Volume Changes Effect Electrical Impedance Measurement Of Body Composition

P. M. Zabetakis, G. W. Gleim, K. E. Vitting*, M. H. Gardenswartz*, M. Agrawal*, M. F. Michelis*, and J. A. Nicholas.

Institute of Sports Medicine and Nephrology Section, Lenox Hill Hospital, New York, NY, 10021

Tetrapolar bioelectrical impendance TBI) is used as a measurement of body composition. To assess the effect of acute volume changes on this measurement, we performed TBI immediately before and after hemodialysis in 16 patients. Blood samples for electrolytes, and plasma renin activity (PRA) were obtained prior to and after dialysis. Based on our previous experience (J Lab Clin Med 96: 734, 1980) patients were classified as "dry" (without excess fluid) if initial PRA was > 1 ng/ml/hr and increased with dialysis (n=9) or "wet" (having excess fluid) if PRA did not meet both criteria (n=7). Resistance (RS) changed significantly more in dry than in wet patients (82 vs 45 ohms, p=.009) while the change in reactance (Xc) showed a similar trend (13 vs 5 ohms, p=.106). Using both the change in RS and Xc in a discriminant analysis, 14 of 15 patients (p=.016) could be classified correctly as to their fluid status. We conclude that acute changes in volume effect TBI measurements and that the magnitude of this effect is dependent on the initial volume status. Thus, when assessing alterations in body composition, the effect of baseline volume status on TBI should be considered.

Med. Sci. Sports Exerc., Vol 19:s40, 1987.