever the period of fasting, water should only be withheld for 1 hour prior to anesthesia.
This may be combined with a sedative tranquilliser such as midazolam (see below) for minor procedures where full anesthesia is not required, such as minor skin wound repairs and cloacal prolapse replacements. The local anesthetic of choice is lidocaine hydrochloride with epinephrine. The epinephrine component is necessary to prevent rapid absorption of the lidocaine, which may induce fatal arrhythmias. It is advised that the stock solution (which is generally 2% concentration) be diluted with sterile water to create a 0.5% solution, particularly if dealing with smaller species. Even then, parrots below 200g should only be given small doses (average 0.1-0.15ml / 100g of a 0.5% solution).
These are used in cats and dogs to provide cardiopulmonary and central nervous system stabilisation, a smooth anesthetic induction, muscle relaxation, analgesia and a degree of sedation. Pre-anesthetic medications are used infrequently in birds. However, fluid therapy is important as it can make the difference between a successful procedure and failure.
Atropine and glycopyrrolate will reduce vagally induced bradycardia and oral secretions that may block endotracheal tubes. They both however may have unwanted side effects, the main ones being an unacceptably high heart rate, increasing myocardial oxygen demand and making oral / respiratory secretions so tenacious that they make endotracheal tube blockage even more likely.
Diazepam or midazolam may be used as premedicants in waterfowl, as these species may exhibit periods of apnoea during mask induction of anesthesia. This is due to a stress response (often referred to slightly inaccurately as a ‘diving response’) mediated by the trigeminal receptors in the beak and nares, whereby the breath is held and blood flow is preferentially diverted to the kidneys, heart and brain. Dosages of diazepam of 0.2-0.5 mg / kg intramuscularly or 0.05-0.15 mg / kg intravenously, or of midazolam at 0.1-0.5 mg / kg intramuscularly or 0.05-0.15 mg / kg intravenously, have been suggested. Midazolam is more potent and recovery is quicker than following diazepam and it tends not to affect mean arterial blood pressure and gases to the same extent.