Evaluation of Isoflurane, Sevoflurane and Nitrous Oxide Anesthesia in Dumeril’s Monitor (Varanus dumerili)
Abstract
Isoflurane is the most commonly used inhalant anesthetic in reptiles,3 but sevoflurane has shown promise as a rapid and safe alternative4,7. Despite their popularity only limited information on their performance in reptiles is available.
The induction and maintenance of anesthesia in 10 Dumeril’s monitor (Varanus dumerili) with isoflurane (I), sevoflurane (S), and sevoflurane with 66% nitrous oxide (N2O), and the effects of the inspired oxygen (O2) concentration on anesthetic induction were investigated in a randomized prospective study.
During anesthetic induction with the desired gas mixture delivered through a facemask, the tone and reactivity of the tail, hind limbs, front limbs, and neck were evaluated every 60 seconds, as was the righting reflex. The tone was judged subjectively on a scale from 0–3 with three being full reaction and strength upon touch and zero being no tone or reaction. The righting reflex was evaluated by placing the animal on its back and observing the reaction. No reaction was scored as zero and full ability to flip itself over was scored a three. Induction was considered complete when all parameters evaluated scored zero. The overall sequence of complete muscle relaxation was consistent: The front limbs always lost tone first, followed by the hind limbs and the neck nearly simultaneously; then the righting reflex would be lost, and finally the tail tone. The pattern was not significantly different among the four treatments.
The mean time to induction for I in 100% O2, S in 100% O2, S in 21% O2:79% nitrogen, and S in 66% N2O:34% O2 was 13.0±4.55 minutes, 11.2±3.77 minutes, 10.4±2.5 minutes, and 9.4±2.8 minutes respectively at 26°C. Mask induction with sevoflurane was faster than with isoflurane. There was no significant difference between the induction time for sevoflurane in 100% O2 and in room air, but sevoflurane combined with N2O resulted in significantly faster inductions than sevoflurane alone.
The minimum alveolar concentrations (MAC) of I, S and S in 66% N2O:34% O2 were determined by a bracketing technique.6 Anesthesia was induced with the desired gas mixture delivered through a facemask. Animals were then endotracheally intubated end-tidal and inspired anesthetic concentration was continuously measured. Animals were mechanically ventilated with a ventilator set to deliver a tidal volume of 25 ml/kg at a rate of 4 breaths/minute. After equilibration at an end-tidal-to-inspired agent concentration ratio of >0.9 for 20 minutes an electrical stimulus of 50 Hz, 50 V was delivered to subcutaneous electrodes on the ventral aspect of the tail at 6.5 msec pulses with 6.5 msec intervals for 1 minute or until purposeful movement was observed. The vaporizer setting was then decreased to affect a 10% decrease in end-tidal agent concentration, and equilibration and stimulation were repeated. The MAC was calculated as the mean of the lowest end-tidal concentration that prevented a positive response and the highest concentration that did not. A blood sample for blood gas analysis was collected from the tail vein at the beginning and end of the anesthetic period.
MAC ± SD of I and S were 1.54±0.17% and 2.51±0.46% respectively at 32°C. A significant reduction (26.4±11.4%) in sevoflurane requirement was found when delivered with 66% N2O:34% O2, and the MAC of N2O was estimated to be 244%.
Mean heart rate at the upper and lower MAC bracket was 32.4±3.1, 30.7±4.5 and 34.4±4.9 and 34±4.5, 34.5±5.2, 36±3.6 beats/minute, respectively during anesthesia with I, S and S in 66% N2O:34% O2. Over the course of the experiment, there was a significant decrease in PaCO2 a significant increase in blood pH and HCO3. For example, for I PaCO2 decreased from to 27.9 mm Hg and blood pH and HCO3 increased from 7.33–7.64 and from 25.3–32.9 mmol/L, respectively.
The pattern of complete muscle relaxation described confirms observations made in several species of lizards induced with halothane2 and in turtles anesthetized with ether1. This pattern may be consistent regardless of the inhalation anesthetic agent used and perhaps even among species.
The MAC of isoflurane in Dumeril’s monitors was similar to that reported in mammals6 but lower than values reported in other reptiles,4,5 possibly reflecting the more mammalian cardiovascular physiology of monitor lizards.
In Dumeril’s monitor anesthetic induction with both isoflurane and sevoflurane was feasible and safe. Sevoflurane offers a slightly faster anesthetic induction than isoflurane, and the use of nitrous oxide at a 2:1 ratio with oxygen further reduces induction time. In both cases, however, it is questionable whether the limited reductions in induction time alone warrants the cost of new vaporizers and additional equipment.
Acknowledgments
The work was supported by the Toronto Zoological Society and the Ontario Veterinary College Pet Trust. We thank Abbott Canada for supplying anesthetics and vaporizers and are grateful to S. Lee for invaluable technical assistance, to W. Sears and S.S. Nielsen for statistical advice, and to the staff at the Toronto Zoo and the Central Animal Facility, University of Guelph for expert care for the animals.
Some of these results were presented at the Meeting of the European Association of Zoo and Wildlife Veterinarians in Ebeltoft, Denmark, May 2004.
Literature Cited
1. Bello, A.A. and A. Bello-Klein. 1991. A technique to anesthetize turtles with ether. Physiol Behav. 50:847–848.
2. Bonath, K. and C. Zschege. 1979. Experimentelle Untersuchungen zur klinischen Anwendung und Überwachung der Inhalationsnarkose bei Reptilien. Zentralblatt Für Veterinärmedizin [A]. 26:341–372.
3. Heard, D.J. 2001. Reptile anesthesia. Vet Clin North Am: Exotic Animal Practice. 4:83–117.
4. Maas, A. and D. Brunson. 2002. Comparison of anesthetic potency and cardiopulmonary effects of isoflurane and sevoflurane in colubrid snakes. In: AAZV Annual Proc. 2002;306–308.
5. Mosley, C.A.E., D. Dyson, and D.A. Smith. 2003. Minimum alveolar concentration of isoflurane in green iguanas and the effect of butorphanol on minimum alveolar concentration. J Am Vet Med Assoc. 222:1559–1564.
6. Quasha, A.L., E.I. Eger II, and J.H. Tinker. 1980. Determination and applications of MAC. Anesthesiology. 53:315–334.
7. Rooney, M.B., G. Levine, J. Gaynor, E. Macdonald, and J. Wimsat. 1999. Sevoflurane anesthesia in desert tortoises (Gopherus agassizii). J Zoo Wildl Med. 30:64–69.