School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI, USA
Introduction
Historically, managing anesthesia in reptiles has been problematic. The diversity of reptiles in terms of natural history, size, anatomy, and physiology presents a unique clinical challenge to the veterinarian. Safe and effective anesthesia of the reptilian patient is a requirement for procedures including physical examination of large or dangerous species, collection of diagnostic samples, application of various imaging modalities, as well as for surgical procedures. The anesthetic management of the reptilian patient, including the design of the anesthetic protocol, represents unique challenges due to marked anatomical and physiological differences between taxa. Responses to frequently used anesthetic agents and dosages are often variable between taxa, and individual differences are also commonly observed. Additionally, anesthetic and analgesic protocols (including monitoring techniques to evaluate cardiopulmonary performance) are often derived from domestic animals and then applied to the reptilian patient. These techniques are often of limited value and should be interpreted by clinicians with caution.
Thermoregulation and the Effect of Environmental Temperature
Reptiles are ectothermic, meaning that their body temperature is directly dependent on environmental temperature and associated behaviors to modify that body temperature. Changes in body temperature significantly affect metabolic rate and many other physiologic processes. Therefore, the temperature at which one maintains a patient is an important factor in reptilian anesthesia due to the fact that the absorption, distribution, metabolism, and excretion of drugs in reptiles are directly related to environmental temperature. Induction, duration, and recovery from sedation or anesthesia induced using injectable drugs will be faster if animals are maintained at higher temperatures, and significantly longer if the body temperature is low.1,2
Hepatic First-Pass Effect
The hepatic first-pass effect following hindlimb administration of anesthetic and other drugs has not received much attention in the literature. However, its pharmacodynamics and pharmacokinetic significance may be more clinically important than that of the renal portal system.3 The venous blood flow from the hindlimb in chelonians, lizards, and crocodilians drains into the ventral abdominal vein(s), which either passes directly to the liver or indirectly via the hepatic portal vein.4 Hence, any liver metabolized or excreted drug administered in the hindlimb first enters the liver before reaching the systemic circulation, resulting in a hepatic first-pass effect. In turtles and crocodiles, the hepatic first-pass effect following hindlimb administration leads to lower bioavailability of drugs, resulting in lower plasma concentrations and/or reduced or no clinical efficacy.5,6 In leopard geckos, the administration of dexmedetomidine-ketamine in the hindlimbs was significantly less effective in inducing sedation compared to the injection in the forelimbs.7 In snakes, administration of alfaxalone in the caudal part of the body was significantly less effective in inducing sedation compared to injection in the cranial part of the body.8 However, despite the published evidence of the clinical significance associated with the hepatic first-pass effect, pharmacokinetic and pharmacodynamic studies continue to be published without taking this anatomical factor into consideration with respect to study design.
Since many drugs commonly used in reptiles, particularly analgesics and anesthetics, undergo hepatic metabolism and/or excretion, one must consider the hepatic first-pass effect, since the majority of venous blood from the hindlimbs and caudal body half drains directly into the liver before entering the systemic circulation. Therefore, it is advised to avoid administration of anesthetic drugs by intramuscular or subcutaneous injection in the caudal body half in reptiles. Intravenous administration of anesthetics in the coccygeal (caudal ventral tail) vein of lizards does not result in a hepatic first-pass effect, because it drains directly into the caudal vena cava. In turtles, the venous drainage from the tail is less consistent, with some blood entering the ventral abdominal vein and some entering the caudal vena cava.
Injectable Anesthetic Agents
Prior to administration of any anesthetic or sedative drugs, reptiles should be maintained within their preferred optimal temperature zone (POTZ), and the core body temperature should be monitored throughout the anesthetic period. A variety of agents representing different classes of drugs are frequently used, either alone or in combination, depending on the desired level of sedation or anesthesia. It is recommended to avoid administration of high dosages of a single anesthetic agent (e.g., ketamine, alfaxalone) and instead consider protocols in which multiple drugs are combined with synergistic actions, thereby requiring lower dosages for each drug. Additionally, using readily reversible drug protocols will provide for more rapid recoveries. Since most deleterious side effects (e.g., prolonged recovery, cardiopulmonary depression) associated with anesthetic and sedative drug administration are dose-dependent, individual drug dosage reduction and reversibility will result in fewer complications and improved recoveries. Similar drugs and drug combinations can also be chosen for premedication/sedation prior to induction of general anesthesia.
