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.

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.