analgesia

Anaesthesia and analgesia

Introduction

Physical restraint alone, although sometimes more economical and faster than anaesthesia, is often problematic. Many reptiles are capable of maiming or killing handlers. Persistent struggling will result in muscle contraction and consequent damage, possible hypercalaemia and lactic acidaemia. Although large reptiles that are bound appear immobile, they may still be contracting skeletal muscle isometrically.

The ideal anaesthetic agent should provide restraint, muscle relaxation and analgesia, with ease of reversal or recovery, and be safe both for the patient and for veterinary personnel. Unexpected recovery, which may occur with some anaesthetic protocols, may be extremely hazardous if working with venomous or otherwise dangerous species. The only anaesthetic agent currently licensed for reptiles in the UK is isoflurane. However, many other anaesthetic agents have been used successfully in a variety of reptile species and the choice of anaesthetic regimen, whilst paying due regard to current legislation, must also consider the needs of the patient and personnel.

Correct identification of the reptile to the species level is important, as species-specific susceptibilities to certain anaesthetic agents, even significant mortality, have been identified in some cases. Correct species identification will also allow the correct preferred body temperature for that species to be maintained during general anaesthesia.

Cardiopulmonary anatomy and physiology

Anatomy

Lung anatomy varies between species: lizards and snakes have simple sac-like lungs, whereas the chelonian lung has a more complex structure that increases surface area (Figure 11.1).

The glottis is positioned at the base of the tongue in lizards (Figure 11.2) and snakes (Figure 11.3), and more caudally in chelonians (Figure 11.4). The chelonian trachea bifurcates quite rostrally, approximately half way along the neck, a feature which must be taken into account when intubating these animals (Figure 11.5).

Blood shunting

The incompletely divided ventricle in non-crocodilian reptiles means that these animals can shunt blood across the heart to bypass the pulmonary circulation. This may occur in response to changes in the ratio of the pulmonary and systemic resistances or to adrenergic changes to the vascular resistance in either circulation. During the normal apnoeic phase of respiration, it is cholinergic contraction of the pulmonary artery that prevents a perfusion-ventilation mismatch.

Heart rate

Heart rate varies with species and is also dependent upon factors such as temperature and body size. In lizards an increase of bodytemperature of 10°C over the range of 20-40°C increases heart rate by a factor of approximately 2.

Respiration

Mouth breathing is possible in all reptiles, yet healthy reptiles usually breathe via the nostrils. Both inspiration and expiration are active processes. Chelonians lack intercostal musculature, so ventilation is achieved using the movement of the abdominal viscera, limbs, pelvic and pectoral girdles, and two groups of abdominal muscles that compress the abdominal viscera. In lizards and snakes the negative pressure to begin inspiration is provided by the intercostal, thoracic and abdominal musculature. There is also smooth muscle in the lung airways to assist ventilation, this occurs even if the coelom is opened.

Respiratory cycle

Reptiles breathe in one of two breathing patterns:

- Terrestrial animals tend to take single breaths followed by variable periods of breath-holding

- Aquatic species tend to have periods of breathing followed by extended periods, lasting minutes to hours, of no ventilation.

Reptiles may vary breathing frequency (respiratory rate) and depth (tidal volume) and also the duration of apnoea (non-ventilatory periods).

Control of respiration

Respiration is controlled by responses to arterial partial pressures of carbon dioxide and oxygen, acid-base balance and lung stretch receptors, with differences between species.

Oxygen pressure: The respiratory drive in reptiles is different to that in mammals, in that reptiles are more sensitive to a low partial pressure of oxygen (pO2) than to a high partial pressure of carbon dioxide (pC02). Reptiles are also "capable of prolonged anaerobic metabolism. These factors may contribute to a prolonged recovery time (relative to normal respiratory rates) from general anaesthesia when 100% oxygen is administered.