If the patient is scheduled for a painful procedure, it is recommended to include an analgesic agent such as an opioid in the preanesthetic protocol. Many injectable protocols in reptiles include the administration of an alpha-2 agonist (e.g., [dex]medetomidine) with ketamine and/or midazolam. A mu-opioid receptor agonist (e.g., morphine, hydromorphone) should be added to the protocol if additional analgesia is required in chelonians and lizards. One must keep in mind that many factors (e.g., site of injection, underlying disease, dehydration, body temperature, gravidity, etc.) may affect the onset, recovery from, and efficacy of anesthetic drugs. Therefore, an individualized approach to choosing appropriate drug combinations and dosages should be considered.
Injectable agents alone can be used to facilitate a variety of short diagnostic and surgical procedures, as well as for induction of general anesthesia. Maintenance of a surgical plane of anesthesia for prolonged, invasive procedures is frequently based on administration of an inhalational agent, such as isoflurane or sevoflurane. In aquatic freshwater turtles and terrestrial chelonians, injectable protocols containing (dex)medetomidine-ketamine, with or without opiates and local anesthetics, were successfully used to induce and maintain surgical anesthesia without the need of gas anesthesia, and these protocols are preferred by the authors.
Routes of Parenteral Drug Administration
While historically the intramuscular route of drug administration is recommended in reptiles, it is considered more painful, and less space is provided in small muscle bellies for larger drug volumes when compared to the subcutaneous route. In snakes, the epaxial muscles are used, while in lizards and chelonians, muscles of the front limbs are a common intramuscular injection site but provide only limited space to administer anesthetic drugs. The subcutaneous administration of anesthetic and analgesic drugs offers several advantages over intramuscular injection, including access to a variety of subcutaneous sites requiring minimal manual restraint or patient manipulation, suspected less discomfort on injection, and the capability of administering larger volumes in a single location.
The intravenous route offers an excellent route of administration for anesthetic agents and results in rapid onset of anesthesia. However, depending on the species and patient size and cooperation, intravenous access may be challenging or impossible to obtain, making the subcutaneous or intramuscular route more feasible. In lizards, intravenous injections are most commonly administered into the caudal (ventral tail) vein. In most tortoises and non-marine turtles, the jugular vein is preferred for intravenous drug administration, but the brachial plexus is also practical. The subcarapacial sinus and dorsal coccygeal vein cannot be recommended for intravenous drug administration in chelonians, due to the risk of accidental administration of anesthetic agents into the intrathecal space, which may lead to severe neurologic complications including permanent paralysis and death. In addition, the dorsal coccygeal vein is not consistently developed across chelonian species, and while present in red-eared sliders, it is absent or poorly developed in several tortoise species. Therefore, it should be assumed that a well-developed dorsal coccygeal vein is the exception, rather than the rule, amongst chelonians. Hence, the author advises against drug administration into the subcarapacial sinus or the dorsal tail vein in chelonians. Intravenous injection sites in snakes are limited to the caudal (ventral tail) vein and the jugular vein following a cut-down procedure. Intracardiac administration of propofol has also been reported in snakes and results in rapid onset with no significant complications reported in a controlled study.9 However, cardiac tamponade after cardiocentesis in snakes has been reported, and the increased risk of intracardiac injections should be considered.10
Drugs
Ketamine, a dissociative agent with dose-dependent anesthetic, sedative, and analgesic properties, is frequently used in reptile anesthesia. However, when administered alone, muscle relaxation is considered inadequate and recoveries excessively prolonged, especially when high dosages (>50 mg/kg) are used. Administration of an alpha-2 adrenergic agonist (e.g., ([dex]medetomidine) and/or a benzodiazepine (e.g., midazolam), in conjunction with ketamine, allows for reduction of ketamine dose. Additionally, these protocols have the benefit of partial reversibility, leading to more rapid recoveries and increased safety. Even at lower, subanesthetic dosages (<5 mg/kg), ketamine can provide additional sedation and analgesia if combined with other anesthetic drugs.
