A bridge from bioimpedance spectroscopy to 50 kHz bioimpedance analysis: application to total body water measurements.

This paper presents a method for extrapolating the total body water (TBW) resistance R(t50) from the resistance measured at 50 kHz (R(50)). A DXA examination and impedance measurements were carried out in a 1st group of 57 healthy volunteers with a Xitron 4200 multifrequency impedancemeter, in order to determine their values of R(t50) by comparison with resistances extrapolated at an infinite frequency by the Xitron (R(infinity)). TBW volumes were calculated using our modified BIS method (Jaffrin et al 2006 Med. Biol. Eng. Comput. 44 873-82) from R(infinity), R(t50) and from the fat-free mass measured by DXA, assuming a hydration rate of 73.2%. The same protocol and calculations were also carried out on a 2nd group of 21 subjects for independent validation. Data of the 1st group showed that values of R(t50), not significantly different from those of R(infinity), could be obtained by dividing R(50) by 1.231 in men and by 1.224 in women. Applying this method to the 2nd group yielded also values of R(t50) not significantly different from R(infinity). TBW volumes V(t50) obtained from R(t50) were not significantly different from those of our modified BIS method V(tn), or from TBW volumes obtained from DXA in both groups. A comparison with three BIA methods of TBW determination showed that our new method gave results in better agreement with TBW from DXA and from our modified BIS method.