November 2023 33 What makes a bird anaesthetic different? There are four fundamental differences between avian and mammalian anaesthesia: the anatomy of the bird; its metabolism; its tendency to lose heat quickly; and finally, that most birds requiring anaesthesia are chronically ill. Anatomy • Trachea o Complete tracheal cartilage rings and therefore not expandable. For this reason, only non-cuffed endotracheal tubes should be used. (If cuffed tubes are used, they must never be inflated.) o Additionally, some species (notably waterfowl) have either very long, coiled tracheas or diverticula (bullae). These are anatomical adaptations allowing these birds to hold their breath under water. It also may make it difficult to mask induct these species, making i/v or i/m induction advisable. o Voice production arises from the syrinx – the bifurcation of the trachea. This is located within the body, and is a common site for obstructions (aspergillus granulomas or inhaled birdseed). Air sacs, non-expandable lungs and lack of a diaphragm. o Respiration is achieved by movement of the sternum (up and down) and the ribs (in and out). If these movements are restricted, or if enlarged organs, fat or fluids within the body compress the air sacs, respiratory compromise can quickly develop. Therefore: Wherever possible place the bird in ventral or lateral recumbency. Birds in dorsal recumbency may require IPPV to properly oxygenate them. If any anaesthesia is likely to be prolonged more than 5-10 minutes, the patient should be intubated and ventilated, regardless of its positioning. Birds with ascites should be handled very carefully, and may need diuretic therapy or coeliocentesis before anaesthesia. o If the coelom is opened during surgery, the bird may be able to breathe through the abdomen, via the air sacs. This may require increasing the oxygen flow rate and deepening the anaesthesia to over come this. Catheterisation of the caudal thoracic air sac can allow anaesthesia to be maintained without tracheal intubation, leaving the head free to work on. Metabolism • The avian metabolic rate is much faster than mammals. For this reason, lengthy fasting is potentially dangerous, as hypoglycaemia may develop. Conversely, regurgitation and aspiration of crop and proventricular contents is a very real risk. It is generally recommended to fast avian patients for 3-4 hours before anaesthesia. If necessary, crop contents should be removed by lavage prior to anaesthesia. Heat loss • Birds begin to cool down within 20 minutes of induction. Profound hypothermia, resulting in the patient’s death, can easily occur. Various techniques have been developed to maintain body temperature: o warmed i/v fluids o heating pads o warmed anaesthetic gases o radiant heat >>> 34 Technical I Article Anaesthesia Dr Coetzee de Beer BVSc (Hons) MANZCVS (Avian Health) This article is sponsored by V-Tech
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