Alpha-2 adrenergic agonists, such as medetomidine and (dex)medetomidine, provide sedation, muscle relaxation, and analgesia in reptiles. Dose-dependent cardiovascular depression was documented in reptiles after alpha-2 agonist administration. Alpha-2 adrenergic agonists are commonly used in combination with ketamine (especially in chelonians) for safe, reliable, and reversible anesthesia but can also be combined with benzodiazepines for procedural sedation. Combining ketamine with medetomidine, or (dex)medetomidine, allows reduction of both drug dosages, and reversibility of the alpha-2 adrenergic agonists with atipamezole will lead to faster and more predictable recoveries. Morphine or hydromorphone can be added to alpha-2 adrenergic agonist plus ketamine protocols in cases in which analgesia is needed. If morphine is used, reversal with naloxone (0.04–0.2 mg/kg SC) should be considered if respiratory depression persists after termination of the anesthetic procedure or recovery is delayed. Concentrated formulations of medetomidine (e.g., 20 mg/ml, ZooPharm Inc.) are available and are more suitable for giant species, such as large chelonians or crocodilians.
Benzodiazepines, such as midazolam and diazepam, have sedative and muscle relaxant properties. Midazolam is water-soluble; can be administered SC, IM, and IV; and is considered a more appropriate choice than diazepam, which is not recommended for IM or SC injection. Midazolam, used alone, is anxiolytic and provides mild, but highly variable sedation, which may be sufficient for minor clinical procedures. Midazolam, combined with ketamine and/or an alpha-2 adrenergic agonist, reduces all drug dosages and attenuates the dose-dependent cardiovascular depressant effects and prolonged recoveries commonly observed with high dosages of ketamine. The effects of benzodiazepines can be antagonized using flumazenil (0.05 mg/kg SC, IM), which shortens recovery from sedation or anesthesia. Concentrated formulations of midazolam (e.g., 50 mg/ml, ZooPharm Inc.) are available and are more suitable for administration of midazolam in giant reptile species.
Propofol, a short-acting, non-barbiturate anesthetic agent can be administered intravenously or intraosseously to provide short-time anesthesia, facilitating procedures such as placement of an esophagostomy tube or abscess debridement, or to induce general anesthesia and allow for endotracheal intubation and maintenance of anesthesia with an inhalational agent. Respiratory depression is also more profound when propofol is administered rapidly. Since propofol does not accumulate in tissues and is rapidly metabolized, recovery can be expected with assisted ventilation in cases of overdose. Propofol requires intravascular administration, which can be challenging. Complications secondary to accidental extravascular injection of propofol intrathecally and into the subcarapacial sinus or the coccygeal vein of chelonian species have been reported.11,12 Complications associated with intrathecal propofol injection included forelimb and hindlimb paralysis, coma, and spinal necrosis.
