Application of a new mobile segmental bioimpedance spectroscopy device for tracking fluid shifts during different g-levels

Bioelectric impedance analysis has been shown to be useful for tracking segmental fluid shifts in health and diseases as well as different gravity conditions. However, commercial devices are rather bulky and limited to laboratory settings. Moreover, they typically operate a single frequency (e.g. 50 kHz), which neither allows distinguishing extra- and intracellular spaces nor reflects a constant proportion of fluid spaces between subjects and within subjects if the relation between intra- and extracellular water is altered. In this work we explored the assessment of body fluids shifts by means of a small portable bioimpedance spectroscopy (BIS) prototype designed to minimize weight, size and power consumption, and to monitor different body segments simultaneously for long periods of time. Moreover, this prototype has the capability of differentiating extra- and intracellular fluid space and overcomes the biophysical shortcomings of single-frequency devices. The prototype consists of a digital board with a Digital Signal Processor (DSP) and a custom analog board. The DSP generates the stimulus waveforms, and digitalizes the voltages across three body segments to compute magnitude and phase of body impedance at 10 frequencies between 1 kHz and 770 kHz. The analog board interfaces the DSP with two injecting electrodes and four sensing electrodes that read the voltages across three body segments. The prototype was tested by monitoring BIS in 3 segments of the leg during accelerations between 1G and 3G on a short arm human centrifuge (SAHC) at the German Aerospace Center (DLR). Current injecting electrodes were placed on the metatarsal area of the foot and 5 cm above the patellar area of the knee; sensing electrodes were equispaced in the area below the knee and above the ankle, allowing to estimate BIS in segments S1, S2 and S3, with S1 closest to the ankle and S3 to the knee. Tests were performed on 20 volunteers in two experimental sessions, each of about 4-h duration, with a 1-month break in between. Results demonstrated the capability of our prototype to detect meaningful effects upon bioimpedance by imposing different gravity levels, likely to be secondary to morphological differences of each body segment following a fluid redistribution in the lower limbs. Results also suggest a good reproducibility of the BIS measures. For instance, in a representative subject we measured decreasing values of the impedance modulus at 8 kHz from the ankle to the knee (S1=61 Ω, S2=41 Ω, S3=34 Ω) at baseline. During 1G exposures segmental impedance decreased (S1=59 Ω, S2=40 Ω, S3=32 Ω), with even more pronounced reductions at 3G (S1=56 Ω, S2=38 Ω, S3=29 Ω). After one month, we observed very similar baseline data for all three segments (S1=67 Ω, S2=41 Ω, S3=34 Ω) and analogous impedance changes at 1G (S1=62 Ω, S2=38 Ω, S3=31 Ω) as well as 3G (S1=59 Ω, S2=37 Ω, S3=29 Ω). Our results therefore support the use of DSP-based mobile, multi-segmental BIS devices for studying the physiological adaptations during altered gravity conditions.