For prolonged anesthetic procedures, gaseous maintenance is required with isoflurane being the agent of choice. For avian patients larger than 100 g it is advisable to intubate the bird for this period as more effective control of rate and depth of anesthesia can be achieved. Also isoflurane causes dose-dependent respiratory depressive effects, hence with prolonged procedures, the patient may become apnoeic and require positive pressure ventilation.
Intubation is easily achieved once the patient is induced via the face mask. Once resistance is reduced, the beak may be opened using an avian gag, or simply by attaching one piece of bandage material to the lower beak and one to the upper, whilst the other operator places the endotracheal tube through the glottis which is easily visible per os at the base of the tongue on midline. Visibility is improved with a directional light source and by grasping the tongue, in those species with a mobile tongue, with a pair of atraumatic forceps and pulling it and the attached glottis rostrally. It is better not to inflate the cuff on endotracheal tubes as previously mentioned, due to the risk of causing severe damage to the lining of the rigid avian trachea. A good fitting tube is thus necessary. However, if prolonged positive pressure ventilation is required or if performing a crop wash to remove foreign bodies, the endotracheal tube cuff may be inflated carefully to prevent air escaping when the patient is ventilated or to prevent refluxed fluid being aspirated.
Anaesthetic breathing systems
Anaesthetic circuits used must minimise dead space and be of a non-rebreathing nature as many avian patients are much smaller than the more routinely seen cats and dogs. Modified Bain circuits, Mapleson C circuits or even the Ayres T-piece may be used for larger parrots and waterfowl. In any instance a 0.5 litre rebreathing bag is necessary.
Air sac catheterisation
In the event of an airway obstruction, or if head or oral cavity surgery is required, it may be necessary to deliver anesthetic gases via a tube placed directly into one of the air sacs. This is possible due to the close proximity of many of the air sacs to the skin surface, and the fact that they take no part in gaseous exchange, but as previously mentioned simply shunt the air back and forth through the rigid lung structure.
The clavicular and the abdominal / caudal thoracic air sacs may be catheterised. A small stab incision is made in the skin over the air sac, and the underlying muscle bluntly dissected with a pair of mosquito haemostats. The endotracheal tube is then inserted to a depth of 4-5 mm and sutured in place.
The clavicular air sacs lie at the thoracic inlet, just dorsal to the clavicles; the caudal thoracic air sacs may be entered between the sixth and seventh ribs, while the abdominal air sacs may be entered by raising the leg cranially and incising just caudal to the stifle through the lateral body wall.