Abstract

The field of reptile medicine and surgery has expanded considerably over the past decade. In addition to a more thorough understanding of reptile pathophysiology, a variety of surgical procedures, including microsurgical techniques are now routinely performed. In order to successfully anesthetize a reptile, a thorough understanding and knowledge of their unique anatomy and physiology is essential. It is also necessary to be familiar with the cardiopulmonary and anesthetic effects of the anesthetic agent(s) being used. The anesthetic effects of drugs used for the immobilization of reptiles show species and individual differences. Additional factors that may influence the response to anesthetic agents include season, age, reproductive status, wild vs. captive animals and health status of the animal.

In many cases, especially when handling large or potentially dangerous species, or in order to minimize stress from handling or to facilitate sample collection, it is essential to restrain and anesthetize reptiles for a variety of diagnostic procedures. In order to reduce mortality and morbidity it is necessary to select a safe and appropriate anesthetic regimen for the species and for the procedure being performed. The veterinarian should select drugs or drug combinations he or she is familiar with and apply his or her knowledge directly to the reptilian patient. Although the response to and the effects of certain drugs may be very different from those seen in domestic mammals, the standards and principles employed in domestic animal anesthesia should be directly applied to the reptilian patient. Several reviews of reptile anesthesia have been published recently.^1,3,5

Anesthetic Agents

A variety of different agents have been used in reptile anesthesia with variable success. Many injectable agents are associated with prolonged induction and recovery times as well as profound respiratory depression. For this reason, opioid agents and barbiturates are rarely used in reptile anesthesia and most anesthetic regimen are based on the administration of the dissociative agent ketamine HCl combined with other drugs such as the benzodiazepines (e.g., diazepam and midazolam) or opioid agents (e.g., butorphanol and buprenorphine). Ketamine alone is associated with poor muscle relaxation and poor analgesia and if used at high dosages sometimes prolonged recoveries. Ketamine has been used in all orders of reptiles and is most useful for sedation or induction of anesthesia to facilitate endotracheal intubation. Maintenance of anesthesia with an inhalational agent such as isoflurane is recommended. The administration of the dissociative agent tiletamine combined with zolazepam (Telazol) will improve muscle relaxation when compared with ketamine alone but recovery times may be prolonged. At lower dosages (2–6 mg/kg i.m.) it is adequate for minor diagnostic procedures or for tracheal intubation.

Recently, ultra-short acting induction agents and reversible agents have been investigated for their use in reptile anesthesia. While published data on their cardiopulmonary effects are limited at present, these drugs have the potential to improve anesthetic management of the reptilian patient. Propofol, a non-barbiturate short acting induction agent is commonly used in human and veterinary anesthesia. The main characteristics of this drug are rapid induction and recovery times and minimal cumulative effects, even after prolonged administration. The pharmacokinetics of propofol do not appear altered in patients with renal disease, making it especially useful in reptile patients, commonly diagnosed with impaired renal function. The cardiopulmonary depressant effects of propofol are comparable to those of the barbiturates but they are more transient. A major disadvantage of propofol is the need for i.v. administration making it unsuitable in very small or potentially dangerous reptiles. A recent study investigated the cardiopulmonary effects of propofol administered intraosseously in green iguanas for induction and maintenance of anesthesia.² At the dosages used in this study (5 mg/kg and 10 mg/kg i.o.) administration of propofol resulted in rapid inductions but was associated with marked respiratory depression and endotracheal intubation and assisted ventilation and supplemental administration of oxygen was recommended. Further studies are needed to investigate the cardiopulmonary effects of propofol in other reptile species and determine effective and safe dosages.

Reversible agents such as the α2-agonist medetomidine are currently investigated for their potential use in reptile anesthesia. Medetomidine offers the advantage over other agents that it is reversible with the specific antagonist atipamezole. In addition, when combined with other anesthetic agents such as ketamine the dose of ketamine can be reduced, therefor resulting in faster recovery times. However, medetomidine has been associated with pronounced cardiopulmonary depressant effects in dogs and cats, including bradycardia and respiratory depression. A study in leopard and yellow foot tortoises showed medetomidine (100 µg/kg i.v.) combined with ketamine (5 mg/kg i.v.) produced rapid induction times and reversal with atipamezole (400 µg/kg i.v.) was complete within 30 min.⁴ In the same study Aldabra tortoises were immobilized with medetomidine (25–80 µg/kg i.v.) and ketamine (3–8 mg/kg i.v.) showing similar results. The above dosages were adequate to allow minor procedures and facilitate endotracheal intubation. All tortoises showed a decrease in heart rates when compared to baseline values. Studies investigating more reptile species and determining effective dosages are needed.

