Fluid Status/Body Composition
BIS is a safe, rapid, noninvasive technique for assessing multiple fluid compartments and body composition simultaneously without laborious dilution techniques or underwater weighing.
Note: This device is intended to be used on healthy individuals
Van Loan et al., 1993 1.
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Bioimpedance analysis (BIA) has become a widely accepted method in healthy populations, for the determination of body composition due to its simplicity, speed and noninvasive nature. While BIA is recognized as one of the more reliable methods, not all devices or methods are able to achieve the level of accuracy required. The accuracy of a BIA device depends primarily on the number of frequencies at which measurements are taken. This is why the choice of instrument is so important.
BIA was first used over 30 years ago to measure the total water content of the body. The method involves passing an extremely low strength electrical current through the body and measuring the impedance to the flow of this current.
BIA is based on two key concepts:
- The Fact That the Body Contains Water and Conducting Electrolytes.
When a current is passed through the body, the water-containing fluids primarily conduct the electrical current. Water is found both inside the cells, intracellular fluid (ICF) and outside the cells, extracellular fluid (ECF). At low frequency, current passes through the ECF space and does not penetrate the cell membrane. At high frequencies however the current passes through both the ICF and ECF (Figure 1).
- That Impedance of a Geometrical System is Related to Conductor Length, its Cross Sectional Area and Signal Frequency.
Based on these concepts a value for impedance can be calculated from a fixed strength current being passed through the body, which is inversely proportional to the amount of fluid. By appropriate choice of signal frequency, this can be made specific for extracellular fluid or for total fluid determinations.
The various types of bioimpedance include, single frequency BIA (SF-BIA), multiple frequency BIA (MF-BIA) and the latest advancement offered by ImpediMed, bioimpedance spectroscopy (BIS).Single Frequency BIA (SF-BIA)
SF-BIA is generally performed at a frequency of 50 kHz. At this frequency the current passes through both the intracellular and extracellular fluid and consequently total body water (TBW) may be calculated. However, as the current passes through both intra and extracellular compartments, differences in intracellular fluid alone are unable to be determined 2.
SF-BIA relies on prediction equations and algorithms to calculate results 2. These algorithms have in general been determined from healthy subjects. The one algorithm is not suitable for all subjects. To be accurate devices will offer a choice of three on-board algorithms (General, Child and Obese) therefore improving the accuracy of measurement at 50 kHz.Multiple Frequency BIA (MF-BIA)
MF-BIA involves taking impedance measurements at less than seven frequencies. These limited frequency devices employ empirical linear regression models to estimate subject results.Bioimpedance Spectroscopy (BIS)
BIS takes measurements at 256 different frequencies and uses mathematical modeling to calculate the resistance at zero and infinite frequencies (R0 and Rinf respectively). These values are utilized through Hanai mixture theory to derive fat-free mass and fat mass. The determination of impedance at zero frequency is highly significant as this value represents the impedance of the ECF alone while establishing Rinf allows reliable prediction of the TBW 3.
There have been over 1600 publications on BIA in the English medical literature since 1990 4. Medical researchers, under IRB approved protocols, have been investigating the application of BIA & BIS technology in the areas of cancer, HIV, obesity, anorexia, renal failure and cirrhosis 4. There are many more possibilities for clinical research under approved IRB protocols.References
- Van Loan MD, et al. Use of bioimpedance spectroscopy (BIS) to determine extracellular fluid (ECF), intracellular fluid (ICF), total body water (TBW), and fatfree mass (FFM). pp. 6770. In Ellis, K. (ed.) Human Body Composition: In Vivo Measurement and Studies. Plenum Publishing Co., New York. 1993.
- Kyle UG, et al. (2004) ESPEN Guidelines. Bioelectrical impedance analysis - part 1: review of principles and methods. Clin. Nutr. 23:1266-43.
- Cornish BH, et al. (1993) Improved prediction of extracellular and total body water using impedance loci generated by multiple frequency bioelectrical impedance analysis. Phys. Med. Biol. 38:337-46.
- Kyle UG, et al. (2004) ESPEN Guidelines. Bioelectrical impedance analysis - part 2: utilization in clinical practice. Clin. Nutr. 23:1430-53.