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Estimation of Rat Body Composition by Means of Electromagnetic Scanning Is Altered by Duration of Anesthesia^1,2,3,4,

Brian W. Tobin^*,,5 and Diane T. Finegood^*

^* Departments of Medicine and Physiology and the Surgical Medical Research Institute, University of Alberta, Edmonton, Alberta, Canada T6G 2S2 Division of Basic Medical Sciences and the Department of Pediatrics, Mercer University School of Medicine, Macon, GA 31207-0003

We determined the effect of anesthesia on estimation of fat-free mass and body fat in rats using electromagnetic scanning (EMS). Male Wistar Furth rats (n = 7, 226 g) were injected with 66 mg/kg ketamine hydrochloride and 6.6 mg/kg xylazine intramuscular anesthesia. EMS measures were repeated every 4 min, up to 80 min post-anesthesia injection. From 4 to 44 min post-injection, the EMS signal and consequently the estimation of fat-free mass decreased from 198 ± 5 to 180 ± 5 g (mean ± SD, P < 0.05). Conversely, the estimation of body fat increased by 63% during this period of anesthesia (12.9 ± 2.9 vs. 21.1 ± 2.6 g/100 g body wt, P < 0.05). In cohort animals (n = 6), body temperature and respiration rate declined following anesthesia (P < 0.05), and may have correspondingly suppressed EMS signal via reduced ion flux and/or muscular activity. In another study the effects of food deprivation, tail position, and anesthesia duration were demonstrated to alter estimation of fat-free mass (P = 0.0001), but these effects were not interactive (P > 0.05). Proximate analysis of body composition in cohort rats indicated that EMS predicted fat-free mass with a 3.5% error when estimated at 4 min post-anesthesia injection. Taken together, these data suggest that standardized EMS protocols should be adopted to account for effects of anesthesia, animal position and food deprivation. When manufacturer's equations are used, body composition should be measured immediately after induction of anesthesia. However, laboratories that choose to internally generate EMS regression equations with proximate analysis should determine and utilize the period of minimal variability in EMS measures. Such precautions would minimize interlaboratory differences in the reporting of EMS body composition measures in anesthetized animals.

KEY WORDS: • electromagnetic scanning • anesthesia • body composition • rats

¹ Presented in part at the 75th Annual Meeting of the Federation of American Societies for Experimental Biology, April 1991, Atlanta, GA [Tobin, B. W. & Finegood, D. T. (1991) Duration of anesthesia alters TOBEC measurement of lean body mass in rodents. FASEB J. 5: A960 (abs.)].

² Funded in part by the Medical Research Council of Canada (ME-11166 and MT-10574). B.W.T. was a postdoctoral fellow of the Canadian Diabetes Association, and D.T.F. is a scholar of the Alberta Heritage Foundation for Medical Research.

³ Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product and does not imply its approval to the exclusion of similar products.

⁴ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁵ To whom correspondence and reprint requests should be addressed, in Georgia.

Manuscript received 24 August 1994. Revision accepted 15 November 1994.