Studies on turtles have shown that the respiratory rate increases, as does the heart rate, if the animal becomes hypoxic. Right-to-left cardiac shunting also occurs. If the hypoxia continues for >60 minutes, heart rate may fall to 50% of original, and a right-to-left shunting of up to 80% of cardiac output occurs. These abnormalities are rapidly reversed when normoxic air is offered. Turtles have been shown to exhibit hypoxic pulmonary vasoconstriction at a lower threshold than mammals, and then to perform a rightto left intracardiac shunt to prevent a perfusion-ventilation mismatch. This has relevance to the use of gaseous general anaesthesia: despite apparent ventilation, the lack of perfusion together with cardiac shunting will prevent absorption of sufficient gas to maintain general anaesthesia.

Chemoreceptors and stretch receptors: Many snakes have both intrapulmonary chemoreceptors and stretch receptors, whereas many chelonians have only stretch receptors. In periods of high inspired carbon dioxide, snakes can over-ride the volume-related feedback. If snakes are breathing a gas with a high carbon dioxide level, respiration slows and a high tidal volume is seen. Hypercapnia leads to an increase in tidal volume by suppressing lung stretch receptors. Hypoxia increases the breathing frequency by reducing or eliminating the non-breathing periods. These effects are exacerbated at higher temperatures. In snakes studies have shown that respiration is primarily governed centrally, with less reliance on respiratory sys:em mechanics such as stretch receptors. The respiratory system compliance (elasticity and flexibility) of snakes is also higher when compared with 7ammals of similar bodyweight. This is important as snakes may assume many different postures that affect the mechanics of the lungs, and this mechanism allows the breathing to remain unchanged.

Tidal volume

The normal tidal volume of reptiles varies between species, e.g. 12.5 mg/kg in Boa spp., 45 ml/kg in Trachemys spp. Reptiles also commonly have ventilation-perfusion mismatch and right-to-left pulmonary shunts, and this may be exacerbated by voluminous coelomic contents such as eggs and ingesta. Such shunting may complicate the use of arterial blood gas measurements. Although occurring naturally, espe

cially in aquatic species, ventilation-perfusion mismatch may be exacerbated in anaesthetized reptiles that are placed into lateral or dorsal recumbency.

General considerations for anaesthesia

Many of the physiological systems of reptiles are affected by changes in temperature, sometimes quite dramatically. Therefore, it is recommended that reptiles are maintained at their preferred optimum body temperature during the perianaesthetic period. Parenteral (injectable) anaesthetics are removed from the body by biotransformation (metabolism) and/or renal excretion. Decreased body temperature, by decreasing enzyme activity and renal perfusion, will delay drug removal and prolong anaesthetic duration. The relative potency of inhalational anaesthetics is generally the same across most animal species, but at temperatures below a reptile's preferred optimum temperature, potency is lower. Additionally, the uptake and removal of inhalational anaesthetics is slower than in mammals and birds, due to a lower respiratory efficiency and slower circulation time. A further complication occurs in those reptiles that can shunt blood away from the lungs for variable time periods as these shunts can effectively block the uptake and/or removal of inhalational anaesthetics.

Hypothermia ('cold narcosis')

Hypothermia or the induction of `cold narcosis' is neither humane nor an appropriate substitute for reptile anaesthesia. Although it will produce immobility, it is questionable whether it induces analgesia. Furthermore, hypothermia impairs drug metabolism and depresses immune function. Necrotic changes to the brain of snakes and tortoises have been described following hypothermic episodes (Bennett, 1998). Rarely, hypothermia may be used in conjunction with anaesthetic techniques to facilitate cardiopulmonary surgery such as removal of pentastomids from the lung and intra-atrial thrombi (Bennett, 1998).

Pre-anaesthetic considerations

Clinical examination

A thorough clinical examination to ensure that the animal is free from clinical disease, especially with regard to respiratory and cardiovascular function, is essential prior to inducing general anaesthesia (see Chapter 5). Any abnormalities should preferably be corrected prior to general anaesthesia or at least permit stabilization of the patient in the perianaesthetic period. Any animal that is compromised, e.g. by dehydration, blood loss, cachexia, anorexia or infection, will pose a greater anaesthetic risk than a clinically normal animal. A complete pre-anaesthetic assessment and stabilization is therefore especially important. Food and water intake should be recorded preoperatively and used to assess the postoperative recovery.

