Anaesthesia in exotics is an area which often appears intimidating to both vets and nurses, owing to the historically high mortality rate of these patients.

Unlike cats and dogs, our exotic patients will hide signs of illness for significant periods, so when presented at the clinic, not only are we often dealing with an unfamiliar species, but this animal may also be suffering from a chronic underlying disease process, which may not always be obvious on initial examination.

This is the first article in a three-part series which will look at anaesthesia in exotics, starting with anaesthesia of the reptile patient.

Relevant anatomy and physiology

The term ‘reptiles’ encompasses a massive group of animals, but this article will focus on the three most common groups – snakes, lizards and chelonians (tortoises, terrapins and turtles). Each group has vastly different anatomical and physiological variations, but there are a few key points which they all have in common.

Unlike mammals and birds, reptiles are ectothermic, relying on an external heat source to maintain their body temperature. Each species of reptile will have an optimum temperature range within which their body will function best, so it is important to maintain our patient at this temperature before, during and after anaesthesia.

Reptiles do not have a diaphragm, so breathing is controlled by small movements of their intercostal, pectoral, and abdominal muscles, +/- limb movements. It is, therefore, important not to restrict these movements under anaesthesia.

Most reptiles have a three-chambered heart with a specialised shunting system, to separate oxygenated and deoxygenated blood. In addition, this system allows blood to bypass the lungs if necessary during periods of apnoea. Many reptiles, especially chelonians, are able to function for a significant period of time by anaerobic metabolism, which can make apnoea a problem during induction, as the intake of gaseous anaesthetic may be variable. Once anaesthetised, however, their ability to bypass the lungs is reduced, so gaseous anaesthetic should be effective.

Pre-anaesthetic considerations

A full pre-anaesthetic examination – including obtaining an accurate weight – should be attempted with every patient. The nature of some patients may mean that sedation is required in order to fully complete this examination (for example, the unco-operative box turtle which shuts itself into its shell (Figure 1).

Figure 1: An unco-operative box turtle demonstrates the need for pre-anaesthetic sedation!

Once the patient has been assessed initially, it should be stabilised. In the case of a reptile, the first stage is generally to warm it up to its preferred temperature, which can make a significant difference to its demeanour.  

Fluid therapy – and even nutritional support – may be required. This can be by warm water baths, administration of oral fluids by stomach tube, subcutaneous, epicoelomic (Figure 2) or intracoelomic fluids. Technique and positioning for intracoelomic fluids are similar to performing an intraperitoneal injection in a small mammal. Intravenous fluid therapy is possible in some cases, but may be technically challenging owing to the size of many reptilian patients.

Figure 2: Administration of epicoelomic fluids to a tortoise between the plastron and pectoral muscles

Pre-anaesthetic starvation is not generally necessary in herbivorous species, but insectivores should be fasted for 24 hours to allow digestion of live food prior to anaesthesia. Snakes should, ideally, be fasted for two to three days – or longer in larger species – prior to anaesthesia, to avoid a full stomach compressing the lungs under anaesthesia.

Induction

The preferred method of induction will depend on the species, drug availability and personal experience.

Snakes and very small lizards may be most easily anaesthetised by placing them in an induction chamber or sealed plastic bag filled with isoflurane and oxygen (Figure 3). Induction is slower than mammals or birds, but after 10 to 20 minutes when the righting reflex has been lost, the animal is usually sufficiently sedated to allow intubation.

Figure 3: A fat-tailed gecko undergoing induction in a plastic bag pre-filled with anaesthetic gas

In snakes, it is even possible to induce anaesthesia by conscious intubation and ventilation with isoflurane. The glottis is positioned relatively rostrally in the mouth and usually easily visualised (Figure 4). Specialised, small endotracheal tubes are commercially available (Cook Veterinary Products) or, alternatively, urinary or intravenous catheters may be adapted for the purpose. Length of tube will depend on individual species anatomy, but for most species a short tube inserted 1-2cm beyond the glottis should be sufficient.

Figure 4: This snake's glottis is easily visualised and intubated

For larger snakes and lizards, an injectable induction agent may be preferred, and although none are licensed in reptiles, both propofol and alfaxalone have been used successfully. Administration is generally via the ventral tail vein, and after a few minutes animals are usually sufficiently sedated to allow intubation (Figure 5).

Figure 5: Induction via the ventral tail vein in a water dragon

Chelonians provide the greatest challenge to anaesthesia because of their impressive ability to breath-hold, and the inaccessibility of many veins. If possible, induction with propofol or alfaxalone via the intravenous route is preferred, and the jugular vein (Figure 6), subcarapacial sinus (Figure 7) or dorsal tail vein (Figure 8) may all be used. Alternatively, if intravenous access is difficult, an intramuscular combination of ketamine and midazolam, ketamine and medetomidine, or even ketamine alone may be used, for initial sedation to allow relaxation for better access.

