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ISSN: 0163-5581 (Print) 1532-7914 (Online) Journal homepage: http://www.tandfonline.com/loi/hnuc20
Nutritional assessment: Whole body impedance and body fluid compartments R. Gregg Settle , Kenneth R. Foster , Benjamin R. Epstein & James L. Mullen To cite this article: R. Gregg Settle , Kenneth R. Foster , Benjamin R. Epstein & James L. Mullen (1980) Nutritional assessment: Whole body impedance and body fluid compartments, , 2:1, 72-80, DOI: 10.1080/01635588009513660 To link to this article: https://doi.org/10.1080/01635588009513660
Published online: 04 Aug 2009.
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Nutritional Assessment: Whole Body Impedance and Body Fluid Compartments by R. Gregg Settle, Kenneth R. Foster, Benjamin R. Epstein and James L. Mullen
Introduction Abstract Measures of total body water and extracellular fluid volume would provide valuable information for the assessment of nutritional status in patients. Time and cost factors make the use of tracer dilution techniques impractical in routine nutritional assessment batteries. Some studies suggest that measurements of the electrical impedance properties of the body can be used to estimate total body water and extracellular fluid volume. The observed and predicted relationships between whole body impedance and body fluid compartment volumes are discussed, and problems which may limit the accuracy of prediction of body fluid compartment volume from whole body impedance measures are discussed. With further improvements, whole body impedance measurement could provide a practical bedside method for measurement of body fluid compartment volumes.
R. Gregg Settle is Staff Scientist, Medical Research Service, Veterans Administration Medical Center, Philadelphia, PA and Research Associate, Department of Surgery, School of Medicine, University of Pennsylvania. Kenneth R. Foster is Assistant Professor and Benjamin R. Epstein is Research Assistant, Department of Bioengineering, University of Pennsylvania, Philadelphia, PA. James L. Mullen is Assistant Professor, Department of Surgery, School of Medicine, University of Pennsylvania; Chief of the University of Pennsylvania Surgical Service, Veterans Administration Medical Center, Philadelphia; and Director, Nutrition Support Service, Hospital of the University of Pennsylvania, Philadelphia, PA. Parts of this paper are based on a presentation given by Dr. Settle at the "Third Clinical Congress," ASPEN, Jan. 31-Feb. 3, 1979.
Total body water (TBW) and extracellular fluid volume (ECFV) change with a person's nutritional state. In undernourished patients, most body components decrease.2'4'293035 However,, as a fraction of body weight, there Is usually an increase in ECFV which often results in an increase in TBW. This relative increase in ECFV and TBW leads to an underestimation of the loss of body solids (particularly, loss of protein) based on body weight measurements in nutritionally deprived patients. In catabolic states, "the body cell mass quickly melts away into a hypotonie ocean of extracellular fluid."20 Many substances have been used as tracers to measure TBW and ECFV by dilution techniques.14 These techniques are expensive and time consuming. They require several hours for equilibration of the tracer before blood or urine samples are collected and analyzed for the tracer substance; many require repeated blood or urine collections over several hours. Some tracer techniques expose the subject to radioactivity, and most require expensive equipment to measure concentrations of the tracer substance. Thus, dilution techniques are generally impractical for routine nutritional assessment. In contrast, whole body impedance (WBI) measures might be used to estimate TBW and ECFV safely, rapidly, and with little sophisticated equipment. The use of electrical impedance measurements to estimate TBW and ECFV is suggested by the variation of the electrical properties of individual tissues with changes in measurement frequency and in water content. Measurement of impedance changes across parts of the body has been used to detect changes in
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blood volume (impedance plethysmography)1-21 as well as in other body fluid compartments.13 The literature on electrical impedance measurements on tissues contains thousands of papers, some written in the 19th century. However, few studies have tried to correlate WBI with ECFV or TBW in order to develop a useful clinical technique for measuring these fluid compartments. Such studies have been only partially successful. We will discuss first the instrumentation required for these measurements and the safety of the measurements for the subject. We will summarize the relevant WBI studies and suggest areas where further development is needed.
Figure 1. A schematic showing two techniques for measuring WBI. The current source passes alternating current of amplitude I through the body, while the voltmeter measures the corresponding voltage V produced by this current. The complex impedance is then the ratio V/l exp (jö), where 0 is the phase angle between the current and voltage, and j =
Measurement of the Whole Body Impedance Whole body impedance is measured by passing a small alternating current I (less than 1 mA) through the body and measuring the voltage V developed across the body by this current (Figure 1). The whole body impedance magnitude is the ratio of the magnitudes of V to I. A complete measurement would also determine the phase shift (0) between the voltage and current; the complex whole body impedance is then V/l exp (jô), where j = V ~ 1 - T . ne concept of impedance as a complex quantity is illustrated in Figure 3b. Although for body tissues the phase shift between V and I is usually small and difficult to measure accurately, it contains information that is potentially useful for interpreting data. The WBI is not an intrinsic property of the body, but rather the data derived from a particular measurement procedure. Thus, the experimental variables (most importantly, electrode placement and measurement frequency) must be specified with the WBI data. The frequency range of interest is from 1 kHz to 1MHz; WBI measurements at the low end of this range apparently reflect ECFV and, at the high end of this range, TBW. One important variable is the electrical impedance of the skin that is in the current path. The skin impedance is wide-ranging, from a few hundred to a million ohms per square centimeter of electrode area, depending on the measurement frequency, skin wetness, and integrity of the stratum corneum.13 In contrast, if the impedance of the skin is bypassed, the whole body impedance is about 500 ohms. Two techniques have been used to bypass the skin impedance (Figure 1), one using two needle electrodes inserted subcutaneously, and the other using four electrodes linearly arranged on the surface of the body. In this second technique, current is passed between the outer pair of electrodes, and the voltage difference between the inner two is measured using a high-input impedance amplifier. Since negligible current is drawn
TWO ELECTRODE TECHNIQUE
FOUR ELECTRODE TECHNIQUE
through the skin by the voltage amplifier, the impedance of the skin, in principle, should not affect the WBI measurement. The two-electrode technique has several serious disadvantages, in addition to the discomfort to the subject. Although the skin impedance is bypassed by the subcutaneous electrodes, the electrode polarization impedance, resulting from electrochemical reactions at the electrode surface, still adds to the total measured impedance. This impedance can be large at low frequencies, and somewhat nonreproducible.11'13'27 Since the subcutaneous electrodes (usually hypodermic syringe needles) are of small diameter, the current density near the electrodes is much larger than in the rest of the body between the electrodes. This means that the total impedance between the electrodes depends upon the electrode size and upon the properties of the tissues near the electrodes, in addition to the total volume of conductive material in the body.1528 For this reason, perhaps, the measurements by Thomasset's group (using the two-electrode technique with
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hypodermic needle electrodes) gave WBI values approximately twice as high as those obtained by Hoffer1516 and in our study (eg, approximately 800 ohms vs about 400 ohms at 100 kHz), employing the four-electrode technique. Hoffer noted that the twoelectrode technique suffered from poor reproducibility, perhaps due to tissue trauma.15 The four-electrode technique largely avoids these problems; the impedance of the skin and the electrode polarization impedance in principle do not affect the WBI measurement, and the choice of electrodes is much less critical. We have found, however, that errors still occur in WBI measurements at low frequencies (
