Performing anaesthesia in a reptile patient is not something that a general practitioner will encounter often. But when faced with the prospect, it is important to be prepared and aware of the intricacies and complexities of performing anaesthesia in these species.
Broadly speaking, reptilian patients are separated into three categories: chelonia (tortoises, turtles and terrapins), lizards and snakes. Anatomically and physiologically, these all differ from mammals and birds, and it is important to keep this in mind when performing anaesthesia or sedation.
Anaesthesia is not routine in reptilian patients, and they do not undergo routine procedures such as neutering, so any reptile receiving general anaesthesia will have some degree of pathology. This could be minor pathology, such as dental disease, or major pathology, such as septic coelomitis or a bone fracture.
A quick-fire summary of the differences in reptile anatomy
Several anatomical differences should be kept in mind when treating reptilian patients.
Firstly, reptiles lack a diaphragm, so air movement requires movement of the abdominal, pectoral and intercostal muscles (Perpiñán, 2018). When a patient is anaesthetised, we achieve muscle relaxation, so in reptilian patients that require skeletal muscle movement for respiration, adequate depth of anaesthesia for surgery results in apnoea. Because of this, the anaesthetist must be able to provide intermittent positive pressure ventilation (IPPV) to provide oxygen to the patient.
Lung anatomy is also different in reptiles versus mammals, where reptiles often have an increased pulmonary capacity compared to mammals, but a smaller surface available for gas exchange. This is because in most species the right lung has an air sac attached to the caudal aspect that can be inflated at will to allow the body to puff up and look menacing during times of conflict (Bertelsen, 2014).
Reptiles lack an epiglottis, so visualisation of the glottis at the base of the tongue is easy and facilitates easy intubation with a non-cuffed endotracheal (ET) tube (Figure 1). In smaller patients, wide-bore intravenous cannulas can be used to intubate, which can be attached to an anaesthetic circuit with a 2.5mm attachment.
Lizards and snakes have incomplete tracheal rings, but chelonia and crocodilians have complete tracheal rings, which influences the choice of ET tube (Perpiñán, 2018). In addition, chelonia tend to have a very proximal bifurcation of the trachea, which means an ET tube cannot be inserted very far as there is a risk of intubating only one primary bronchus.
Non-crocodilian reptiles have an incomplete ventricular septum in the heart, meaning there is potential for a left-to-right or right-to-left shunt and mixing of oxygenated and deoxygenated blood, and the ability for reptiles to bypass blood flow to the pulmonary circulation entirely (Hedley, 2022). This is a consideration for the use of inhalational agents.
A quick-fire summary of the differences in reptile physiology
Physiologically, one of the most important considerations with reptile anaesthesia is that reptiles are ectotherms, which means they do not produce their own body heat and, instead, absorb heat from the surrounding environment. Each species of reptile has a preferred optimum temperature zone (POTZ) at which its metabolic function is at its most efficient (Edis, 2017).
The body temperature of a reptile patient determines the rate of absorption, distribution, metabolism and excretion of any drug given, even those not being used for anaesthesia. If the patient sits below its POTZ, the onset of action of a drug can take longer and the duration of action can increase. Similarly, if a patient is hotter than its POTZ, there may be an increase in the onset and duration of action of certain drugs, and it may also cause stress to the patient.
Heat can be provided to reptile patients undergoing anaesthesia using vivarium heat sources, overhead heat lamps, heated rooms and heat pads. However, care must be taken to prevent thermal burns caused by prolonged contact with heat pads.
Crucial considerations for reptile anaesthesia
The hepatic first-pass effect
In reptiles, venous blood flow from the caudal limbs drains into the ventral abdominal veins, which then drain into the liver (Benson and Forrest, 1999). This means that any drug that is metabolised by the liver may never be absorbed systemically before it is excreted (Mans, 2014).
For example, a study comparing the effectiveness of a ketamine dexmedetomidine sedation in leopard geckos showed that with the same dose, sedation was less pronounced when drugs were administered in the hindlimbs compared to the forelimbs (Fink et al., 2018). This is the reason why it is recommended to administer injectable drugs into the cranial half of a reptile patient’s body, rather than the renal portal system which has been shown to have a minimal effect on the absorption and distribution of drugs in reptiles (Holz, 1999). However, the ventral tail vein of lizards and snakes drains directly into the caudal vena cava, meaning that intravenous injections into this site will not undergo the hepatic first-pass effect (Mans, 2014) (Figure 2).
Patient factors
Patient factors should also be taken into consideration when determining which drugs will be used for induction and maintenance of general anaesthesia. Many young reptiles are very small and larger volumes of intramuscular injections will be painful (Figure 3). Intravenous access is difficult in smaller patients, but also in much larger patients such as leopard tortoises (Stigmochelys pardalis) and African spurred tortoises (Centrochelys sulcata), which require deep sedation before intravenous access is able to be obtained (Figure 4).
