Anesthetic Depth in Exotic Animals: Monitoring the Degree of Central Nervous System Depression

Key Points

  • Anesthetic depth is the degree to which the central nervous system is depressed by a general anesthetic agent.
  • Exercise caution when evaluating anesthetic depth in exotic animal patients, particularly when monitoring a novel species, as response to anesthetic agents can defy generalization in any given individual, particularly when anesthesia is achieved using a combination of drugs.
  • As a general rule of thumb, as anesthetic depth increases, muscle tone decreases and the respiratory pattern may become regular and even.
  • Respiratory rate and depth are important guides to anesthetic depth in birds and mammals, but respiration is a poor indicator in reptiles since apnea is often observed at a surgical plane of anesthesia.
  • As anesthetic depth increases, reflexes become attenuated. At a surgical plane of anesthesia, pedal withdrawal reflexes are often absent and the corneal reflex is present but sluggish in those patients possessing eyelids.
  • Although the palpebral reflex is often lost at a surgical plane of anesthesia in reptiles possessing eyelids and birds, this reflex may not be lost until dangerously deep levels of anesthesia in many mammals.
  • Excessive anesthetic depth is associated with severe, life-threatening cardiovascular depression manifested by bradycardia, hypotension, and hypercapnia. This stage can also be recognized by the loss of all reflexes, including corneal and palpebral. Death will ensue if the patient’s status is left unchecked.
  • The anesthetist must remain vigilant for any changes in patient status during the perianesthetic period. Changes in anesthetic depth can occur particularly quickly in birds.
  • This article is part of a RACE-approved Anesthetic Monitoring teaching module. Visit the articles on physiologic monitoring of vital signs, blood pressure, capnometry, pulse oximetry, and electrocardiography for additional information in exotic animal patients.
  • The content presented here is also available as an approximately 23-minute slideshow recording. More detailed information is available in the article version of this content, however it is not necessary to read the article to complete the teaching module quiz.

A dedicated anesthetist should be assigned to monitor every patient during the perianesthetic period. The anesthetist is fundamental to patient safety because she assures the patient is not aware, not moving, and not in pain, all while maintaining stable anesthetic depth. A deep plane of anesthesia can lead to hypoventilation and hypoxemia, reduced cardiac output, hypotension, inadequate tissue perfusion, central nervous system (CNS) depression, and prolonged recovery. This review article first explores the stages of anesthesia and then discusses assessment of anesthetic depth in exotic companion mammals, birds, and reptiles . . .

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1. Bello AA, Bello-Klein A. A technique to anesthetize turtles with ether. Physiol Behav 1991;50:847-848. DOI: 10.1016/0031-9384(91)90028-m..

2. Bertelsen MF. Squamates (snakes and lizards). In: West G, Heard D, Caulkett N (eds). Zoo Animal and Wildlife Immobilization and Anesthesia, 2nd ed. Ames, IA: Wiley Blackwell; 2015:654, 658.

3. Bertelsen MF, Mosley CA, Crawshaw GJ, et al. Inhalation anesthesia in Dumeril’s monitor (Varanus dumerili) with isoflurane, sevoflurane, and nitrous oxide: effects of inspired gases on induction and recovery. J Zoo Wildl Med 2005;36:62-68. DOI: 10.1638/04-033.

4. Bonath K. Halothane inhalation anaesthesia in reptiles and its clinical control. International Zoo Yearbook 1979;19:112-125.

5. Borkowski GL, Danneman PJ, Russell GB, Lang CM. An evaluation of three intravenous anesthetic regimens in New Zealand rabbits. Lab Anim sci 40(3):270-276,1990.

6. Colville T. Monitoring anesthetized animals – how general anesthesia affects normal anatomy and physiology. Proc Annu Conf Atlantic Coast Vet Conf 2011.

7. Fish RE, Brown MJ, Danneman PJ, Karas AZ. Anesthesia and Analgesia in Laboratory Animals, 2nd ed. ACLAM, Academic press: London; 2008.

8. Flecknell P. Laboratory Animal Anaesthesia, 4th ed. Boston: Elsevier; 2015: 103-105.

9. Girling SJ. Veterinary Nursing of Exotic Pets. Ames, Iowa: Blackwell Publishing; 2003.

10. Gunkel C. Training day: avian and reptile anaesthesia and analgesia. AVA/ECVAA Meeting Leipzig 2007. Available at

11. Harkness JE, Turner PV, Van de Woude S, Wheler CL. Harkness and Wagner’s Biology and Medicine of Rabbits and Rodents, 5th ed. Ames, IA: Wiley-Blackwell; 2010.

12. Haskins SC. Monitoring anesthetized patients. In: Grimm KA, Lamont LA, Tranquilli WJ et al (eds). Veterinary Anesthesia and Analgesia: The Fifth Edition of Lumb and Jones. Ames, Iowa: Wiley Blackwell; 2015: 86-88.

13. Hawkins MG, Pascoe PJ. Anesthesia, analgesia, and sedation of small mammals. In: Quesenberry KE, Carpenter JW, eds. Ferrets, Rabbits, and Rodents Clinical Medicine and Surgery, 3rd ed. St. Louis, MO: Elsevier; 2012: 442.

14. Hawkins MG, Wright BD, Pascoe PJ, et al. Pharmacokinetics and anesthetic and cardiopulmonary effects of propofol in red-tailed hawks (Buteo jamaicensis) and great horned owls (Bubo virginianus). Am J Vet Res 64(6):677-683, 2003. DOI: 10.2460/ajvr.2003.64.677.

