sources of variability in the duration of anesthesia in snakes dan preston mentors: dr. robert...
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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 proceduresIhooq38/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
C
B
A
0
20
40
60
80
100
120
140
21 C 26 C 31 C
Tim
e to
Rig
hti
ng
Ab
ility
(m
in)
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
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
A
A,B
B
0
20
40
60
80
100
120
140
Thin Medium Fat
Tim
e t
o R
igh
tin
g A
bilit
y (
min
)
y = -97.19x + 230.51
R2 = 0.2527P < 0.001
0
50
100
150
200
250
0.8 1 1.2 1.4 1.6 1.8 2
Mass/SVL
Tim
e t
o R
igh
tin
g A
bili
ty (
min
)
Thin
Medium
Fat
Results: Time Post-feeding
0.95
1
1.05
1.1
1.15
1.2
1.25
1.3
1.35
-2 -1 0 1 2 3 4 5 6 7 8 9 10
Days Post-feeding
Re
lati
ve
Bo
dy
Ma
ss
0
10
20
30
40
50
60
70
80
90
1 3 10
Days Post-feeding
Tim
e to
Rig
hti
ng
Ab
ilit
y (m
in)
One way repeated measures ANOVA:
F2,17 = 3.091
P =0.057
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
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
1.00 10.00 100.00 1000.00 10000.00 100000.00
Time (seconds)
% D
os
e
Blood Pool
Viscera
Lean Tissue
Fat
Thurmon, Tranquili and Benson. 1999. Essentials of Small Animal Anesthesia and Analgesia.
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