anest rep

ANAESTHESIA IN REPTILES
Z. KNOTEK
Affiliation:
Avian and Exotic Animal Clinic, Univ. Veterinary and Pharm. Sci. Brno, C Z – 612 42, Czech Republic
Abstract
When establishing an anaesthetic protocol, the the weight of the patient must be accurately
established, his water balance assessed, and basic blood tests taken. In reptiles, cardiac
activity and blood oxygen saturation can be monitored by pulse oxymetry. From the practical
point of view, distinguishing between five stages of anaesthesia in reptiles has
provenappropriate: induction, sedation, surgical anaesthesia, risk stage and recovery. Local
anaesthesia is only rarely used in reptiles. General anaesthesia is more appropriate in big
and dangerous reptiles. Ketamine is used in reptiles primarily in combination with other
injection or inhalation anaesthetics, the aim being to achieve better analgesia and
myorelaxation and to lower the necessary dose of ketamine. When combined with
zolazepam, tiletamine induces analgesia and myorelaxation free of side effects. Thanks to its
rapid onset it is a popular choice for intubation preparation before inhalation anaesthesia. In
our veterinary practice, lower doses of tiletamine/zolazepam are commonly used to achieve
mild sedation of patients during clinical and specialised testing (radiography,
ultrasonography). An ideal inhalation anaesthetic for use in reptiles is isoflurane. It rapidly
induces a deep surgical anaesthesia . The recovery is fast and free of complications. The
anaesthetic can also be used in patients with renal or hepatic damage. In turtles and
tortoises, the concentration of isoflurane during surgery often has to be kept at a higher level
(3.5 – 4%) when compared to mammals. Thanks to the interest in new, efficient and reliable
anaesthetics shown by clinical veterinarians, a situation has been established where reptile
anaesthesia no longer poses a any serious risk.
Key words: local anaesthesia, general anaesthesia, tiletamine/zolazepam, isoflurane
Introduction
A veterinarian involved in clinical practice focused on reptiles is commonly faced with new
patient species, often of exotic origin, requiring specialised care. Thanks to the progress of
the pharmaceutical industry and the interest in new, efficient and reliable anaesthetics shown
by clinical veterinarians, a situation has been established where reptile anaesthesia no
longer poses a serious problem. Considering the enormous variety among reptiles (around
6,000 species), it is natural that most anaesthetics being launched have been tested in only a
negligible number of representatives of this family. The indications for anaesthesia in reptiles
include: prevention of stress and the ensuing pathological alterations in the patient; surgeries
that cannot be performed in full consciousness; sedation and immobilization forsome
potentially stressful examinations and examinations using special methods (radiography,
magnetic resonance imaging, endoscopy, ultrasonography, biopsy, etc.); safe handling and
transportation of dangerous species. Preparation of reptiles for anaesthesia always includes
a clinical examination. A well-prepared patient history is always essential. A decision must be
made as to whether the surgery is indispensable and what the risk of general anaesthesia in
each particular case is. In the process of choosing from among the injection anaesthetics,
the weight of the patient must be accurately established, his water balance assessed, and
basic blood tests taken – low PCV values are considered to be associated with reptile
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infections; the assessment of individual groups of blood cells provides a rough measure. As
far as the biochemical parameters are concerned, total protein, uric acid, ALT, AST, ALP,
phosphorus, and calcium concentrations should be established. Other specialised tests
include: parasitologic faecal examination, urine examination or tracheal wash testing. Prior to
the operation the reptiles must be kept in an environment with an optimal temperature
guaranteeing physiological cardiac and respiratory rate. Stress stimuli must be eliminated.
Inappropriate or rough handling of the patient increases muscle tone, decreases blood
pressure, and affects oxygen and carbon dioxide blood concentrations in an undesirable
way. There is a risk of cardiac arrythmia. The main reason for depriving the patients of food
is preventing the digestive system from causing pulmonary compression. The digestive
system must not contain any live food (insect larvae). Depending on the health status of the
patient, application of antibiotics may be started several days before the surgery.
Sedation
Distinction is made between majortranquillisers (phenothiazine derivatives – chlorpromazine
10 mg/kg IM) and minor tranquillisers (benzodiazepines – diazepam 0.20 – 0.60 mg/kg IM).
Smaller doses of dissociativeanaesthetics (tiletamine/zolazepam) have proved efficient and
reliable in practice.
Bronchial secretion inhibition
The drugs of choice are the anticholinergics atropine (0.01 – 0.04 mg/kg IM or IV, 10 minutes
prior to anaesthesia) or glycopyrrolate (10 mg/kg IM, SC, IV) with capacity to reduce
salivation, bronchial and gastric salivation, and reduce brachycardia.
Patient monitoring during general anaesthesia
If the patient is operated in dorsal recumbency, there is a risk that the lungs may be
exposed to pressure from the internal organs. In addition, there is a risk of tympania in big
reptiles. Assessment of mucosal membranes alone cannot be relied on. An apnoic pause
seems natural to some extent. In reptiles, cardiac activity and blood oxygen saturation can
be monitored by pulse oxymetry.