Preanaesthetic fasting
Herbivores do not generally require more than 12 hours of preanaesthetic fasting, but omnivores and insectivores should be fasted for at least 24 hours to allow digestion of food; snakes should be fasted for several days (Hedley, 2022).
Analgesia
If anaesthesia is planned in order for a painful procedure to be carried out, consideration should also be given to the timing of analgesia administration. Some clinicians advocate for analgesia to be given 12 hours prior to a procedure, as there are limited data for most species on the distribution and metabolism of analgesic agents. Allowing an extended period of time between administration and when the analgesic effect is required means that there is more time for the patient to absorb, distribute and metabolise the analgesia before the advent of the noxious stimulus.
What should be inside a reptile anaesthetist’s toolkit?
A comprehensive exotics formulary is an important tool for any clinician treating reptiles, as there are several studies showing that commonly used analgesics do not work in certain species at specific doses
Pharmacokinetic and pharmacodynamic studies for drugs in reptiles are vast, but species-specific data are often hard to come by. This means that clinicians may need to extrapolate data from other reptile species when faced with even commonly treated reptile species. A comprehensive exotics formulary is an important tool for any clinician treating reptiles, as there are several studies showing that commonly used analgesics do not work in certain species at specific doses.
A quick-fire guide to anaesthetising reptilian patients
Premedication and induction
Premedication should include a sedative, together with an analgesic drug if the procedure to be performed is deemed painful. As it is difficult to determine whether a reptile patient is experiencing pain, clinicians should consider whether the procedure is painful to other species and always assume that a procedure will elicit pain if there are no data.
Local anaesthetics should also be considered and can be given locally, as well as intrathecally, to provide regional analgesia to the hindlimbs and cloaca (Mans, 2014).
Induction can be administered as intramuscular or intravenous injections (Figure 5), or with inhalational agents. However, many species will hold their breath when they smell an inhalational agent; due to intracardiac shunting, many reptiles can hold their breath for a prolonged period of time.
Once the patient has been induced, intubation should be performed and the ET tube secured in place (Figure 6).
Maintenance
Once the patient has been induced, anaesthesia can be maintained with an intravenous or inhalational anaesthetic. Due to ease of administration, inhalational anaesthesia is often used; however, consideration needs to be given to how inhalational agents are distributed throughout the body.
Inhalational agents work by diffusing from the inhaled agents in the lungs to the circulation and ultimately to the brain along a partial pressure gradient. Cardiac shunting can affect the use of inhalant anaesthesia because if a reptilian patient initiates a right-to-left cardiac shunt, systemic blood bypasses the pulmonary circulation. This means that changes in the percentage of vapour delivered via the vaporiser on the anaesthetic circuit may not mirror the partial pressure of that inhalational agent in the blood. Therefore, changes in vaporiser percentage may not result in a deepening or lightening of the anaesthetic plane (Mans, 2020).
The direction of cardiac shunting can be under sympathetic or parasympathetic control. Increased parasympathetic tone results in decreased pulmonary blood flow, whereas increased sympathetic tone increases pulmonary blood flow (Sladky and Mans, 2012). Fortunately, this can be manipulated: the administration of adrenaline increases the sympathetic tone, resulting in increased pulmonary blood flow and cardiac output. The administration of intramuscular adrenaline at the cessation of anaesthesia has also been shown to significantly reduce recovery times in crocodilian patients (Gatson et al., 2017).
Monitoring
While under anaesthesia, vital parameters can be monitored.
- Heart rate can be monitored via direct visualisation or a Doppler device, which can be positioned directly over the heart (Figure 7)
- As IPPV is required when the patient is at an adequate depth of anaesthesia, the respiratory rate can be manually determined
- Because of cardiac shunting, exhaled carbon dioxide is not always representative of the partial pressure of carbon dioxide within the circulation, so capnography is not always accurate in reptile patients
- The withdrawal reflex is often used and is often the most reliable method to determine the anaesthetic plane. In snakes, the righting reflex can be used in place of limb withdrawal reflexes
Recovery
During anaesthesia and recovery, care should be taken to keep the patient at their POTZ.
When a patient is in the recovery phase of anaesthesia, it is important to keep in mind that oxygen is a ventilatory depressant in reptile patients (Sladky and Mans, 2012). Therefore, keeping them on oxygen alone at recovery is likely to result in breath holding. Instead, room air should be provided via IPPV until the patient is consistently spontaneously ventilating (Figure 8).
Once anaesthesia has been completed, patients often take several hours to recover and should be monitored for re-narcotisation until they are conscious and ambulating without assistance.
Conclusion
Anaesthesia can be carried out relatively easily and safely in reptiles, as long as clinicians are aware of the anatomical and physiological oddities of each species. Proper preparation for an anaesthetic in a reptile patient is essential for the best outcome.
