ESTIMATION OF RAT-BODY COMPOSITION BY MEANS OF ELECTROMAGNETIC SCANNING IS ALTERED BY DURATION OF ANESTHESIA

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, similar to 226 g) were injected with 66 mg/kg keta mine hydrochloride and 6.6 mg/kg xylazine intramuscular anesthesia. EM S measures were repeated every 4 min, up to 80 min post-anesthesia inj ection. From 4 to 44 min post-injection, the EMS signal and consequent ly 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 i ncreased 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 io n 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 w ere 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 togeth er, these data suggest that standardized EMS protocols should be adopt ed to account for effects of anesthesia, animal position and food depr ivation: When manufacturer's equations are used, body composition shou ld be measured immediately after induction of anesthesia. However, lab oratories that choose to internally generate EMS regression equations with proximate analysis should determine and utilize the period of min imal variability in EMS measures. Such precautions would minimize inte rlaboratory differences in the reporting of EMS body composition measu res in anesthetized animals.