Fasting

Starvation is rarely necessary prior to anaesthesia. Aspiration is unlikely, as regurgitation is rare with the possible exception of a snake that has recently fed. General anaesthesia should be avoided in snakes with a full stomach, as the mass may lead to cardiopulmonary disturbances if the lungs are compressed. Starvation of herbivorous reptiles is to be avoided, as many of these species have enteric flora that may be disrupted by prolonged starvation, leading to digestive disturbances.

In general, a period of time sufficient for digestion of the last meal to have been completed is adequate for reptiles. For non-herbivorous species, the starvation period may vary from 18 hours in chelonians and smaller lizards to 72-96 hours in larger carnivorous lizards and snakes. This time should be adequate to avoid the presence of live invertebrates in the gastrointestinal tract of insectivores.

Hydration

All reptiles should be adequately hydrated prior to surgery.

Immersion in water or in oral electrolyte replacement or rehydration fluids can be used several days prior to general anaesthesia. The reptile is placed in a shallow water bath at its preferred body temperature, to encourage drinking. Chelonians are also able to absorb water via the cloaca. The water should be deep enough to allow the head to be fully submerged when required, but shallow enough to allow the animal to easily keep its head above water. Sick, weak or debilitated animals should be constantly monitored to pret vent drowning.

Oral fluid therapy with electrolyte replacement or rehydration fluids may be given several days prior to general anaesthesia (Figure 11. 7)

For parenteral administration, equal volumes of 5% glucose in 0.9% sodium chloride, Ringer's solution and water may be given via the intravenous, intraosseous, intracoelomic or subcutaneous routes (see Chapter 10). All injections should be given aseptically to avoid introducing infection.

Preparation for medication

An intravenous or intraosseous catheter may be preplaced for peri-anaesthetic care. The patient should be weighed immediately before surgery to enable correct dosing with medications.

Handling and positioning

The animal should be handled correctly to minimize trauma and stress. Care should be taken when positioning or moving the patient during general anaesthesia. For example, elevating the head will increase the heart rate in lizards. The dorsal position of the lungs in chelonians means that positioning these animals in dorsal recumbency will lead to lung compression and respiratory embarrassment.

Safety

Reptiles are resilient animals and capable of surviving physiological disturbances that would rapidly kill a mammal or bird; therefore it must be remembered that the apparent safety of a particular anaesthetic regimen may be merely a reflection of this physiological resilience. Impaired blood flow (secondaryto marked hypotension and dehydration) and severe hypoxaemia may induce renal tubular necrosis that manifests as renal failure and visceral gout several days to weeks after an anaesthetic episode. Therefore, survival in the immediate post-anaesthetic period does not necessarily mean that the general anaesthetic regimen used was safe and efficacious.

Premedication

Figure 11.8 lists drugs used for premedication and sedation in reptiles.

Atropine/glycopyrrolate

A reduced heart rate under general anaesthesia may occur with many reptile species, though it is not known whether this is a significant clinical problem and therefore the use of drugs to combat this apparent bradycardia may not be indicated. Atropine and glycopyrrolate have been suggested to reverse profound bradycardia.

Atropine is also used in mammalian patients to reduce excessive secretions from the respiratory tract. As reptiles produce little, if any, respiratory secretions likely to interfere with general anaesthesia, this use of atropine is not indicated.

The use of atropine in reptiles has been associated with prolonged ileus, requiring protracted therapy. The administration of atropine to the hypoxic reptile leads to an increase in heart rate; this may be inappropriate in hypoxic reptiles and lead to organ damage as it interferes with the physiological adaptive mechanisms.

Opioids

Opioids used alone do not appear to produce sedation or general anaesthesia in reptiles, but their analgesic and anaesthetic-sparing properties recommend their use in combination with other agents to provide enhanced general anaesthesia. Butorphanol or buprenorphine may be administered preoperatively for analgesia..

Miscellaneous

Phenothiazines are avoided because they produce prolonged tranquillization and hypotension. The benzodiazepine midazolam has been investigated in several species of aquatic turtle and provides variable sedation at relatively high dosages.

Local anaesthesia

Reptiles have sensitive skin and procedures likely to cause pain in other animals should not be performed in reptiles without the benefit of general anaesthesia or adequate local anaesthesia. Local anaesthesia, e.g. infiltration of 2% lidocaine (lignocaine) or 1 % procaine, may provide a good alternative to general anaesthesia for minor procedures in those reptiles that are easily restrained. Figure 11.9 lists local anaesthetic agents and indications for use in reptiles.

