The Johnson Space Center (JSC) body-fluids monitor advances the state of the art of measuring hydration levels in humans during spaceflight. Neither bulky nor heavy, this noninvasive instrument is built around a commercial inductance-capacitance-resistance meter, which is used to obtain electrical-impedance-vs.-frequency data for equivalent-circuit/electrical-components analyses. The instrument is expected to prove invaluable not only in space-flight settings but also in such other settings as veterans' hospitals, clinical facilities, and medical research laboratories.
The nearest commercial competitor is an instrument, based on a resistance-capacitance meter, that is used to perform bioelectrical response spectroscopy (BERS) for determining hydration levels. Going beyond the commercial instrument, the JSC body-fluids monitor also rates cerebral and other regional blood flows and cardiac outputs by use of time-based assessments of resistance and/or inductance, noninvasively gauges blood pressure by use of resistance and inductance alone, and estimates the lean and muscle mass of a subject's upper and lower limbs in terms of the values of resistors, capacitors, and/or inductors of equivalent circuits derived from electrical-impedance measurements of the human body. A major feature that distinguishes the JSC body-fluids monitor from the commercial instrument is the capability to obtain and utilize data on inductances in addition to resistances and capacitances. Moreover, unlike the commercial instrument, the JSC body-fluids monitor can discriminate between vascular and extravascular water.
Some other methods of measuring hydration levels involve the dilution of tracer substances in the subject's body: In the total-body-water (TBW) method, one uses deuterium-labeled water; in the extracellular-fluid-volume (ECF) method, sodium bromide; and the plasma-volume (PV) method, radioactive iodine attached to albumin. In another method, total body volume is calculated from hematocrit and PV, and bioelectric-impedance analysis (BIA) [which involves measurement of impedance at a single frequency] is used to estimate TBW and ECF. Many of these methods are invasive, and most are time-consuming (a typical measurement takes between 1 and 6 hours), and cannot be repeated until the diluted tracer substances leave the body. These methods also cannot provide information during on-orbit operations. Worse, BIA produces measurement errors greater than those of dilution techniques. Moreover, the performance of the resistance-capacitance-meter-based BERS commercial instrument mentioned above has been challenged because its design makes no provision for modeling inductances; this is critical because inductors are key to the modeling of total blood volume (TBV) and PV.
In 1928, K. Cole originated the concept of using alternating-current measurements to characterize dielectrics dispersed in a conducting medium. In 1941 K. Cole and R. Cole refined this concept into the Cole-Cole principle, which has been applied to biological problems since the early 1960s. For the purpose of Cole-Cole analysis, the human body is treated as both a cylinder and an insulator (cell membranes) surrounding small conductors (the intracellular fluid) embedded in a larger conductor (extracellular water).
Therefore, the human body is treated as a parallel circuit, one limb of which is a purely resistive element that represents extracellular fluid. The other limb, representative of the intracellular component, consists of a resistance and capacitance in series. The commercial instrument mentioned above, operating over the frequency spectrum from 5 Hz to 1 MHz, generates BERS data that are used to compute the resistances and the capacitance.
The JSC body-fluids monitor operates over the frequency spectrum from 5 to 300 kHz. It functions similarly to the commercial instrument except that, as noted above, its design and method of operation account for inductance in addition to resistance and capacitance. More specifically, an inductor is added to the resistance limb; thus, the equivalent-circuit model used to analyze the BERS data is one limb (consisting of a resistor in series with a capacitor) in parallel with another limb (consisting of a resistor in series with an inductor). The inductor represents the inductance of the blood vasculature.
The JSC body-fluids monitor differentiates between vascular and extravascular water even as it noninvasively provides data for an assessment of a subject's TBW, ECF, TBV, PV, and changes in TBV and PV (ΔTBV and ΔPV). These assessments can be performed quickly and safely, and can be repeated frequently. The JSC body-fluids monitor could also be used to estimate the percentage of body fat, given the assumption that the lean tissues of the body are normally hydrated.
For operation of the JSC body-fluids monitor, a subject is instrumented with four standard electrocardiogram electrodes. Two electrodes are placed on the hand (wrist and knuckles), and two are placed on the foot (ankle and base of toes). A small electric current (below the human ability to feel) is introduced, and the magnitude and phase angle of impedance are recorded as the frequency is varied from 5 to 300 kHz. Electrical-component analysis of the measurement data produces three values of resistance (representing TBW, ECF, and intercellular water content), self and mutual inductance, and capacitance. These electrical-component values, along with the height and weight of the subject, are inserted in computational models developed to assess TBW, ECF, TBV, PV, ΔTBV, and Delta;PV. Thus, the JSC body-fluids monitor and the associated data-analysis method yield a greater assortment of data than does any similar instrument/method combination heretofore used to measure a subject's hydration levels.
This work was done by Steven F. Siconolfi of Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Test and Measurement category.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Johnson Space Center
Refer to MSC-22491.