Alfaxalone is a short-acting steroid anesthetic, which is labeled for induction of anesthesia in dogs by intravenous administration. However, alfaxalone can be administered by either the intramuscular or subcutaneous route at higher doses. Alfaxalone is rapidly cleared, and its metabolism is independent of organ function; however, pronounced temperature-dependent differences in anesthetic induction, plateau, and recovery times have been reported.1 Similar to propofol, alfaxalone administration is associated with dose-dependent cardiovascular and respiratory depression in mammals. Recovery from alfaxalone-induced anesthesia is dose dependent, and at high dosages, prolonged recoveries are to be expected. The major advantage of alfaxalone over propofol is that it can be administered intramuscularly as well as subcutaneously, in addition to the intravenous route. Due to the high dosages needed for non-intravenous administration in chelonians and lizards, the resulting drug volumes are usually inappropriately large due to the low concentration of the available alfaxalone formulation (10 mg/ml). Therefore, the author recommends SC injection if large volumes of alfaxalone are to be administered, since the subcutaneous administration of alfaxalone in reptiles has been associated with rapid inductions times.8,13,14
Inhalational agents, such as isoflurane and sevoflurane, can be used for both induction and maintenance of anesthesia. Both agents offer the advantages of relatively fast induction and recovery times, as well as limited organ toxicity, especially in patients with renal or hepatic impairment. The low blood solubility allows for rapid changes in anesthetic depth. Administration of inhalational agents follows similar principles as those established for domestic animal anesthesia. Following induction of anesthesia, the trachea should be intubated with an appropriately sized endotracheal tube, which should be connected to a non-rebreathing (body weight <10 kg) or rebreathing system. Intermittent positive pressure ventilation should be performed in all reptiles since they lack a diaphragm and therefore will be apneic while under a surgical plane of anesthesia. The concentration of the inhalational agent necessary to maintain a surgical plane of anesthesia depends on the health status of the patient. Minimum anesthetic concentrations (MAC) of isoflurane and sevoflurane were determined in green iguanas and are 1.8–2.1% and 2.1–4.1%, respectively.15 Maintenance requirements for a surgical plane of anesthesia were reported to be 2–3% for isoflurane and 3.5–4.5% for sevoflurane. Both isoflurane and sevoflurane have a dose-dependent, depressive effect on the cardiovascular system. Isoflurane was shown to significantly reduce blood pressure in green iguanas and ball pythons. Therefore, significant cardiovascular depression can occur with normal maintenance concentrations of isoflurane. No significant differences were detected between sevoflurane and isoflurane in cardiopulmonary function of green iguanas. However, use of sevoflurane resulted in faster induction and recovery compared to isoflurane.
Due to right-to-left cardiac shunting in some reptiles, reduced lung perfusion can occur, and therefore, concentrations of inhalant anesthetic gases in the lungs do not necessarily reflect the concentrations in the blood or brain. Sudden changes in shunting directions can lead to sudden changes in inhalant anesthetic blood concentration, which can lead to significant changes in anesthetic depth, evidenced by slow induction, sudden arousal from anesthesia, inability to deepen a lightly anesthetized reptile, or prolonged recovery from anesthesia. In lizards and snakes, changes in gas concentration can be used to control anesthetic depth, but in chelonians—particularly aquatic turtles—a change in gas concentration will not lead to measurable changes in anesthetic depth as a function of cardiac shunting, breath-holding ability, and ventilation-perfusion mismatch. Intracardiac shunting can be eliminated in chelonians by administration of atropine, which results in lower MAC of isoflurane.16
General Anesthesia
Induction
Induction of anesthesia can be accomplished with injectable or inhalational agents. In patients with established, reliable vascular access, slow administration of propofol (3–10 mg/kg IV, IO) or alfaxalone (5–10 mg/kg IV) will induce rapid induction of anesthesia. Alternatively, intramuscular or subcutaneous administration of various combinations of ketamine, midazolam, alpha-2 agents, alfaxalone, and opioids will induce a dose-dependent plane of anesthesia adequate for endotracheal intubation and maintenance with an inhalational agent, if required. Mask or chamber induction with an inhalational agent can be used in snakes and lizards, and for handler safety reasons is particularly useful in venomous species. Mask or chamber inductions require high concentrations of gas anesthetic, which is associated with environmental contamination and can contribute to unpredictable and prolonged induction times. In monitor lizards, induction times of >10 minutes were noted following isoflurane and sevoflurane delivery by mask. In reptile species that are able to breath-hold for extended periods (in particular, semiaquatic and aquatic turtles and crocodilians), induction times will be prolonged, and/or induction with inhalant agents may be ineffective.
Endotracheal intubation is relatively easy to perform in most snakes and lizards. In these species, the glottis is located rostrally in the oral cavity, and the trachea can be intubated with an appropriately sized endotracheal tube.
Maintenance
During maintenance of anesthesia, the patient should be monitored regularly for depth of anesthesia, cardiopulmonary performance, and evidence of pain. If indicated, the anesthetic level should be adjusted, and additional analgesic therapy may be warranted. Similar to domestic animals, cardiopulmonary depression is dose dependent in reptiles, and a decrease in heart rate and arterial blood pressure will be observed—although blood pressure monitoring is difficult and frequently impractical.