Toxic doses of local anaesthetics do not appear to have been investigated in reptiles. In mammals, the toxic dosage of lidocaine varies from 5 to 20 mg/kg. In small animals this dosage may be unknowingly ex¬ceeded unless care istaken to calculate an appropriate dose. It may be useful to dilute the agent with an equal volume of sterile saline to reduce the likelihood of accidental overdose.

Neuromuscular blocking agents

Neuromuscular blocking agents (NBA) (Figure 11.10) are often used for chemical restraint of reptiles. It must be remembered that these agents produce immobility without analgesia. They should not be used as a substitute for analgesia or general anaesthesia in the performance of surgical or potentially painful procedures. The use of these drugs alone may be used to achieve restraint for non-painful procedures, such as transport of an otherwise dangerous animal, radiography and ultrasonography. Respiratory paralysis is often, though not always, associated with the use of NBA in reptiles and thus intubation and assisted ventilation should be performed as required.

The depolarizing muscle relaxant succinylcholine has been used in several reptile species. Succinylcholine reversibly binds the postsynaptic receptors of nicotinic receptors, initiating depolarization. Succinylcholine has been used extensively in the restraint of large chelonians (>20kg). Immobilization usually occurs in 5 minutes, with recovery taking over 7 hours in crocodiles. Marine turtles may take 20-30 minutes to become immobile and recovery occurs 60-120 minutes later. Respiration is usually, though not always, maintained when this drug is used in reptiles.

Non-depolarizing muscle relaxants that have been used in reptiles include D-tubocurarine, gallamine, atracurium and vecuronium. Non-depolarizing muscle relaxants act by competitively binding the nicotinic receptors, blocking the action of acetylcholine. The competitive binding can be reversed by using acetylcholinesterase inhibitors, such as neostigmine and edrophonium. Gallamine has been used in crocodiles to achieve immobility in 15-30 minutes with recovery in 1.5-15 hours. Reversal with neostigmine, though some times effective, is inconsistent at low doses; the effectiveness of higher doses has yet to be fully evaluated.

Parenteral anaesthesia

Benefits include:

- Ease of administration (especially if intramuscular)

- Availability of commonly used drugs

- Little specialized equipment required.

Disadvantages include:

- Requirement for an accurate bodyweight (before general anaesthesia)

- Reversal may not be possible if overdose occurs

- Intravenous injection is technically demanding

- Prolonged recovery times associated with many parenteral agents.

All reptiles under injectable anaesthesia should be intubated and oxygen supplied.

Injectable agents

Figure 11.11 lists injectable agents for sedation and general anaesthesia.

Ketamine

Ketamine may be administered via the intravenous, intramuscular or intraosseous route. The pharmacokinetics and pharmacodynamics of ketamine in reptiles have not been thoroughly investigated. In mammals, a portion of the recovery from an intravascular bolus injection of ketamine is due to redistribution. Hence, repetitive administration of ketamine will result in prolonged recovery.

The effects of ketamine on the reptile patient depend on both dose and species. Ketamine used alone to produce general anaesthesia requires high doses and high injection volume, and results in prolonged recovery. It produces poor muscle relaxation. Doses >110 mg/kg may result in bradycardia and cardiac arrest. Induction takes 10-30 minutes when given intramuscularly and recovery may take 24-96 hours after a high dose.

Ketamine is painful on injection. It is not recommended in the debilitated patient, where its variable effects and a long recovery period may be detrimental. Ketamine is therefore contraindicated in dehydrated patients or those with renal or hepatic impairment.

Low doses are useful in chelonians to facilitate head extraction to perform gavage (stomach tubing) or jugular venepuncture. Low doses are also used to facilitate intubation in many species.

Benzodiazepines,snch as diazepam and midazolam, are often added to ketamine to reduce the ketamine dosage and improve the quality of anaesthesia.

