Bioelectrical Impedance Analysis

How BIA Has Been Used

Some researchers have sought to replace more difficult methods of body composition with BIA evaluation. In the pediatric population it has been suggested that multi-frequency BIA may be able to replace DXA and TBK measures after taking a sample of 347 children. Using the DXA plus TBK models for predicting body composition, BIA was able to adequately match predictions in populations, but not individual healthy children. The authors suggested that age and sex specific adjustments to the calibrated constants may help to refine the calculations. [55]
In another study, the ability of multifrequency BIA to detect extracellular water changes showed that this technology can respond to such changes induced in a healthy elderly population.[56] Another study suggested that fat mass estimations may change with altered hydration states such as dehydration or edema.[57]
Single-frequency BIA technology was applied to patients with cancer cachexia demonstrating that prediction formulas used (linear equations developed for normal weight patients) may overestimate total body water in underweight patients.[58] The picture that BIA has provided of patient status, regardless of its shortcomings, has been used to predict and monitor outcomes in patients.[59][60][61][62][63]

Comparisons have been made between BIA technologies, usually the single and multifrequency machines. Using both methods, the parallel model of prediction equations using the 50 kHz single frequency machine was suggested to more accurately predict changes in intracellular water (associated with body cell mass). In this same study the multifrequency machine was considered superior (using Cole-Cole model evaluation) when water compartments were altered from normally hydrated status.[64] Comparisons of BIA methods specific to persons infected with HIV were conducted using single 50 kHz frequency, multifrequency, and other similar technologies to determine if one had particular advantages.[65] The authors suggested that there were no significant advantages in determining body composition of the multifrequency over the single frequency method. However, a subsequent study comparing the two technologies suggested that although the single frequency measures may be well-correlated with multifrequency measures, the calculated compartments may not be as accurate using single frequency technology with regression-generated equations.[66]

Multifrequency BIA technology was used to determine the changes in body fluid compartments during inflammatory processes of fever and sepsis. Results suggested that in the 41 patients, tested hydration could remain stable when fever was not present, that dehydration of both intracellular and extracellular fluids may occur transiently during stability, and that during sepsis extracellular fluid quickly increased while intracellular water slowly decreased. The authors suggested that BIA technology, in this case a multifrequency technology, could be used to monitor metabolic stress.[67] Another investigation into the use of multifrequency BIA to monitor fluid shifts during suspected bacteremia was evaluated for the ability to predict outcome. Because fluid shifts can be monitored with this electrical technology providing a more dynamic evaluation criteria than serum albumin changes.[68] Reviews and discussion of bioelectrical impedance technologies are available.[69][70]

In another study of comparisons, multifrequency BIA was compared to isotope dilution and DXA methods of water and body fat compartments.[71] Fluid shifts that affect other indicators, such as serum albumin, have been monitored with multifrequency technology.[72] The authors of this study suggested that multifrequency BIA underestimated total body water and overestimated extracellular water compared to isotope dilution. They also noted that body fat was overestimated when compared to DXA and anthropometry. Another study comparing isotope dilution with multifrequency BIA suggested that BIA measures overestimated total and extracellular water compartments in growth hormone deficiency patients and that the difference between the two methods diminished with growth hormone replacement therapy.[73] Therefore, changes in body water were underestimated using BIA. Bioelectric technology has even been used to estimate bone and total body mineral content.[74]

Many factors and assumptions may be made in order to develop an estimate of body composition.[75] Estimated conductor length (based on height) has been cited as a source of error. Currently under investigation with both single and multifrequency bioimpedance technologies is the value of segmental measures to improve the prediction of fluid shifts.[76] This may be of particular value when truncal shifts occur because of the small contribution the trunk area makes to total body measures using tetrapolar placement of the electrodes.  

There is even potential use of BIA technology to model drug pharmacokinetics studies.[77] Though not currently available, the ability to devise accurate estimations of drug volume and elimination rate based on body cell mass, total body water, and extracellular compartment volume may be the subject of much future research.

Conducting the Test is the next section

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References:

[55] Ellis KJH, Shypailo RJ, Wong WW. Measurement of body water by multifrequency bioelectrical impedance spectroscopy in a multiethnic pediatric population. Am J Clin Nutr. 1999;70(5):847-853.

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