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
Why Anesthetize Reptiles?
• Anesthesia facilitates
invasive procedures
such as surgery
• Veterinary Medicine
– Reptiles are pets
• Research
– Experiments often
involve surgical
procedures
Ihooq38/flickr.com
Challenges Associated with
Reptile Anesthesia
• Risk of Mortality
• Unique Physiology and Anatomy
• Difficult Drug Administration
• Variability in Anesthetic Effect
www.snakegetters.com
Anesthetic Agents used
on Reptiles
• Cold Narcosis, Inhalants and Injectables
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
Questions
1. What factors are responsible for
variability between individuals?
2. How can anesthetic protocols be improved
to create more consistent results?
Red-sided Garter Snakes
(Thamnophis sirtalis parietalis)
• Well-studied
• Large sample sizes
• Easily maintained
in captivity
Potential Causes of Variability in
Duration of Anesthesia
1. Body temperature
-Reptiles are ectotherms; heart rate increases with body
temperature
-High temperatures increase rates of distribution and
metabolism of anesthetics
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
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
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.
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
Results: Body Temperature
140
A
Time to Righting Ability (min)
One way ANOVA:
120
F2,17 = 12.71
100
B
80
P < 0.001
C
60
Post-hoc Tukey:
40
21 vs 31, P < 0.001
20
21 vs 26, P = 0.037
0
21 C
26 C
31 C
26 vs 31, P = 0.042
Results: Body Condition
Time to Righting Ability (min)
250
Thin
y = -97.19x + 230.51
R2 = 0.2527
P < 0.001
200
Medium
Fat
One way ANOVA:
150
F2,42 = 5.026
100
50
P = 0.011
0
0.8
1
1.2
1.4
1.6
1.8
2
Mass/SVL
Post-hoc Tukey:
Time to Righting Ability (min)
140
A
120
Thin vs Fat, P = 0.008
A,B
Thin vs Med., P = 0.300
100
B
80
60
40
20
0
Thin
Medium
Fat
Med. vs Fat, P = 0.230
Results: Time Post-feeding
1.35
Relative Body Mass
1.3
1.25
1.2
1.15
1.1
1.05
1
0.95
-2
-1
0
1
2
3
4
5
6
7
8
Days Post-feeding
10
F2,17 = 3.091
90
Time to Righting Ability (min)
9
One way
repeated
measures
ANOVA:
80
P =0.057
70
60
50
40
30
20
10
0
1
3
Days Post-feeding
10
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
Acknowledgements
Thanks to:
Dr. Robert Mason
Dr. Craig Mosley
Dr. Kevin Ahern
Rocky Parker
Chris Friesen
Howard Hughes Medical
Institute
Pharmacokinetics of Barbiturates
100.00
Blood Pool
% Dose
90.00
Viscera
80.00
Lean Tissue
70.00
Fat
60.00
50.00
40.00
30.00
20.00
10.00
0.00
1.00
10.00
100.00
1000.00
10000.00 100000.00
Time (seconds)
Thurmon, Tranquili and Benson. 1999. Essentials of Small Animal Anesthesia and Analgesia.