Although some of the newer drugs warrant further investigation, at present isoflurane is the anesthetic agent of choice for prolonged procedures requiring a surgical plane of anesthesia. Recovery times are rapid and isoflurane offers the advantage over other inhalational agents that it has limited organ toxicity and limited effects on renal and hepatic function.

Monitoring

Monitoring depth of anesthesia and cardiopulmonary performance is more difficult in reptiles than in mammals and avian species. Some reflexes are difficult or impossible to assess during anesthesia and depth of anesthesia is often determined by the response of the patient to the surgical procedure. Monitoring of cardiopulmonary performance is often difficult due to the size of the patient and the lack of proper monitoring devices or the inaccessibility of suitable vessels for blood pressure measurements or blood gas analysis. In most reptiles, the use of a Doppler flow device allows monitoring of heart rate and rhythm. The device may be placed either directly over the heart or a suitable artery. Monitoring direct arterial blood pressures by placement of an arterial catheter always requires a cut-down procedure and is impractical for most species of reptiles as a routine monitoring tool. The use of pulse oximetry for monitoring relative arterial oxygen saturation is a standard monitoring practice in human and veterinary anesthesia. Recently, different pulse oximeter probes have been made available which allow their use in even small patients. However, the use of pulse oximeters is limited in reptile anesthesia. Pulse oximeters are calibrated for human patients and calculate arterial oxygen saturation based on the human oxygen hemoglobin dissociation curve. Thus, when using pulse oximeters in reptile patients, it must be considered that the saturation values are not absolute numbers. Pulse oximetry is useful in monitoring trends of arterial oxygen saturation.

Arterial blood gas analysis is impractical in most reptiles, especially in small species due to the difficulty of arterial access. Obtaining a blood sample via cardiocentesis and determining blood gas values is of limited value since the obtained sample is in most cases a mixed arterial/venous sample. In larger reptile species measurement of end-tidal CO₂ concentrations is useful for evaluating respiratory performance. In smaller specimen dead-space within the apparatus will compromise results.

Since reptiles are poikilothermic animals, careful monitoring of ambient temperature and initiation of corrective measures, if necessary, is important to assure normal metabolism and excretion of anesthetic agents in reptiles.

Further work is necessary to investigate the cardiopulmonary effects and safety of the newer anesthetic agents in reptile species. Monitoring devices need to be calibrated and validated for their use in reptile anesthesia.

Literature Cited

1.  Bennett RA. 1991. A review of anesthesia and chemical restraint in reptiles. J Zoo Wildl Med. 22(3):282–303.

2.  Bennett RA, J Schumacher, K Hedjazi-Haring, S Newell. 1998. Cardiopulmonary and anesthetic effects of propofol administered intraosseously to green iguanas, Iguana iguana. JAVMA. 212(1):93–98.

3.  Heard DJ. 1993. Principles and techniques of anesthesia and analgesia for exotic practice. In: The Veterinary Clinics of North America, Exotic Pet Medicine I. Quesenberry KE, EV Hillyer, eds. W.B. Saunders Company, Philadelphia, PA, Pp. 1301–1327.

4.  Lock BA, DJ Heard, P Dennis. 1998. Preliminary evaluation of medetomidine/ketamine combinations for immobilization and reversal with atipamezole in three tortoise species. Bulletin Assoc. of Reptilian and Amphibian Veterinarians. 8 (4):6–9.

5.  Schumacher J. 1996. Reptiles and Amphibians. In: Lumb & Jones’ Veterinary Anesthesia. Thurmon JC, WJ Tranquilli, GJ Benson, eds. Williams & Wilkins, Baltimore, Maryland, Pp. 670–685.