Anaesthesia for zoo animals
H McCracken
Zoos Victoria, Melbourne, VIC
Safe and effective anaesthesia and analgesia can be challenging in reptiles and birds because of their unique anatomy and physiology and their variable response to drug dosages. There have been few studies on the cardiopulmonary effects and pharmokinetics of anaesthetic and analgesic drugs in these species, hence doses and techniques commonly in use have principally been determined by clinical experience.
Reptiles
There are numerous unique anatomical and physiological features of reptiles that are ofrelevance to anaesthesia. All reptiles lack a functional diaphragm, the force to move air duringinspiration and expiration coming from movement of the intercostal, pectoral and abdominal muscles. The glottis is located relatively rostrally compared with mammals hence intubation is generally straightforward. Some species have incomplete cartilaginous tracheal rings, while in others these are complete and the level of tracheal bifurcation varies between species, hence one must take care with et tube cuff inflation pressure and depth of tube placement. The position and anatomy of the lungs varies widely between species and in many cases they extend caudally into an air sac lined with non-respiratory epithelium. In snakes and most lizards the lungs are simple thin-walled sacs, hence care must be taken not to rupture them while delivering intermittent positive pressure ventilation (IPPV). (Schumacher and Yelen, 2006).
Many reptiles, especially aquatic species, are capable of long periods of apnoea, converting to anaerobic metabolism at these times. Freshwater tortoises and iguanas have been able to survive in environments of 100% nitrogen for up to 27 and 4.5 hours respectively. In mostreptiles, the stimulus to breathe is hypoxia, not hypercapnia. (Schumacher and Yelen, 2006). Hence most reptiles will not breathe spontaneously during inhalation anaesthesia, requiringIPPV at 2-6 b.p.m throughout the procedure, changing to IPPV with room air at 1 breath per 1-2 mins at the end of the procedure to stimulate return to spontaneous breathing. Reptiles are ectothermic, each species having a specific preferred optimum temperature zone (POTZ) at which metabolism is optimal and organ systems work most effectively. Hence they must be maintained in this temperature zone before, during and following anaesthesia. Gastro-intestinal passage time in reptiles is significantly longer than in mammals (up to 14 days in large boas), hence fasting times for elective procedures are longer (24-96 hours in most species) to avoid regurgitation or putrefaction of undigested foods.
Most anaesthetic regimens in reptiles involve the use of inhalational agents (isoflurane is the agent of choice) either alone or in combination with parenteral agents including dissociatives, alpha 2–agonists and opioids. In the author’s experience, the preferred regimen is premedication with butorphanol (2mg/kg) to provide intra- and post-operative analgesia and mild sedation to reduce the stress of physical restraint during induction.
For snakes, isoflurane is used for both induction and maintenance. Non-venomous species are intubated conscious and induced with 5% isoflurane by IPPV. Venomous species are encouraged to enter a transparent Perspex tube slightly greater than their own diameter. They are then restrained with their heads safely within the tube and induced by 5% isoflurane run into the tube (they generally breathe spontaneously).
For some lizard species mask induction is used, as they will breathe spontaneously; for others, propofol is delivered IV (5-10 ml/kg). All are then intubated and maintained on isoflurane.
For some turtle, tortoise and crocodilian species, induction is by IV propofol, but where intravenous access is not readily achievable, a combination of ketamine (4-10 mg/kg) and medetomidine (0.04 - 0.15 mg/kg) is given IM. Atipamezole is later given as an antagonist of the medetomidine (5 times the dose of medetomidine). Following induction, these species are intubated and maintained on isoflurane.
Monitoring of the reptilian patient during anaesthesia is best achieved using a Doppler flow device positioned over the heart or carotid artery; pulse oximeters have limited value as they are calibrated on the basis of the human oxygen Hb dissociation curve.
Birds
Birds similarly have unique anatomical and physiological features of relevance to anaesthesia. The trachea has complete cartilaginous rings, with bifurcation at the thoracic inlet in most species. The lungs are fixed to the dorsal body wall and not lobulated. The lungs extend into eight air sacs: the unpaired cervical and clavicle, the paired cranial thoracic, paired caudal thoracic and paired abdominal sacs. These extend into the sternum, some long bones and skull sinuses (reducing overall body weight, thus aiding flight). The air sacs have poor vascularity and are not involved in gaseous exchange. Birds have no diaphragm. Muscular contractions, mainly of the abdomen, cause the air sacs to act as bellows, blowing the air back into and through the lungs. The anaesthetised bird may not generate sufficient muscle contractions to achieve this and positioning in dorsal recumbency may compromise the bellows action, hence IPPV is routinely used to maintain adequate oxygenation. The air sacs hold 80% of the volumetric capacity hence will act as a reservoir for anaesthetic gases. Therefore, following gaseous induction, the concentration of anaesthetic within the sacs may lead to further deepening of the anaesthetic plane, even though the vaporiser concentration has been reduced. (Lawton, 1996).
In birds, inspired gases pass from the primary bronchi into the neopulmonic tissues of the lungs (that have only minimal gaseous exchange function), and through into the caudal air sacs. On expiration, the gases pass through the paleopulmonic tissue where the majority of gaseous exchange occurs. (Forbes, 1996). In patients where neither a face mask nor an et tube may be used for maintenance of anaesthetic (eg. with airway obstruction), a tube may be place into an abdominal or caudal thoracic air sac for this purpose.
Birds have a higher metabolic rate than mammals, with a body temperature of 40-44°C. The ability to maintain core temperature is compromised by anaesthesia, further compounded by the removal of feathers and alcohol application in preparation of the surgical site, potentially resulting in hypoglycaemia, particularly in small species. Hence heating pads, lights and insulatory materials are essential to reduce heat loss. Given the high metabolic rate, preanaesthetic fasting may result in hypoglycaemia, and varies according to the size and feeding habits of the patient species.
Inhalation anaesthesia is the method of choice for birds as the use of volatile anaesthetic agents allow for greater control than the use of injectables. In the author’s experience, isoflurane is the agent of choice, although sevoflurane has proven superior in some species of birds (water birds and birds of prey), which may be difficult to maintain on a stable plane with even relatively high concentrations of isoflurane (3.5 – 5.0%). Induction for most species is via facemask (using home-made masks to cater for many difference sizes and shapes). For brief procedures, maintenance may be via mask, but most patients are intubated to permit IPPV throughout the procedure. Monitoring during anaesthesia is by oesophageal stethoscope or thoracic auscultation and pulse oximetry.
Time of Presentation
Sunday 14 May 2006 - 1030-1200
References
1. Forbes, N.A. (1996). “Respiratory Problems”, Chapter 15 in “Manual of Psittacine Birds”, eds.
2. Beynon, P.H., Forbes, N.A. and Lawton, M.P.C., British Small Animal Veterinary Association, Cheltenham, UK.
3. Lawton, M.P.C. (1996). ”Anaesthesia”, Chapter 6 in “Manual of Psittacine Birds”, eds. Beynon,
4. P.H., Forbes, N.A. and Lawton, M.P.C., British Small Animal Veterinary Association, Cheltenham, UK.
5. Schumacher, J and Yelen, T. (2006) ”Anesthesia and Analgesia” , Chapter 27 in “Reptile
6. Medicine and Surgery 2nd ed.”, ed. Mader, D.R., Saunders Elsevier, St. Louis, USA.
