Sources of Variability in the Duration of Anesthesia in Snakes Dan Preston

1. Sources of Variability in the Duration of Anesthesia in Snakes Dan Preston Mentors: Dr. Robert Mason, Department of Zoology Dr. Craig Mosley, College of Veterinary Medicine Oregon State University

2. Why Anesthetize Reptiles? • Anesthesia facilitates invasive procedures such as surgery • Veterinary Medicine • Reptiles are pets • Research • Experiments often involve surgical procedures Ihooq38/flickr.com

3. Challenges Associated with Reptile Anesthesia • Risk of Mortality • Unique Physiology and Anatomy • Difficult Drug Administration • Variability in Anesthetic Effect www.snakegetters.com

4. Anesthetic Agents used on Reptiles • Cold Narcosis, Inhalants and Injectables

5. Brevital Sodium • Barbiturate anesthetic • Work in the brain at GABA receptors; suppress CNS • Widely used in many reptile taxa • Intramuscular or subcutaneous administration • Fast acting; short induction and recovery times • Variable effects across individuals

6. Questions • What factors are responsible for variability between individuals? • How can anesthetic protocols be improved to create more consistent results?

7. Red-sided Garter Snakes (Thamnophis sirtalis parietalis) • Well-studied • Large sample sizes • Easily maintained in captivity

8. Potential Causes of Variability in Duration of Anesthesia • Body temperature -Reptiles are ectotherms; heart rate increases with body temperature -High temperatures increase rates of distribution and metabolism of anesthetics

9. Potential Causes of Variability in Duration of Anesthesia cont. 2. Body condition -Barbiturates distribute from blood to viscera, lean tissue and then accumulate in adipose tissue over time C.R.Friesen

10. Potential Causes of Variability in Duration of Anesthesia cont. 3. Time post-feeding - Snakes have evolved to efficiently digest large meals at infrequent intervals - Upregulate heart, gut and liver after feeding - Brevital sodium distributes to the gut and is metabolized hepatically

11. Methods: Anesthetic Protocol • Anesthetize snakes with 15 mg/kg subcutaneously • Record loss of righting ability, beginning and end of surgical anesthesia, return of righting ability • Measure body temperature every 30 min.

12. Methods: Experimental Designs 1. Body Temperature -20 males, 3 temps. (21C, 26C, 31C) -repeated measures 2. Body Condition -45 females, 3 body condition groups, all at 21 C -60-70 cm SVL, 60-120 g body weight 3. Time Post-feeding -10 males, 10 females, all at 21 C -anesthetized 1, 3, 10 days post-feeding -dosage based on pre-feeding masses

13. Results: Body Temperature One way ANOVA: F2,17 = 12.71 P < 0.001 Post-hoc Tukey: 21 vs 31, P < 0.001 21 vs 26, P = 0.037 26 vs 31, P = 0.042

14. Results: Body Condition One way ANOVA: F2,42 = 5.026 P = 0.011 Post-hoc Tukey: Thin vs Fat, P = 0.008 Thin vs Med., P = 0.300 Med. vs Fat, P = 0.230

15. Results: Time Post-feeding One way repeated measures ANOVA: F2,17 = 3.091 P =0.057

16. Conclusions 1. Body temperature has strong effect • Increased heart rate likely reduces anesthetic duration 2. Body condition has moderate effect • Shortened anesthetic duration with brevital sodium in fatter snakes is probably due to absorption by adipose tissue 3. Time post-feeding did not have a detectable effect • Physiological changes during digestion may not have clinically significant effect on anesthesia • Further studies may clarify whether any effect exists

17. Acknowledgements Thanks to: Dr. Robert Mason Dr. Craig Mosley Dr. Kevin Ahern Rocky Parker Chris Friesen Howard Hughes Medical Institute

18. Pharmacokinetics of Barbiturates Thurmon, Tranquili and Benson. 1999. Essentials of Small Animal Anesthesia and Analgesia.