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.