libro 2

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.