Abstract
Despite the popularity of the inhalant anesthetics, only limited information on their performance in reptiles is available.2,3,7-11 The aim of this study was to determinate the minimum alveolar concentration (MAC) of sevoflurane in two common snake species: Python regius and Python molurus.
Sevoflurane is a potent inhalant anesthetic with a mild odor (which causes minimal breath holding) that has a low solubility and ease of adjustment of anesthetic depth. Its low solubility results in rapid anesthetic induction and recovery. It has bronchodilator activity, doesn’t irritate the airways, is a myorelaxant, and cardiac output decreases with increased concentration of sevoflurane.6 MAC is defined as the concentration of an anesthetic in the alveoli able to induce surgical anesthesia in 50% of the subjects. In this study, an electrical stimulus (4 mA, 12 V, 30 ms, 2 Hz) was delivered to subcutaneous electrodes by an alternating current (AC) electric stimulator (Vygon stimulator, Vygon Group, Padua, Italy).
Fourteen captive-bred snakes, seven Python regius (four males, three females) and seven Python molurus (four males, three females) with known clinical history, were utilized for the study. A blood sample was collected from the ventral tail vein and/or from the heart4 for every snake at the beginning, during, and at the end of the anesthetic period for blood gas analysis.
Anesthesia was induced with 8% sevoflurane in 100% O2 delivered through a facemask. After induction, snakes were endotracheally intubated with uncuffed tubes. The endotracheal tube was then connected to a Bain nonrebreathing circuit and was mechanically ventilated with a ventilator at a rate of six breaths/minute. The snake was kept in dorsal recumbency on an electric heating pad at 30°C. Temperature was monitored using a cloacal thermometer. In addition to blood gases, other measured parameters and methods utilized included: SpO2 using an esophageal pulse oximeter probe (Multiparameter Patient Monitor UT4000f-pro, Goldway Industrial, Smithtown, NY), heart rate using a Doppler apparatus (Ultrasonic Doppler Flow Detector, Model 811b, Parks Medical Electronics, Aloha, OR), cardiac electrical conductivity, end-tidal CO2, and end-tidal anesthetic concentration. Heart rates ranged from 10–16 beats/minute for Python molurus and 20–43 beats/minute for Python regius.
Following intubation, inspired sevoflurane concentration was reduced to 2% and was allowed to equilibrate with end-tidal anesthetic concentration for 20 minutes. Then an electrical supramaximal stimulus was delivered to subcutaneous electrodes on the ventral aspect of the tail for 30 seconds and the animal was observed for purposeful movement. The vaporizer was then decreased by 0.2% and equilibration and stimulation were repeated. This sequence was repeated until electrical stimulation caused observed purposeful movement. The MAC was calculated as the mean of the lowest end-tidal concentrations in which half of the subjects did not respond to the stimulus.
Data were analyzed using an ANOVA analysis and the Mann-Whitney U nonparametric test (p≤0.05: significance level, p≤0.01: high significance level). MAC values of sevoflurane were found to be between 0.9% and 1.3% with a mean and standard deviation of 1.07%±0.21% for Python molurus and were found to be between 1.0% and 1.9% with a mean and standard deviation of 1.31%±0.31% for Python regius.
MAC of sevoflurane in our study is lower than values reported in lizards1 and quite different to that reported in other snakes5 where minimum infundibular concentration (MIC) was studied.
Literature Cited
1. Bertelsen MF, Mosley CAE, Crawshaw GJ, Dyson DH, Smith DA. Anesthetic potency of sevoflurane with and without nitrous oxide in mechanically ventilated Dumeril monitors. JAVMA. 2005;227(4):575–578.
2. Bertelsen MF, Mosley CAE, Crawshaw GJ, Dyson DH, Smith DA. Minimum alveolar concentration of isoflurane in mechanically ventilated Dumeril monitors. JAVMA. 2005;226(7):1098–1101.
3. Hernandez-Divers S, Schumacher J, Read MR, et. al. Comparison of isofluorane and sevoflurane following premedication with butorphanol for induction and maintenance of anesthesia in the green iguana (Iguana iguana). In: Proceedings from the American Association of Zoo Veterinarians Annual Meeting; 2003.
4. Isaza R, Andrews G, Coke R, Hunter R. Assessment of multiple cardiocentesis in ball pythons (Python regius). Contemp Top Lab Anim Sci. 2004;43(6):35–38.
5. Maas A, Brunson DB. Comparison of anesthetic potency and cardiopulmonary effects of isoflurane and sevoflurane in colubrid snakes. In: Proceedings from the American Association of Zoo Veterinarians Annual Meeting; 2002:306-308.
6. Muir W, Hubbell JAE, Skarda RT, Bednarski RM. Manuale di Anestesia Veterinaria (1st Italian edition of Handbook of Veterinary Anesthesia). 2002:179–180.
7. Mosley CA, Dyson D, Smith DA. Minimum alveolar concentration of isoflurane in green iguanas and the effect of butorphanol on minimum alveolar concentration. JAVMA. 2003;222:1559–1564.
8. Mosley CA, Dyson D, Smith DA. The cardiac anesthetic index of isoflurane in green iguanas. JAVMA. 2003;222(11):1565–1568.
9. Mosley CA, Dyson D, Smith DA. The cardiovascular dose-response effects of isoflurane alone and combined with butorphanol in the green iguana (Iguana iguana). Vet Anaesth Analg. 2004;31(1):64–72.
10. Rooney MB, Levine G, Gaynor J, Macdonald E, Wimsatt J. Sevoflurane anesthesia in desert tortoise (Gopheru aghassizii). J Zoo Wildl Med. 1999;30(1):64–69.
11. Schumacher J, Yelen T. Anesthesia and analgesia. In: Mader DR, ed. Reptile Medicine and Surgery. 2nd ed. St. Louis, MO: Saunders Elsevier; 2006:442–452.