- 1 Overview of reptilian anatomy and physiology relevant to anesthesia
- 2 Pre-anesthetic preparation
- 3 Pre-anesthetic medications
- 4 Induction of Anesthesia
- 5 Maintenance of anesthesia with injectable agents
- 6 Maintenance of anesthesia with inhalational agents
- 7 Maintaining Anesthesia
Chemical restraint is often necessary in reptile medicine to facilitate procedures from simply extracting the head of a leopard tortoise or box turtle, to enable a jugular blood sample to be performed, to coeliotomy procedures such as surgical correction of egg-binding.
Before any anesthetic / sedative is administered, an assessment of the reptile patient’s health is necessary. Is sedation / anesthesia necessary for the procedure required? Is the reptile suffering from respiratory disease or septicaemia, i.e. is the reptile’s health likely to be made worse by sedation / anesthesia?
Before any attempt to administer chemical restraint the reptilian respiratory system should be understood.
Overview of reptilian anatomy and physiology relevant to anesthesia
The reptilian patient has a number of variations from the basic mammalian anatomical and physiological systems. Starting rostrally:
(1) Reduced larynx: The reptile patient does have a glottis similar to the avian patient, which lies at the base of the tongue, more rostrally in snakes and lizards and more caudally in Chelonia. At rest the glottis is permanently closed, opening briefly during inspiration and expiration. In crocodiles the glottis is obscured by the basihyal valve which is a fold of the epiglottis that has to be deflected before they can be intubated.
(2) The trachea varies between orders: The Chelonia and Crocodylia have complete cartilaginous rings similar to the avian patient, with the Chelonia patient having a very short trachea, bifurcating into two bronchi in the neck in some species. Serpentes and Sauria have incomplete rings such as is found in the cat and dog, with Serpentes species having a very long trachea. Many Serpentes species have a tracheal lung – an outpouching from the trachea as a form of air sac.
(3) The lungs of Serpentes and saurian species are simple and elastic in nature. The left lung of most Serpentes species is absent or vestigial but may be present in the case of members of the Boid family (boa constrictors etc.). The right lung of Serpentes species ends in an air sac. Chelonia species have a more complicated lung structure, and the paired lungs sit dorsally inside the carapace of the shell. Crocodylia have lungs not dissimilar to mammalian ones and they are paired.
(4) No reptile has a diaphragm: Crocodylia species have a pseudodiaphragm, which changes position with the movements of the liver and gut, so pushing air in and out of the lungs.
(5) Most reptiles use intercostal muscles to move the ribcage in and out, as with birds: The exception being the Chelonia. These species require movement of their limbs and head into and out of the shell in order to bring air into and out of the lungs. This is important when they are anesthetised as such movements, and therefore breathing, cease.
(6) Some species can survive in oxygen-deprived atmospheres for prolonged periods: Chelonia species may survive for 24 hours or more and even green iguanas may survive for 4-5 hours, making inhalation induction of anesthesia almost impossible in these animals.
(7) Reptiles have a renal portal blood circulation system: This means that the blood from the caudal half of the body can pass through the kidney structure before passing into the caudal major veins and entering the heart. Therefore, if drugs that are excreted by the kidneys are injected into the caudal half of the body, then they may be excreted before they have a chance to work systemically (e.g. ketamine). In addition, if a drug is nephrotoxic (e.g. the aminoglycosides) then injection into the caudal half of the body may increase the risk of renal damage.
It is useful to test biochemical and haemocytological parameters prior to administering chemical immobilising drugs. Blood samples may be taken from the jugular vein or dorsal tail vein in Chelonia, the ventral tail vein, palatine vein or by cardiac puncture in Serpentes and the ventral tail vein in Crocodylia and Sauria. Minimal testing advised is a haematocrit, blood calcium levels, blood total protein levels, aspartate transaminase (AST) levels for hepatic function and uric acid levels for renal function.
This is necessary prior to anesthesia in Serpentes (for a period of 2 days in small snakes up to 1-2 weeks for the larger pythons) to prevent regurgitation and pressure on the lungs / heart. Chelonia rarely regurgitate and do not need prolonged fasting. It is important not to feed live prey to insectivores (e.g. leopard geckos) within 24 hours of anesthesia as the prey may still be alive when the reptile is anesthetised!
Midazolam has been used in red-eared terrapins at 1.5 mg / kg as a premedicant and produced adequate sedation to allow minor procedures and induction of anesthesia.
Acepromazine (0.1-0.5 mg / kg intramuscularly (IM)) may be given 1 hour before induction of anesthesia to reduce the dose of induction agent required. Diazepam (0.22-0.62mg / kg IM in alligators) and midazolam (2mg / kg IM in turtles) are also useful.
Alpha-2 adrenoceptor agonists
Xylazine can be used 30 minutes prior to ketamine at 1 mg / kg in Crocodylia to reduce the dose of ketamine required. Medetomidine may be used at doses of 100-150 μ.g / kg, also reducing the required dose of ketamine in Chelonia, and it has the advantage of being reversible with atipamezole at 500-750 µg / kg.
Butorphanol (0.4mg / kg intramuscularly (IM)), can be administered 20 minutes before anesthesia, providing analgesia and reducing the dose of induction agent required. It may be combined with midazolam at 2mg / kg.
Maintenance of anesthesia with injectable agents
This may be used on its own for anesthesia at doses of 55-88 mg / kg intramuscularly (IM). As the dose increases, so the recovery time also increases, in some instances to several days; doses above 110 mg / kg will cause respiratory arrest and bradycardia.
Ketamine may be combined with other injectable agents to provide surgical anesthesia.
Suitable agents include: midazolam at 2 mg / kg intramuscularly (IM) with 40 mg / kg ketamine in turtles; xylazine at 1 mg / kg IM, given 30min prior to 20mg / kg ketamine in large crocodiles and medetomidine at 0.1mg / kg IM with 50mg / kg ketamine in king snakes.
Ketamine at 5mg / kg has been combined with medetomidine at 0.1mg / kg intravenously (IV) to produce a short period of anesthesia in gopher tortoises although some hypoxia was observed and supplemental oxygen is advised.
Propofol may be used to give 20-30 min of anesthesia, which may allow minor procedures. It may be topped up at 1 mg / kg / min IV or intraosseously. Apnoea is extremely common and intubation and ventilation with 100% oxygen are advised.