Medetomidine plus ketamine combinations

The combination of medetomidine and ketamine provides good anaesthesia. Advantages of the comination over either agent used alone are: medetomidine may be reversed (Figure 11.12) to speed recovery; and a reduced ketamine dose prevents prolonged recovery periods. The combination has been evaluated in redeared terrapins and found to produce a general anaesthesia sufficient to permit intubation at low-dose combinations or minor procedures, such as suturing, at higher doses. Atipamezole administered at five times the medetomidine dose leads to full recovery 60 minutes later (Greer et al., 2001). Lower doses are used in larger species, e.g. desert tortoise. In general, the higher the doses of medetomidine and ketamine, the longer the recovery time.

Pentobarbital

There are few reports of the use of barbiturate general anaesthetic agents in reptiles and these drugs are not recommended. The use of pentobarbital has been reported to produce inconsistent results and fatalities when used in skinks.

Tiletamine/zolazepam

The preparation Telazol (USA) or Zoletil (Europe) is an equal combination of the dissociative anaesthetic tiletamine and the benzodiazepine zolazepam. Tiletamine is 2-3 times as potent as ketamine but also has a longer duration of effect. The combined agent has a variable effect in reptiles: 5-10 mg/kg is suggested to facilitate intubation. For animals >50 kg a lower dose of 1-2 mg/kg is recommended, as this agent is very sensitive to metabolic scaling in reptiles. Even at very high doses, the reptile may remain responsive to stimuli; therefore this agent is not to be used as a sole agent of general anaesthesia. The major advantage of using this combination is that it can be made up to a high concentration and requires only a small injection volume. Use is contraindicated in the dehydrated patient or in those with renal or hepatic impairment.

Propofol

Propofol is a non-barbiturate sedative-hypnotic that results in rapid induction and recovery. It should be remembered that the term “rapid” is relative in reptiles induction may take a few minutes even after intravenous injection. Incremental doses should not be given until the initial effect has reached a maximum; otherwise overdosage may occur.

Propofol must be administered either intravenously or by intraosseous injection. The intraosseous route has only been evaluated in a few species to date. Dose rates in lizards and snakes are 5-10 mg/kg. The dose for large or giant chelonians is reduced to 1-2 mg/kg.

Propofol is rapidly metabolized and is non-cumulative. However, it does produce a dose-dependent cardiopulmonary depression. Apnoea is common after initial administration and is dependent on speed of injection and dose. Not all reptiles become apnoeic; slow injection of 10 mg/kg i.v. in the green iguana will often permit 20 minutes of light general anaesthesia sufficient for radiography, and spontaneous respiration usually continues. Intubation and assisted ventilation equipment should always be prepared in advance so that such care can be administered if spontaneous respiration ceases.

The major advantage of propofol is its speed of action and the rapid recovery that occurs spontaneously after 20-40 minutes. Disadvantages are: requirement for intravenous injection, which is technically demanding; and expense.

Gaseous anaesthesia

Advantages over parenteral agents include:

- Ability to control anaesthetic depth more readily

- Concurrent delivery of 100% oxygen

- Ability to provide ventilatory support

- More rapid recovery

- Relative unimportance of obtaining an accurate bodyweight (which may be difficult before general anaesthesia in large, dangerous or aggressive reptiles).

Disadvantages are:

- Requirement for special delivery equipment

- Relative expensive, especially for prolonged general anaesthesia.

The respiratory rate required to maintain gaseous anaesthesia is often greater than the normal respiratory rate of the conscious animal. Therefore intermittent positive pressure ventilation (IPPV) is usually required to maintain anaesthesia, even if the animal is breathing spontaneously. For example, a snake breathing spontaneously may breathe twice per minute, but a rate of 6 times per minute may be required to maintain anaesthesia.

Inhalational agents.

Figure 11.13 lists agents used for gaseous anaesthesia in reptiles. The preferred inhalant anaesthetic for reptiles is isoflurane. Nitrous oxide enhances the rate of induction and recovery when combined with other inhalational anaesthetics. It also provides analgesia that improves the quality of anaesthesia during painful surgical procedures.

