Suggested anaesthetic protocols
Suggested anaesthetic protocols for chelonians, lizards and snakes are given in Figures 11.20-11.22.
Crocodilians are dangerous, and details of general anaesthesia of these species are outwith the scope of this chapter. Small crocodilians may be intubated after sedation or light general anaesthesia with ketamine. Large crocodilians require immobilization using neuromuscular blockers or ketamine ortiletamine/zolazepam. Once immobilized, a wooden mouth gag is placed and taped in place (see Figure 11.17). The animal can then be intubated and maintained on 2% isoflurane and oxygen.
Suggested anaesthetic protocols for chelonians.
For oral examination, stomach tubing, jugular venepuncture or radiography, i.e. to facilitate extraction of the head or moderate restraint:
- 5-10 mg/kg ketamine i.m.
- For general anaesthesia:
- 10-20 mg/kg ketamine i.m.; then intubation and maintenance with 2% isoflurane in oxygen
- OR 5-10 mg/kg propofol i.v.; then intubation and maintenance with 2% isoflurane in oxygen
- Small (
Suggested anaesthetic protocols for lizards.
- Lizards that are tractable, easily physically restrained and do not breath-hold are mask-induced with 5% isoflurane in oxygen, then intubated and maintained on 2% isoflurane in oxygen.
- Small lizards that do not breath-hold are placed into a plastic bag or container that is then filled with 5% isoflurane in oxygen. When unresponsive they are intubated and maintained on isoflurane in oxygen.
- Lizards that are readily restrained (and/or those that breath-hold and so cannot be induced via gaseous means) are induced with 10 mg/kg propofol i.v., then intubated and maintained on 1% isoflurane in oxygen.
- Other lizards that are aggressive, difficult to restrain or breath-holding and therefore resistant to mask induction, are induced/sedated with 20 mg/kg ketamine i.m., then intubated and maintained on 2%isoflurane in oxygen.
Suggested anaesthetic protocols for snakes.
- Small snakes are readily intubated and induced with 5% isoflurane in oxygen via a ventilator set at a rate of 20 breaths/min. The setting is reduced to 4-6 breaths/min for maintenance with 2% isoflurane in oxygen.
- Large or aggressive snakes may be induced in an induction chamber or bag with isoflurane in oxygen. When unresponsive they are intubated and maintained on 2% isoflurane in oxygen.
- Very aggressive snakes may be injected with a low dose of ketamine (5-10 mg/kg i.m.) to facilitate handling before proceeding with intubation and induction.
Perianaesthetic care
Positional changes of the anesthetized reptile patient should be performed slowly and, where possible, the reptile should be maintained in a normal body position, i.e. sterna) and horizontal. This is to prevent orthostatic hypotension. This type of hypotension is caused by positional changes causing pooling of blood. In the healthy conscious patient vascular reflexes act to counteract this effect. However, anaesthetized patients may be unable to counteract this effect, leading to a reduction in venous return and cardiac output. Therefore, if positional changes of the anesthetized reptile are required, they should be performed slowly and carefully. Any hypovolaemia or hypotension should be corrected promptly.
Monitoring the depth of anaesthesia
There are few studies detailing the depth of anaesthesia or how to assess this in reptiles. It is common for reptiles to have a long period of recovery. It may be, therefore, that reptiles are often maintained at a deep plane of general anaesthesia in order to maintain immobility and that the addition of analgesia routinely to the anaesthetic regimens might result in faster recoveries. More research into this area is needed.
Reptiles are notoriously difficult to monitor under general anaesthesia. Once a surgical plane of general anaesthesia has been achieved, few responses remain to distinguish the dangerously 'deep' reptile from an appropriately managed one. Nevertheless, there are responses that should be maintained and monitored during general anaesthesia to enable the anaesthetist to monitor and assess the patient.
Even with careful monitoring, it may sometimes appear that the patient has moved from deep surgical general anaesthesia to moving around, without any change in heart rate or other parameters. Careful attention to the delivery of general anaesthetic agents, particularly the use of assisted ventilation and oxygenation, are essential to ensure general anaesthesia is maintained.
Snakes relax from head to tail and recover in the reverse direction. Return to spontaneous respiration is a useful parameter for recovery. Figure 11.23 describes tests that may be used to assess anaesthetic depth. The pupillary diameter of reptiles appears to be unrelated to the anaesthetic depth. Overstimulation of any response can lead to extinction, especially of the corneal and palpebral responses. Surgical general anaesthesia is characterized by the abolition of all responses except corneal, tongue withdrawal and vent responses.
Cardiovascular system
Useful cardiovascular monitoring devices are the electrocardiogram, Doppler flow apparatus and direct arterial pressure monitors. An oesophageal stethoscope can be placed into position near the heart to allow heart auscultation. The position of the Doppler probe is as close as possible to an artery or the heart e.g. ventral tail base in snakes and lizards or the position of the carotid and femoral arteries (Figures 11.24 to 11.26). Pulse oximetry is a non-invasive method for assessing pulse rate and oxygen saturation. Although the absolute value readings are not validated in reptiles, the trend displayed is a useful monitoring tool. Placement sites are as for the Doppler probe, or a cloacal or oesophageal probe can be used.
