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, approximately 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.(ABSTRACT TRUNCATED AT 250 WORDS)