Isoflurane

Isoflurane is eliminated almost entirely by the lungs and is therefore recommended in debilitated patients. Rapid induction and recovery occurs, with fewer of the cardiopulmonary effects seen with the use of halothane. Generally, induction is achieved using 4-5% isoflurane in 1 litre/min oxygen, taking 5-20 minutes, followed by maintenance using 1-2.5% isoflurane in oxygen. Recovery generally occurs 10-30 minutes after the cessation of isoflurane administration, although it may occur sooner in some species. As the depth of general anaesthesia is hard to define in reptiles, it is advisable to stop the administration of isoflurane only when surgery is completed, as the recovery period (or at least resumption of movement) may be only a few minutes in some cases.

Halothane

More depression of the cardiovascular system is noted with the use of halothane than with isoflurane and there is a longer induction and recovery time because the drug is lipid-soluble. In general, 4-5% is used for induction, which may take 10-20 minutes, with 1.5-2.5% used for maintenance of general anaesthesia. Nitrous oxide and oxygen may be combined in an equal ratio as the carrier gas for the halothane; this has been suggested to speed induction times. Reptiles appear to object to the smell/taste of halothane more than isoflurane, with escape behaviour and breath holding common. To reduce this effect, a slowly rising concentration of halothane is recommend to induce general anaesthesia. Species-specific differences in the response to halothane have been described including the requirement for a higher concentration for induction in venomous compared with non-venomous snakes and in viperids compared with elapids.

Sevoflurane

Sevoflurane appears to be a safe gaseous general anaesthetic in reptiles and produces a rapid induction and recovery. Induction generally takes 3-5 minutes and recovery up to 30 minutes. Heart rate does not appear to be significantly altered by this agent when used in tortoises (Rooney et al., 1999).

Induction of anaesthesia

Reptiles are very resistant to hypoxia; turtles have survived for >6 hours breathing only 100% nitrogen (Wasser et al., 1991). Under certain conditions, reptiles readily convert to anaerobic metabolism and may do this when inhalational general anaesthesia is offered, thereby avoiding the absorption of the gas. While chelonians seem resistant to induction by mask, the clinical relevance varies in lizards and snakes. The author has anaesthetized many snakes by placement into a plastic bag that is then filled with a mixture of isoflurane/oxygen and sealed to induce general anaesthesia in less than 5 minutes. This method avoids over-handling and restraint of the animal. Similarly, lizard species or individuals that are easily restrained with minimal resistance can be sensitive to mask induction with gaseous anaesthetic (Figure 11.14). Attention must be paid to the respiratory rate of the animal; if apnoea occurs before general anaesthesia is induced, this is often normal apnoea and gently stroking the flanks or gently compressing the ribs may stimulate voluntary breathing. Tractable lizards and snakes may also be induced using gaseous anaesthetic by intubation (with or without a degree of sedation)l and assisted ventilation.

Intubation

Intubation is relatively easy in reptiles compared to mammals. The glottis is easily visualized in most species.

- The glottis of snakes is readily identified in the anterior oral cavity immediately above the lingual recess (Figures 11.3, 11.15).

- The lizard glottis is positioned at the back of the tongue (see Figure 11.2). It is sometimes difficult to see in animals with a large fleshy tongue; pressing beneath the chin externally may raise the glottis.

- The chelonian possesses a large fleshy tongue that obscures the view of the glottis (see Figure

- 11.4). Pressing upwards below the chin raises the glottis and extending the head fully aids visualization.

- Crocodilians possess a basihyal valve that must be displaced to view the glottis on the floor of the pharynx (Figure 11.16).

- The glottal opening in green turtles is obscured by the presence of caudally directed pharyngeal spines.

Local anaesthesia

To facilitate intubation in conscious reptiles it may be necessary to provide topical anaesthesia with direct application of local anaesthetic (2% lidocaine injection or lidocaine gel) to the glottis. The use of neuromuscular blocking agents (see above) will also aid intubation. Light sources used for visualization of the glottis can include a laryngoscope, fibre optiscope and penlight.

Breathing cycle

The glottis is closed for variable and prolonged periods as part of the normal breathing cycle. It may be preferable to wait for the glottis to open naturally for inspiration;this necessitates waiting for over a minute in some cases. Extreme care must be exercised if the endotracheal tube is to be advanced through a closed glottis.

