Post-Anesthetic Hypoxemia In Freshwater Fish
IAAAM 2000
Elizabeth J Chittick

Abstract

In a previous study evaluating the physiologic effects of MS222 and eugenol anesthesia in freshwater fish, hypoxemia was noted in five pacu (Piractus brachypomus) 1 hr after recovery. Our study evaluates this phenomenon in tilapia (Oreochromis niloticus). Tilapia weighing 458-830 g were housed in a North Carolina State University (NCSU) fish production facility and maintained with adequate nutrition and water quality. All holding, anesthetic, and recovery tanks were aerated and supplied with water from the original fish tanks. Six fish were anesthetized with tricaine methanesulfonate, 200 mg/L (MS222, Argent, Redmond, WA), and six with eugenol, 200 mg/L (Sigma, St. Louis, MO) until ventilation ceased, or a 10-min immersion in anesthetic was completed, whichever came first. Blood gases were collected from the caudal vein using lithium heparin Micro A.B.G. syringes (Marquest Medical Products, Englewood, CO) before and during anesthesia, and 1 hr into recovery. Samples were analyzed within 1 min after collection using EG7+ cartridges (i-STAT Corporation, East Windsor, NJ) in an i-STAT portable clinical analyzer (Heska, Waukesha, WI). Dissolved oxygen and temperature of the water were recorded at each sampling using a Y.S.I. dissolved oxygen meter (YSI Incorporated, Yellow Springs, OH). The pH and blood gas values were corrected to body temperature which was assumed to be equal to the ambient water temperature at the time of blood sampling. Given the disparity of water oxygen saturation between sample collections, partial pressure of oxygen in the water (PwO2) was calculated and used to express the ratio of sample venous partial pressure of oxygen (PvO2) to PwO2. This ratio, which we termed the venous blood oxygen index (VBOI), standardized comparisons within individuals and between groups. Statistical analysis included two-way analysis of variance and Tukey's posthoc test for differences between groups. A P value # 0.05 was used to define statistical significance.

Venous blood oxygen index values decreased significantly (P < 0.001) during anesthesia and recovery, regardless of the anesthetic agent used (Table 1). Neither anesthetic group recovered to their pre-anesthetized oxygenated state after experiencing hypoxemia during anesthesia (P < 0.001). Recovery of VBOI toward the pre-anesthesia value tended to be more complete after 1 hr in tilapia anesthetized with MS222 (88.7% of pre-anesthesia value) than in tilapia anesthetized with eugenol (55.8% of pre-anesthesia value), however this difference was not statistically significant. Factors affecting PvO2 at a given FIO2 include minute ventilation, gas exchange efficiency at the respiratory interface, cardiac output, and oxygen extraction supporting tissue metabolism. Prolonged hypoxemia can have detrimental, if not life-threatening, effects on fish. Further research is currently being conducted to evaluate post-anesthetic hypoxemia and its related factors in other freshwater fish species.

Table 1. Venous blood oxygen index (VBOI) responses in tilapia anesthetized with MS222 or eugenol.

Anesthetic Pre-agent

Sampling Period

 

VBOI

% of anesthetic
mean

n

Mean

SEM

Median

MS222 and
Eugenola

Pre-anesthesia

12

0.113

0.012

0.117

--

During anesthesia

12

0.043b

0.004

0.039

38.1

Recovery (1 hr)

12

0.080bc

0.008

0.080

70.8

MS222

Pre-anesthesia

6

0.106

0.014

0.103

--

During anesthesia

6

0.042b

0.003

0.040

39.6

Recovery (1 hr)

6

0.094bc

0.012

0.090

88.7

Eugenol

Pre-anesthesia

6

0.120

0.021

0.124

--

During anesthesia

6

0.043b

0.007

0.037

35.8

Recovery (1 hr)

6

0.067bc

0.006

0.064

55.8

a Either drug, 200 mg/L in freshwater.
b Significantly different from pre-anesthesia value (P < 0.001).
c Significantly different from anesthesia value (P < 0.001).

Acknowledgments

The authors thank Maureen Trogdon, Dennis Delong, Jonathan Bridges, Dr. Anthony Blikslager, Dr. Neil Blair, and Dr. Michael Stoskopf for their assistance with and support for this study.

Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

Elizabeth J. Chittick, DVM
Environmental Medicine Consortium
and Departments of Companion Animal and Special Species Medicine and Microbiology, Pathology, and Parasitology
North Carolina State University, College of Veterinary Medicine
Raleigh, NC, USA


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