Assesment of anaesthesia dept based on reflex evaluation.
Righting reflex – fast correction of position in response to being turned into the dorsal
position. Its disappearance is a sign of onset of sedation in snakes, but is not a sufficient
evidence of analgesia. There is not much point in evaluating this reflex in lizards and
crocodiles.
Head, neck, and frontal body lifting reflex – snakes lift the front part of the body when held by
the tail.
Skin sensitivity – is well preserved and disappears only in deep anaesthesia.
Finger sensitivity – is also preserved in lizards until the onset of deep anaesthesia.
Pinprick test followed by tail movement – a very good reflex, which is of practical use in
reptiles.
Hiding head, neck, and legs in the carapace upon touch or painful stimuli. Disappearance of
this reflex is an evidence of sufficiently deep anaesthesia in turtles and tortoises. The neck
and legs are floppy and can be easily pulled out under surgical anaesthesia.
Jaw tone – that the oral cavity can easily be opened is an evidence of the onset of deep
anaesthesia in turtles and tortoises.
“S position“ in snakes – can be observed in snakes after painful stimuli.
Some of the reflexes described in reptiles (corneal reflex, pupilary reflex, tongue -withdrawing
reflex) are only of limited practical significance.
From the practical point of view, distinguishing between five stages of anaesthesia in reptiles
has proven appropriate:
Induction – slow movement, muscle tone and pain perception are preserved, the head-lifting
reflex is preserved.
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Sedation (immobilization, shallow anaesthesia, onset of analgesia) - diminished
spontaneous movement, mild muscle relaxation, restricted response to painful stimuli,
diminished head -lifting reflex.
Surgical (deep) anaesthesia – the patient does not move, significant muscle relaxation,
disappearance of response to painful stimuli, disappearance of most reflexes (only the
corneal reflex in turtles and tortoises is partly preserved).
Risk (toxic) stage – risk of death, respiratory and cardiac arrest.
Recovery – recovery of the head-lifting reflex, recovery of spontaneous movement,
perception of painful stimuli. This last stage often lasts for hours until full activity of the
patient, muscle tone, and entirely natural movement are restored.
In reptiles, the first as well as the last stage take much longe r than they do in mammals, due
to the temperature of the environment and the metabolism of the patient. To accelerate the
effect of anaesthetics, reptiles should be kept in temperatures corresponding to their
preferred temperature optimum. Excessive temperatures are, however, to be avoided as they
increase the oxygen demand of tissues, which is in conflict with decreased pulmonary
ventilation during anaesthesia.
Local anaesthesia is only rarely used in reptiles. Infiltration with lidocaine or procaine is
possible. The lethal dose following subcutaneous application of procaine is 250 mg/kg.
General anaesthesia is more appropriate in big and dangerous reptiles. In our practice we
have repeatedly demonstrated the applicability of 1% procaine and 2% trimecaine in minor
surgeries in agamas, geckos, green iguanas, black iguanas, and cockatrices. The surgeries
mostly involved necrotic tissue removal in distal parts of legs and the tail and/or skin
biopsies. In snakes belonging to Lampropeltis, Elaphe, Python, Coluber, and Boaedon
genera local infiltrative anaesthesia with 1% procaine or 2% trimecaine was used to provide
anaesthesia during cloacal prolapse reposition. One half of the dose was used to infiltrate
the cloaca, the other half was used to anaesthesize the tissue to be repositioned.
General anaesthesia
Phenyl and arylcyclohexamine dissociative anaesthetics
These anaesthetics induce a state characterized by diminished tone and reduced impulse
transmission without muscle relaxation. These substances have a limited capacity of
inducing visceral analgesia and are not appropriate for use as stand -alone anaesthetics in
larger and abdominal surgeries. In some reptiles, doses of ketamine over 10 mg/kg may
result in respiratory and cardiac arrest. Its elimination requires unimpaired renal function. For
this reason, ketamine should not be used in patients suffering from renal or hepatic function
failure. Ketamine is used in reptiles primarily in combination with other injection or inhalation
anaesthetics, the aim being to achieve a better analgesia and myorelaxation and to lower the
necessary dose of ketamine. Inappropriate myorelaxation after administration of ketamine
can be compensated by combining the drug with diazepam. Other possible combinations are
with xylazine (Hellabrunn mixture, 1 ml = 100 mg of ketamine + 125 mg of xylazine),
midazolam or zolazepam. In our practice we use ketamine occasionally when preparing
intubation before inhalation anaesthesia. Tiletamine has an effect similar to that of ketamine
while being 2-3 times more potent , which amounts to approximately 50% of the effect of
phencyclidine. When combined with zolazepam, tiletamine induces analgesia and
myorelaxation free of side effects. Although using this combination in reptiles is safe, it
should be pointed out that these substances are eliminated by the kidneys. Combinations of
tiletamine and zolazepam are available as Telazol and Zoletil. These drugs can be prepared
in various concentrations as they are lyophilised. Thanks to its rapid onset this combination
is a popular choice for intubation preparation before inhalation anaesthesia. The fact that the
susceptibility of reptiles to the above-listed preparations varies sometimes fails to be
expressed clearly enough in literature.