All reptiles require intermittent positive pressure ventilation (IPPV) at a surgical plane of anesthesia to maintain EtCO2 between 10 and 25 mm Hg. Over-ventilation (decreased EtCO2) causes respiratory alkalosis and delays return to spontaneous ventilation during recovery. Ventilation can be performed manually or more accurately with a mechanical ventilator. Any anesthetized reptile that is breathing spontaneously is light. While anecdotally it has been recommended to ventilate an anesthetized reptile at 4–6 breaths per minute, the rate of IPPV needs to be adjusted to each individual patient based on the preanesthetic examination and based on EtCO2 values. For example, in anesthetized neotropical rattlesnakes (Crotalus durissus), 5 breaths per minute led to respiratory alkalosis and was considered excessive, while 0.67 breaths per minute led to mild hypercapnia and mild respiratory acidosis.17 Therefore, a respiratory rate of 1–2 breaths per minute at a tidal volume of 30 ml/kg was recommended for this species.
Monitoring Techniques
Accurate monitoring of anesthetic parameters is essential to safely and effectively anesthetize the reptilian patient. Application and interpretation of many monitoring techniques in reptiles are in their infancy, and some devices and techniques may only be of limited value. Monitoring of cardiopulmonary performance in the anesthetized reptilian patient presents challenges, and further work is needed to establish reference data for common reptile species. One should be familiar with the applications and limitations of various monitoring devices developed for human and veterinary anesthesia. Anesthesia monitoring techniques such as pulse oximetry, arterial blood gas analysis, and capnography are commonly used in human and veterinary anesthesia, but not all have been validated for use in reptiles. In addition, reference values for common reptile species and common anesthetic protocols are scant. Therefore, knowledge of the anatomy, physiology, and pathophysiology is essential. Also, the pharmacology of sedative and anesthetic agents being used, as well as their effects on cardiopulmonary performance and organ function, must be known. In order to accurately interpret cardiopulmonary performance, there is a need for normal values in conscious reptiles; however, few studies have investigated normal cardiopulmonary parameters in conscious, non-anesthetized reptiles.
Reflexes—Determination of the degree of sedation and depth of anesthesia is commonly performed by assessment of the presence or absence of reflexes, such as head or limb withdrawal, righting reflex, palpebral and corneal reflexes, toe/tail pinch reflex, as well as cloacal tone. However, significant anatomic differences exist between snakes, chelonians, and lizards. The corneal and palpebral reflex is present and useful in many reptiles but cannot be assessed in snakes and most geckos (one exception being leopard geckos), due to lack of eyelids and presence of spectacles. The righting reflex should be absent in snakes and lizards during a surgical plane of anesthesia, but it is not as useful in turtles and tortoises. Assessment of head withdrawal and neck tone may provide useful information in turtles and tortoises. Snakes tend to lose muscle tone from head to tail during induction and regain muscle tone from tail to head during recovery. In general, a surgical plane of anesthesia, including effective analgesia, is determined when there is no response to a painful stimulus (such as a skin incision), and consequently, no changes in cardiopulmonary parameters such as tachycardia, hypertension, or an increase in respiratory rate.