Tubes

Endotracheal tubes of internal diameter of as little as 2 mm can be purchased commercially. Smaller tubes are made out of over-the-needle catheters or rubber catheters. Reptiles produce minimal respiratory secretions, so blockage of the tube by mucus is not a common occurrence. However, kinking or crushing, resulting in blockage, of the smaller tubes may occur easily and so every effort must be made to keep the head and neck straight and to prevent the jaws closing on to the tube. Small gags made of wood or rubber blocks may be used to keep the mouth open (Figure 11.17); care must be taken not to damage delicate teeth. The head and neck may be taped to a tongue depressor with the beginning of the anaesthetic circuit to keep all connections in line. This technique is especially useful in snakes (Figure 11.18), small lizards and chelonians.

The length of endotracheal tube should be appropriate for the species. Chelonians have a relatively short trachea before it bifurcates into two bronchi (see Figure 11.5). Thetube length should be little furtherthan half the length of the neck in these species. If too long, a single bronchus may be intubated, resulting in ventilation of only one lung. An uncuffed tube should be used as the trachea may be damaged by inflation of a cuff.

Forced positive pressure ventilation

It is possible, and sometimes preferable, to intubate conscious snakes and lizards and induce general anaesthesia by forced positive pressure ventilation. This negates the effect of breath-holding, which would make mask or chamber induction a prolonged affair.

Induction of general anaesthesia using this method is generally achieved in 2-5 minutes and is the author's method of choice for snakes. Chamber induction may be prolonged, especially in animals that maintain apnoea for period of minutes at a time, and should therefore be avoided in chelonians. However, it may be - a safer alternative to handling very aggressive or venomous species of lizards and snakes.

Anaesthetic circuits

It is recommended that inhalational anaesthetics be administered using an anaesthesia machine with precision flow meters and vaporizer. The choice of anaesthetic circuit is similar to that in mammals:

- Patients <5 kg may be maintained on a nonrebreathing system, such as a T piece, with a minute volume of 300-500 ml/kg/min

- Patients >5 kg may be maintained on a Lack or circle system.

Assisted ventilation

It is recommended that all reptiles are intubated and artificially ventilated under general anaesthesia, as apnoea is common, whether a gaseous or parenteral agent is used for the induction or maintenance of general anaesthesia (Figure 11.19). This apnoea under general anaesthesia is possibly due to a combination of high oxygen saturation, the absence of a diaphragm and a reliance on the voluntary skeletal muscle movement for ventilation.

The conscious respiratory rate of reptiles varies with species, bodyweight, activity level and environmental temperature. It is a useful to observe the conscious patient before general anaesthesia, to compare the pre- and postanaesthetic respiratory rates as a measure of recovery. The conscious respiratory rate may be too slow to produce general anaesthesia using an inhalational agent, therefore as a general rule the mechanical ventilator or manual IPPV rates are 2-4 breaths per minute. This rate may be increased to 10-30 per minute for induction purposes. A low pressure is advisable, e.g. <20 cm H₂O, to avoid damage to the delicate lungs. It is advisable that the anaesthetist observes the depth and body wall movements in the conscious animal and attempts to replicate this when gauging the pressure to apply when ventilating the anaesthetized animal. It is a common mistake to overpressurize, producing body wall movements far in excess of the natural breathing movements; this may lead to lung damage and even rupture.

The normal breathing cycle is characterized by expiration, then inspiration, then a postinspiratory period of breath-holding with the glottis closed. Both the inspiratory and expiratory phases require active muscle activity, i.e. expiration is achieved by pressing air out of the lungs, not merely a relaxation of the ribcage as in mammals. Therefore, after a period of assisted ventilation (which relies on forcing air in but not pulling it out) it may be beneficial to ensure the lungs are emptying, gas is circulating and lungs are not overfilling, by gently compressing the ribs of the snake or lizard along the length of the body, or moving the legs in and out for chelonians. This is especially useful in the recovery phase to ensure that general anaesthetic gas is `washed out' of the lungs to achieve recovery.

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 (0.5 kg) tortoises may be anaesthetized with alfaxolone/alfadolone at 9-12 mg/kg i.m. or intracoelomically. Induction takes 10-30 minutes; general anaesthesia lasts for approximately 10-20 minutes. A lower dose may be used to facilitate intubation.

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.