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Non-barbiturate hypnotics
The effect of xylazine consists in sedation, analgesia, and myorelaxation. The results of its
clinical use have been conflicting. A prolonged induction period (up to 60 minutes) and
recovery as long as 12 hours are often described. The antagonist is yohimbine or
atipamezole. Medetomidine can be used in combination with ketamine 100 mg/kg + 50 mg/kg
IM). In practice medetomidine is used for induction of inhalation anaesthesia. Its antagonist
is atipamezole.
Opioid analgesics
The drawbacks of using etorphine for analgesia in reptiles include high price, toxicity to
humans, and the need for relatively high doses even for simpleimmobilization. There have
been cases where the achieved anaesthesia was inadequate for more complicated
surgeries. Diprenorphine has been used as a postoperative analgesic in turtles and tortoises.
In order to achieve the maximum analgesic effect after finishing the operation, it is applied
immediately before the start or during the operation.
Intravenous injection anaesthetics
Repeated application of injectable anaesthetics involves the risk of over-dosage. This is why
injection anaesthetics tend to be used as induction agentsbefore intubation and connecting
the patient to the inhalation apparatus. The access to reptile blood vessels is a complication.
Propofol is a safe and reliable anaesthetic for use in reptiles. Since its effect is only shortlasting
(20 minutes), propofol is used primarily for induction into anaesthesia. It is very easy
to apply including into fine vessels (turtles and tortoises: 14-15 mg/kg IV; snakes: 10 mg/kg
IV; lizards: 13 mg/kg IV).
Inhalation anaesthesia
In crocodiles, turtles and tortoises, intubation is performed in patients premedicated by one
or another of the injection anaesthetics. Inserting and removing a cannula both require
maximum care. Inadequate manipulation of a cannula increases the risk of postoperative
infections – tracheitis and pneumonia. To make the work as easy as possible and the larynx
accessible in crocodiles, the tongue and the soft palate have to be pushed back. A surprise
that may be encountered in turtles and tortoises is a cranial bifurcation. Intubating small
patients requires making all kinds of adjustments to the tubesand catheters of small
diameters. Intubating lizards and snakes is very easy.Inhalation anaesthetics can be
administered to reptiles using a mask. All kinds of improvisation and adjustments are needed
to arrive at a suitable size and shape of the mask (e.g. high-volume syringes (20 ml) can be
used). As reptiles can hold their breath for a relatively long period, inhalation anaesthesia is
not suitable for induction and is best preceded by premedication in the injectable form. Nonrebreathing
systems – T, Y – are appropriate for patients up to 5 kg of weight. The oxygen
inflow is a double of the minute volume (approx. 300 – 500 ml/kg/min) and the respiratory
rate is 2 - 4 cycles per minute. Respiratory arrest is prevented by assisted ventilation. In big
patients, this system however does not meet the demand for oxygen and the use of a
rebreathing system is advisable. The oxygen inflow during the induction stage is 2 – 4 l/min.
The oxygen inflow during deep anaesthesia is 1 – l/min. An ideal inhalation anaesthetic for
use in reptiles is isoflurane. It has a minimum rate of transfer into blood, is not lipophilic, and
only 0.3% of the substance is metabolised in the body. It causes a very fast onset and
adeep surgical plane of anaesthesia. The recovery is fast and free of complications. The
anaesthetic can be used in patients with renal or hepatic damage. Isoflurane does not irritate
the myocardium. Myorelaxation is excellent. Isoflurane decreases blood pressure and partly
inhibits spontaneous respiration. In green iguanas, concentrations of 1.5 – 2.5% of isoflurane
are sufficient during surgery. In turtles and tortoises, the concentration of isoflurane during
surgery often needs to be kept at a higher level (3.5 – 4%).
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Recommended doses of tiletamine/zolazepam combinations for reptile anaesthesia
Reptiles Dose
(mg/kg
b.w.)
Induction
stage
(min)
Surgical
anaesthesia
(hrs)
Recovery
(hrs)
Indications
Crocodiles 2 - 8 5 - 8 - 3 - 6 RTG, USG
Turtles/
tortoises
15 - 20 5 - 7 - 1 - 4 Manipulation, RTG
60 - 90 5 1 - 2 2 - 6 Surgery
Lizards 1 - 4 5 - 10 - 1 - 3 RTG, USG
15 - 25 3 - 5 1 - 2 2 - 5 Surgery, MRI
Snakes 5 - 10 5 - 10 - 2 - 4 RTG, USG
15 - 30 3 - 5 1 - 2 2 - 4 Surgery
In our veterinary practice, lower doses of tiletamine/zolazepam are commonly used to
achieve mild sedation of patients during clinical and specialised testing (X-ray,
ultrasonography).
Acknowledgements
This project was supported by the grant of the Ministry of Education, Youth and Physical
Education of the Czech Republic (No. 161700002).