Recovery
Following cessation of the procedure, inhalant anesthesia should be discontinued, and any reversal drugs administered (taking care to provide alternative analgesia if using naloxone). Room air administered with an Ambu bag™ instead of 100% oxygen has, historically, been recommended for ventilation of reptiles during recovery. It was suggested that high oxygen concentrations in the lungs delay return to spontaneous respiration. However, recent studies could not demonstrate any statistical or clinically significant difference in recovery times in monitor lizards or bearded dragons ventilated with 100% oxygen, compared to animals ventilated with room air during recovery from gas anesthesia.18
As is standard protocol for domestic animals, fluid therapy and pain management should be continued during the recovery period, and additional analgesia should be provided as necessary. Prolonged anesthetic recoveries are common in reptiles, especially if high dosages of non-reversible injectable drugs are administered (e.g., tiletamine/zolazepam). Isoflurane anesthesia in chelonians is also frequently associated with slow recoveries, suspected to be due to the hypotension, right to left shunting, and reduced lung and tissue perfusion. Administration of epinephrine (syn. adrenalin, 0.1 mg/kg IM) in common snapping turtles (Chelydra serpentina) significantly reduced time (>50%) to return of spontaneous breathing and complete recovery following isoflurane anesthesia. It was demonstrated that an increase in adrenergic tone of the pulmonary vasculature leads to shunting of blood away from the pulmonary circulation during episodes of apnea in turtles, and that by increasing the sympathomimetic tone of the systemic vasculature through administration of epinephrine, pulmonary shunting is reduced, leading to accelerated recovery in turtles anesthetized with isoflurane.19 Stimulation of the acupuncture point GV-26 also hastens snapping turtle recovery from isoflurane, comparable to the administration of epinephrine.19
During recovery, the patient should be kept in a temperature-controlled environment. For most reptilian patients, dedicated reptile or small animal incubators are ideal since they are designed for accurate temperature maintenance. It is not recommended to exceed the species-specific POTZ since this will result in an increased metabolic rate and oxygen demand. Reptiles recovering from general anesthesia will be in a state of respiratory compromise, and it may be difficult to meet the increased demand of oxygen by spontaneous respiration. During the recovery period, it is essential to assess respiratory status of the patient and continue assisted ventilation until the patient is breathing spontaneously, has regained reflexes, and can be extubated.
Spontaneous breathing and ambulation are reliable indicators of anesthetic recovery. Reptiles should be extubated once spontaneous breathing is observed and oropharyngeal reflexes (e.g., jaw tone, tongue movement) have returned. If spontaneous breathing has not returned, IPPV should be continued. Even following extubation, monitoring should continue, as it is not unheard of for a spontaneously breathing reptile to slip back into unconscious apnea.
Regional Anesthesia and Analgesia
Regional anesthesia and analgesia are currently underutilized in reptile medicine but offer significant benefits; for example, the ability to reduce injectable, inhalant anesthetic, and sedative drug requirements, and to produce preemptive analgesia in the perioperative period without deleterious side effects (such as respiratory depression). In addition, drugs used for regional anesthesia are widely available and do not require controlled drug scheduling. Finally, in mammals, local anesthetics have the potential to blunt a surgery-induced neuroendocrine (stress) response. Knowledge of anatomical landmarks and basic pharmacology of local anesthetics is required for safe and effective nerve blocks. Regional anesthesia can be used as a sole anesthetic modality in manually restrained or sedated reptiles for minor surgical procedures. Common indications for the use of local anesthetics as part of an anesthetic protocol in reptiles include: distal limb surgery, cloacal procedures (e.g., prolapse, cloacoscopy, egg removal), phallectomy, and celiotomy and celioscopy. Local anesthetics can be administered as part of topical or incisional (infiltration) anesthesia, peripheral or cranial nerve blocks, or neuraxial (intrathecal, spinal) anesthesia.
Clinical experience demonstrates that local anesthetics have similar onset and duration of action in reptiles when compared with mammals. Dosages, volumes, and concentrations of local anesthetics are crucial in determining the magnitude and quality of the anesthetic block. High concentrations are preferred, but sufficient volume of tissue distribution should be considered. Site of injection will be an important factor in inducing the onset of block. For shorter onset of anesthesia, local anesthetics should be administered as close as possible to, but not directly into, a nerve. Physical and chemical characteristics of the local anesthetic explain the slow onset, but some lipophilic drugs (e.g., bupivacaine) provide a long duration of action.
Local anesthetics can produce irreversible or reversible nerve damage. Neurotrauma occurs during intraneural injections. A local anesthetic should never be injected if resistance to injection is observed. Systemic toxicity is produced by accidental intravascular injection of local anesthetics. Toxic dosages associated with adverse effects have not been determined in reptiles, but maximum dosages used in mammals—particularly in small reptiles—should be used as a guide. Adverse effects can include seizures, cardiorespiratory depression, and death. Only preservative-free drug formulations should be used for intrathecal injections.
