Reptiles may be anesthetized using a variety of parenteral and/or inhalant anesthetics; however, it is important for veterinarians working with these animals to understand that most of the literature pertaining to anesthetics in reptiles is limited. The purpose of this presentation is to review the common anesthetics used for reptiles.
The dissociatives were some of the first anesthetics evaluated in reptiles. These compounds are so-named because they "dissociate" the patient from its environment. Although these compounds are considered to provide general anesthesia, the author considers them to be incomplete. There are two primary dissociative anesthetics used in veterinary medicine: ketamine hydrochloride (Ketaset®; Ft. Dodge Animal Health, Ft. Dodge, IA, USA) and tiletamine (Telazol®; Ft. Dodge Animal Health, Ft. Dodge, IA, USA). Tiletamine is considered 2–3 times as potent as ketamine. In Telazol®, tiletamine is combined with zolazepam. The zolazepam provides protection against seizures. The primary advantages associated with these compounds are that they are inexpensive, readily available (in the United States), and provide somatic analgesia. The primary disadvantages associated with these compounds are that they do not provide visceral analgesia, have cardiopulmonary depressant effects, do not provide muscle relaxation, and leave the patient with some knowledge of the experience. In reptiles, an additional disadvantage has been noted: prolonged recoveries. In some species of reptiles, 24–72 hour recoveries are not uncommon. Part of this is related to the dosing recommendations for these compounds. In the literature it is possible to find a range of recommended doses. For example, the author has found doses for ketamine hydrochloride to range from 5–88 mg/kg. This is a huge disparity in dosing regimens. It should not be surprising, given these ranges of doses, for veterinarians to experience variable outcomes using these compounds. Because these compounds do not provide muscle relaxation or visceral analgesia, the author does not recommend them as general anesthetics. Instead, the author generally utilizes these compounds as pre-anesthetics.
The alpha-2 agonist anesthetics have been used in reptiles with some success. The two primary alpha-2 agonists used in veterinary medicine (in the United States) are xylazine and medetomidine. Xylazine has been found to have limited value. The primary advantages associated with these compounds are that they are reversible and provide muscle relaxation and visceral analgesia. The primary disadvantage associated with these compounds is that they can cause cardiopulmonary depression. The majority of the research evaluating these compounds has been limited to the chelonians; however, the author has evaluated medetomidine in green iguanas and American alligators (Alligator mississipiensis). In green iguanas, doses as high as 1 mg/kg intramuscularly provided no sedation, but did induce a mild bradycardia. In the alligators, 0.15 mg/kg medetomidine intramuscularly provided sedation. Mild bradycardia was also observed in the alligators. In chelonians, medetomidine is primarily used in combination with ketamine to provide surgical anesthesia. Dosing regimens for this combination of drugs vary: ketamine 5–10 mg/kg intramuscularly, medetomidine 0.075–0.15 mg/ kg intramuscularly. These drugs can be combined in the syringe and administered together.
Propofol is a non-barbiturate hypnotic. This drug can be used to provide general anesthesia for reptiles. The primary advantages associated with this drug are that it is associated with rapid recoveries and can be given to effect. The primary disadvantages associated with propofol are that it can cause apnea and must be given intravenously or intraosseously. If there is perivascular leakage of this compound, the induction may be inconsistent. Perivascular leakage of propofol is not associated with any significant irritation. Fortunately, there is no cumulative effect and additional drug can be given. The author has used this drug extensively in the field to perform surgeries when inhalant anesthetic agents were not available, and found it to be efficient and effective.
The inhalant anesthetics remain the preferred method for providing general anesthesia to reptiles. The two most common inhalant anesthetics used in the United States are isoflurane and sevoflurane. These compounds are rapidly metabolized by reptiles, and can be used to provide smooth inductions and recoveries. Preliminary research comparing these two compounds in reptiles generally shows that sevoflurane does have more rapid induction and recovery periods, but the differences may be inapparent (1–3 minutes) to most clinicians. For the most part, it is the difference in price (sevoflurane costing significantly more) that separates the two inhalants. The primary advantages associated with these compounds are that they provide rapid induction and recoveries, and allow the clinician to have direct and immediate control over the delivery of the anesthetic. The primary disadvantages are that they can induce mild cardiopulmonary depression and require attachment to a vaporizer with a source of oxygen.
Reptiles can be induced with inhalant anesthetics via a facemask; however, the author finds that this can be a lengthy proposition for animals capable of breath-holding. In those instances, the author prefers to use any of the previously mentioned parenteral agents as induction agents. The author highly recommends intubating reptiles when delivering an inhalant (or after using any parenteral for that matter). Ensuring that an animal has a patent airway is important, and enables the veterinarian to ventilate the animal during the procedure. Intubating reptiles is straightforward. The glottis is located at the base of the tongue in these animals, and can directly visualized once the mouth is opened. For larger reptiles, pediatric sized endotracheal tubes can be used. For smaller reptiles, intravenous catheters can be used. Crocodilians and chelonians have complete tracheal rings, while snakes and lizards have incomplete tracheal rings. Because of this, it is important not to insufflate a cuff on the endotracheal tubes placed into crocodilians or chelonians.
Our current understanding of measuring and controlling painful stimuli in reptiles is limited. The primary question being asked by most scientists is whether or not reptiles have the cognitive ability to understand pain. Some will argue that when a reptile is injected with a medication, and it responds by retracting its leg, that its response confirms that they sense pain; however, others will argue that the example only confirms that these animals can respond to a noxious stimuli via a spinal reflex, and that it is not a cognitive experience. Reptiles have the basic anatomic and physiologic structures required for pain perception found in higher vertebrates, so the author prefers to err on the conservative side and provide analgesics when appropriate. This, however, also poses a problem in reptiles, as there has been even less study attempting to evaluate the pharmacokinetics of analgesics in reptiles. Suffice it to say that much work is left to be done.
The provision of analgesics has several inherent benefits. First, pain can actually slow healing times. Not controlling pain will increase recovery and treatment times for an animal. Second, pain can increase anesthetic time and provide inconsistent results. A reptile with a contusion on its lateral body wall may take shallow breaths, limiting the amount of inhalant anesthetic delivered to the animal. Finally, analgesics can reduce the amount of anesthesia required for a procedure. This can shorten overall anesthesia times and the risks associated with some anesthetics.
When considering an analgesic plan for a reptile it is important to consider the situation. The author generally separates these events into three categories: local analgesia, surgical analgesia, and post-operative analgesia. A local analgesic may be used to alleviate the discomfort associated with a focal injury or surgical procedure. The author routinely administers lidocaine or bupivacaine prior to removing an abscess or making a coeliotomy incision. These local analgesics are presumed to provide some analgesia after the completion of a procedure. Analgesics delivered pre- or peri-operatively for a surgical procedure may reduce the amount of other anesthetics required for a procedure. Many of the anesthetics described previously (e.g., dissociatives, alpha-2 agonists, propofol, inhalant anesthetics) provide analgesia during a procedure. Other analgesics commonly used during a surgical procedure are the opioids and non-steroidal anti-inflammatories. The opioids and non-steroidal anti-inflammatories are also commonly used in the post-operative period to control pain.
References are available upon request.