15. Hawkins MG, Zehnder AM, Pascoe PJ. Cagebirds. In: West G, Heard D, Caulkett N (eds). Zoo Animal and Wildlife Immobilization and Anesthesia, 2nd ed. Ames, IA: Wiley Blackwell; 2015:770-772.

16. Heard D. Anesthesia. In: Speer BL (ed). Current Therapy in Avian Medicine and Surgery. St. Louis: Elsevier; 2016: 611.

17. Heard D. Anesthesia and analgesia. In: Avian Medicine and Surgery. 807-828, 1997

18. Joubert K. Monitoring. Proc Annu Conf World Small Animal Vet Assoc World Congress 2014.

19. Knotek Z, Ermáková E, Johnson R, et al. Anaesthesia and analgesia in tortoises, terrapins and turtles. Proc iCARE 2017: 146

20. Lierz M, Korbel R. Anesthesia and analgesia in birds. J Exotic Pet Med 21(1):47, 2012.

21. Longley L. Saunders Solutions in Veterinary Practice: Small Animal Exotic Pet Medicine. Philadelphia, PA: Saunders Elsevier; 2010: 223.

22. Mans C, Sladky KK, Schumacher J. General anesthesia. In: Mader’s Reptile and Amphibian Medicine and Surgery. St. Louis, MO: Elsevier; 2019.

23. McArthur S. Anaesthesia, analgesia and euthanasia. In: Medicine and Surgery of Tortoises and Turtles. McArthur S, Wilkinson R, Meyer J (eds). Ames, IA: Blackwell Publishing; 2004:384-386.

24. Mosley CA. Anesthesia and analgesia in reptiles. Semin Avian Exot Pet Med. 2005;14:243-262.

25. Muir WW, Hubbell JAE, Bednarski FRM, Lerche P. Handbook of Veterinary Anesthesia, 5th ed. St. Louis, MO: Elsevier Mosby; 2013.

26. Olsson A, Simpson M. Analgesia and anaesthesia. In: Doneley B, Monks D, Johnson R, Carmel B (eds). Reptile Medicine and Surgery in Clinical Practice. Hoboken, NJ: Wiley Blackwell; 2018: 379

27. Ozeki L, Caulkett N. Monitoring. In: West G, Heard D, Caulkett N (eds). Zoo Animal and Wildlife Immobilization and Anesthesia, 2nd ed. Ames, IA: Wiley Blackwell; 2015: 149-150, 161-164.

28. Rani DD, Harsoor SS. Depth of general anaesthesia monitors. Indian J Anaesth 56(5):437-441, 2012. DOI: 10.4103/0019-5049.103956.

29. Schumacher J, Yelen T. Anesthesia and analgesia. In: Mader DR (ed). Reptile Medicine and Surgery, 2nd ed. St. Louis, MO: Saunders Elsevier; 2006: 448-449.

30. Silva A, Campos S, Monteiro J, et al. Performance of anesthetic depth indexes in rabbits under propofol anesthesia: prediction probabilities and concentration-effect relations. Anesthesiology 115(2):303-314, 2011. DOI: 10.1097/ALN.0b013e318222ac02 .

31. Sladky KK, Mans C. Clinical anesthesia in reptiles. J Exot Pet Med. 2012;21:17-32.

32. Vigani A. Chelonians (Tortoises, turtles, and terrapins). In: West G, Heard D, Caulkett N (eds). Zoo Animal and Wildlife Immobilization and Anesthesia, 2nd ed. Ames, IA: Wiley Blackwell; 2015: 701.

33. Wenger S. Anesthesia and analgesia in rabbits and rodents. J Exot Pet Med 2012;21(1):7-16.

34. West G, Heard D, Caulkett N (eds). Zoo Animal and Wildlife Immobilization and Anesthesia, 2nd ed. Ames, IA: Wiley Blackwell; 2015: 659, 701.

Further reading

Diven K. Inhalation anesthetics in rodents. Lab Anim (NY) 32(3): 44–47, 2003. DOI: 10.1038/laban0303-44..

Imai A, Steffey EP, Farver TB, Ilkiw JE. Assessment of isoflurane-induced anesthesia in ferrets and rats. Am J Vet Res 60(12):1577-1583, 1999.

Imai A, Steffey EP, Ilkiw JE. Comparison of clinical signs and hemodynamic variables used to monitor rabbits during halothane- and isoflurane-induced anesthesia. Am J Vet Res 60(10):1189-1195, 1999.

Korbel R. Vergleichende Untersuchungen zur Inhalationsanästhesie mit Isofluran (Forene) und Sevofluran (Sevorane) bei Haustauben (Columba livia Gmel., 1789, var. dom.) und Vorstellung eines Referenz-Narkoseschemas für Vögel. Tierärztl Prax 26:71-83, 1998.

Read MR. Evaluation of the use of anesthesia and analgesia in reptiles. J Am Vet Med Assoc 224(4):547-552, 2004. DOI: 10.2460/javma.2004.224.547..

Sandmeier P. Evaluation of medetomidine for short-term immobilization of domestic pigeons (Columba livia) and Amazon parrots (Amazona species). J Avian Med Surg 14(1):8-14, 2000.

Schumacher J, Mans C. Anesthesia. In: Mader DR (ed). Current Therapy in Reptile Medicine and Surgery, 3rd ed. St. Louis: Elsevier; 2014: 308.

To cite this page:

Pollock CG, Nugent-Deal J. Anesthetic depth in exotic animals: monitoring the degree of central nervous system depression. Aug 29, 2020. LafeberVet Web site. Available at