Lidocaine (2%) is an aminoamide local anesthetic that is commonly used in reptile medicine. Dosages associated with toxicity were reported to be between 5 and 20 mg/kg in mammals. In reptiles, lidocaine was administered at up to 4 mg/kg intrathecally without side effects. Concentrations of 0.5% and 1% are also available. In addition, a commercial mixture of lidocaine (2.5%) and prilocaine (2.5%) in gel preparation (EMLA® cream) is available and used for venipuncture in small animals. Its use was reported in turtles and tortoises undergoing phallectomy. Infiltration of the prefemoral fossa in chelonians with lidocaine has been reported and is frequently used by biologists for coelioscopy. However, it has been documented that this technique using 1 mg/kg of lidocaine results in insufficient anesthesia and analgesia compared to general anesthesia, and is therefore not recommended.20
Bupivacaine is a highly lipophilic, aminoamide local anesthetic that produces significant cardiotoxicity when injected intravenously. It is not clear what dosages will produce these deleterious effects in different reptile species, but negative aspiration of blood into the needle hub is always recommended prior to bupivacaine administration.
Regional Anesthetic Techniques
Tissue infiltration (incisional anesthesia)—Local anesthetics are used for incisional anesthesia and can be infiltrated in the subcutaneous tissue in association with a surgical field; for example, prior to celiotomy (“line block”), skin biopsies, or laceration repair. However, local anesthetics should not be administered in proximity to neoplastic tumors since the process can spread neoplastic cells. In small animals, this technique is used most frequently before laparotomy and as part of a multimodal analgesic approach, during which NSAIDs are also administered systemically. Aseptic techniques are always employed.
Nerve blocks—Local anesthetic techniques of the head and limbs are not commonly reported in the reptile literature. This is likely due to the large number of reptilian species and the paucity of information on their specific neuroanatomy and the efficacy of locoregional techniques. In crocodilians, dental blocks have been reported.21
Spinal anesthesia—Spinal (also referred to as intrathecal or subdural) anesthesia and analgesia of the spinal cord are currently in their infancy in reptiles and have only been reported in turtles and tortoises. In chelonians, epidural anesthesia is not possible due to the lack of a sufficiently developed epidural space. However, a well-developed intrathecal (subdural) space, which directly surrounds the spinal cord and is filled with cerebrospinal fluid (CSF), has been published and allows for intrathecal administration of various anesthetic and analgesic drugs. The presence of the carapace, and the fusion of the vertebral column to the carapace, limits access only to the cervical and coccygeal intrathecal space in chelonians, but only intrathecal injections in the coccygeal region are of clinical interest.
Spinal anesthesia has been evaluated in several species of chelonians. In red-eared sliders (Trachemys scripta elegans), spinal anesthesia was successfully induced in turtles of both sexes, with body weights ranging from 0.5 to 1.0 kg.22 Injections were performed in sedated turtles, which allowed for proper restraint of the tail and correct intrathecal administration at the level of the mid-to-proximal coccygeal vertebrae. The turtles were positioned in ventral recumbency, and the tails were aseptically prepared before insertion of 28-G needles, attached to 0.5-mL insulin syringes. Successful intrathecal injection and induction of spinal anesthesia were confirmed by complete motor block (relaxation) of the tail, cloacal sphincter, and hindlimbs. The onset to motor block ranged from 1 to 5 minutes. Following the initial injection of either preservative-free lidocaine (4 mg/kg, 2%) or bupivacaine (1 mg/kg, 0.5%), spinal anesthesia was successfully induced in approximately half of the turtles. In turtles showing no evidence of spinal anesthesia after the initial injection, a second intrathecal injection (of the same drug and dosage) was administered approximately 15 minutes following the first. The repeated intrathecal injection increased the overall success rate to approximately 80–90%. In male and female red-eared sliders, the duration of motor block of the tail, cloacal sphincter, and hindlimbs was approximately 1 hour following intrathecal lidocaine administration (4 mg/kg, 2%) and about 2 hours following intrathecal bupivacaine injection (1 mg/kg, 0.5%). There was a large variation in duration of spinal anesthesia in turtles of both genders. A possible explanation for this variability was associated with the well-developed and prominent intravertebral venous plexus, which is located within the spinal canal directly overlying the intrathecal space, both dorsally and laterally.
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