Figure 6: Blood sampling from the jugular in a tortoise

Figure 7: Induction via the subcarapacial sinus in a tortoise

Figure 8: Induction via the dorsal tail vein in a tortoise

When intubating these species, it is important to be aware that their trachea bifurcates particularly cranially, so a short endotracheal tube inserted only just beyond the glottis should be used to avoid only ventilating one lung.

Maintenance

Once intubated, reptiles may be maintained on gaseous anaesthesia. Isoflurane is currently the preferred anaesthetic, as it appears to be the most reliable in reptiles; but further research needs to be performed in this area. Maintaining isoflurane at a high concentration is often advisable until surgical stimulation when levels can be reduced if there is no voluntary movement.

As mentioned earlier, most reptiles are prone to long periods of apnoea under general anaesthetic. This, therefore, needs to be overcome by intermittent positive pressure ventilation (IPPV), by either ventilating manually or ideally by a mechanical ventilator (Figure 9). Respiratory rates may be initially set at around six breaths per minute, although it may be increased to deepen anaesthesia. The appropriate pressure will depend on the size of the individual patient but it is best to start with a low pressure and then to increase this slowly until small breathing movements are seen, resembling those of the conscious animal. As with mammals, peri-anaesthetic fluid therapy is advised at similar rates.

Figure 9: This small animal ventilator is ideal for reptile anaesthesia

Monitoring

Monitoring the depth of anaesthesia can be challenging in a reptile, as we are normally controlling respiratory rate by IPPV, and auscultation with a stethoscope is often unrewarding. The most important piece of anaesthetic monitoring equipment is therefore a Doppler probe, to detect blood flow and allow the heart/pulse rate to be easily heard (Figure 10). This may be placed directly over the heart in snakes and lizards, or at the thoracic inlet in chelonians.

Figure 10: Placement of a Doppler probe in a snake

Reflexes such as the toe pinch, tail pinch, palpebral reflex and jaw tone should also be regularly checked. Temperature of the reptile patient is dependent on environmental temperature, so it is important to monitor this carefully by the use of a long temperature probe – room temperature probes can be useful – and to prevent hypothermia, which would lead to a slow recovery.

Post-anaesthetic procedure

Post-anaesthetic, reptiles can often have prolonged recoveries owing to their slow metabolism. Unlike mammals, the respiratory drive in reptiles is more sensitive to hypoxia than hypercapnia. Recovering reptiles should, therefore be ventilated with room air and not 100% oxygen, in order to stimulate voluntary breathing.

Obviously environmental temperature should be maintained within the reptile’s optimum temperature range, in order to allow effective metabolism of anaesthetic agents. Extubation should only occur when jaw tone has increased and voluntary breathing is occurring.

Analgesia

Finally, it is important to remember that most anaesthetic agents we use in reptiles have little or no lasting effect in providing analgesia, so additional analgesics should be used for any potentially painful procedures. The physiology of pain in reptiles is still poorly understood, but NSAIDs and local anaesthetics should be used, although they are currently unlicensed. The effectiveness of opioids is more debatable, but anecdotally butorphanol may provide some degree of analgesia.

By understanding the relevant differences between reptiles and mammals, it should be possible to provide the same standard of anaesthetic care for reptiles as for our traditional companion animal patients.

Author

Joanna Hedley BVM&S MRCVS

Joanna Hedley has had a varied clinical background since graduation from Edinburgh vet school in 2003, having worked in mixed, small animal and exotic practice, and wildlife rehabilitation both in the UK and abroad. She is currently senior clinical training scholar in exotics and wildlife medicine at the Royal (Dick) School of Veterinary Studies and is working towards her RCVS Certificate in Zoological Medicine.

Further reading

GIRLING, S. and RAITI, P. (2004) Manual of Reptiles (2nd Edition). BSAVA, Gloucester MCARTHUR, S., WILKINSON, R. and MEYER, J. (2004) Medicine and Surgery of Tortoises and Turtles. Blackwell Publishing, Oxford.

GIRLING, S. J. (2003) Veterinary Nursing of Exotic Pets. Blackwell Publishing, Oxford.

LONGLEY, L. A. (2008) Anaesthesia of Exotic Pets. Saunders Elsevier

 

Vetecinaiy Nursing Journal. • VOL 25 • No8 • August 2010 •