The heart rate should be constant throughout the general anaesthetic period. An increase may signify a response to pain or recovery from general anaesthesia. It should be noted, however, that it is not unusual for the heart rate to stay exactly the same between surgical general anaesthesia and the reptile `suddenly' appearing fully conscious.
Chelonians
Carotid artery Femoral artery
Lizards
Carotid artery Femoral artery Heart
Caudal tail base
Snakes
Carotid artery (difficult to locate) Heart
Caudal tail base
Sites for positioning pulse Doppler for monitoring pulse/heart rates.
Respiratory system
Auscultation of the respiratory tract is not easy, although the use of a moist cloth between the stethoscope and the reptile's skin may enhance the acoustics. Generally, respiratory rate can be measured by observing thoracic expansion or using an in-circuit respiratory monitor attached to the endotracheal tube. As most reptiles stop breathing under anaesthesia I PPV is usually required, either manually or, preferably, using a ventilator. The aim of assisted ventilation is to ensure adequate delivery of gaseous anaesthetic, prevent the conversion to anaerobic metabolism, to maintain sufficient oxygenation to prevent cardiac shunting or pulmonary blood flow constriction, and to maintain a normal acid-base balance.
Blood gases
Blood gas analysis can be used to monitor oxygenation and adequacy of ventilation. The preferred blood sampling site is the carotid artery as this more accurately reflects the blood supply to the brain. However, arterial samples are difficult to obtain from many species. Venous blood samples may indirectly reflect the PaC02 and therefore the adequacy of ventilation, although the venous PC02 may be high because of impaired tissue perfusion or increased metabolic activity and not merely because ventilation is inadequate.
The measurement of carbon dioxide concentrations in expired gases (capnometry) is problematic in reptiles. Analysers with high sampling rates (e.g. 250 ml/min) result in contamination with room air in small patients, leading to an erroneously low value. In human paediatric patients the rate is 50 ml/min, yet this is still too high for many small reptile patients. Cardiac shunting in reptiles further reduces the accuracy of capnography, as the expired carbon dioxide levels do not reflect the arterial levels.
Resuscitation
The low metabolic rate of most reptiles means that there is a longer period of cardiac and/or respiratory arrest from which an animal can be resuscitated and be functional. As with all animals prevention of a problem is better than a need for resuscitation and this should be achieved by adequate monitoring and supportive care in the perioperative period, as described above.
The principles of resuscitation are the same as for other vertebrates, namely the ABC rule:
A: Airway: provide a clear airway
B: Breathing: ventilate if necessary
C: Circulation: provide circulatory support: fluids, adrenaline (for asystole), atropine (for vagally
associated bradycardia).
Recovery from anaesthesia
It should be noted that recovery in reptiles, even with a `rapid recovery' agent, takes 10-30 minutes. The animal should not be over-handled or overstimulated during this period, to prevent cardiovascular disturbances. If the patient has been maintained or induced with a long-acting injectable agent (e.g. ketamine) the animal may take hours to regain consciousness completely. Excessively prolonged recoveries are primarily due to hypothermia, unrecognized hypoglycaemia and impaired drug excretion (including inhalational anaesthetics).
Assisted ventilation with oxygen or room air will be required until the reptile has begun spontaneous breathing. Ventilation with room air, using an Ambu-bag, may hasten recovery from inhalational anaesthesia compared to ventilation with 100% oxygen. Care must be
exercised to prevent excessive lung expansion using such bags or pulmonary damage may occur. Breathing may be stimulated by pinching the toe or tail. In exceptional circumstances a respiratory stimulant (e.g. doxapram 15 mg/kg i.m. or i.v.) may be administered.
Postoperative support and monitoring should continue until full normal function returns. The animal's core temperature must be monitored until the patient has recovered fully. The reptile should be maintained at the higher end of its POTZ throughout the recovery period.
The reptile may begin to move around and then appear 're-anaesthetized' and rest immobile for prolonged periods during the recovery phase, even appearing to be dead. It is important to administer oxygen (via IPPV if respiratory rate is slow or not detected) and fluids and to warm the animal slowly over 10-30 minutes; this often leads to a revival.
Overheating should be avoided in the recovery phase or the metabolic rate may be raised but not the respiratory rate in a corresponding manner. The increased tissue oxygen demand may exceed supply, leading to necrosis and potentially fatal metabolic derangements. The use of overhead heat lamps should be avoided, as the animal may move into a hot zone and not move away when it becomes too hot. Similarly, the animal should not be placed with access to a water source it could crawl into and drown in.
It is important to note that full recovery from all the effects of general anaesthesia may take over 24 hours. Where possible, reptiles should be kept away from large bodies of water until they are fully recovered. The period out of water is kept to a minimum for fully aquatic species, such as soft-shelled turtles, whose leathery shell may be compromised by drying out. Spraying with water rather than a premature return to deep water may be safer in the recovery period.
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