How Does BIA Work?
How Does BIA Work?
Surface electrodes are attached to the hand and foot which introduce a small alternating electrical current into the body. With a multi-frequency BIA device, two or more frequencies are used rather than a single frequency. The different body tissues and structures will cause opposition to the flow of this current.
The voltage drop between the electrodes is measured, which provides us with a measure of Impedance (usually abbreviated with the letter ‘Z’). The Impedance data can then be used to gain an understanding of the patient’s body composition and fluid distribution.
The basic principle of BIA is that lean tissue, which consists essentially of electrolyte-containing water, conducts the electrical current. On the other hand, fat contains a relatively small amount of water, so fat will impede the current or act more like an insulator compared to lean tissue.
An individual with more fatty mass will therefore be more resistant to the flow of the current than someone else with more muscle mass.
Prediction equations are used to convert the measured Impedance value to a corresponding estimate of Total Body Water (TBW).
Lean Body Mass is then calculated from the TBW and Fat Mass is calculated as the difference between body weight and Lean Body Mass. Other parameters can be calculated using specific validated equations.
The VLA Quadscan
The manufacturers of the VLA Quadscan device have been established since 1990 and specialise solely in BIA technology. The VLA Quadscan device is manufactured in Europe, made to the highest specifications and uses only the best electrical components to ensure the device is of high quality and produces accurate results that are both reproducible and reliable. These high standards have led to the use of the VLA Quadscan in over fifty published research trials.
The History of BIA
BIA has been used commercially since the early 1980’s, however the electrical properties of tissues have been described since 1871. In 1962 physician Thomasset was convinced that electrical resistance reflects the fluid content of the human body. He and his colleagues developed one of the first impedance analysers for measurements on biological tissues.
Other pioneers in the field included EC Hoffer and J. Nyboer who in the 1960’s first introduced the four-surface electrode BIA technique. Since then there have been many significant developments and technological advancements.
Why Are Multiple Frequencies Used for BIA?
More than one frequency is used to improve the accuracy of the BIA device. The flow of current through the body is dependent on the frequency applied. At low frequencies, the current cannot bridge the cellular membrane and will pass predominantly through the extracellular space.
For this reason the low frequency of 5 kHz is used to estimate the Extracellular Water (ECW).
On the other hand, the 200 kHz frequency will pass through cellular membranes and is conducted by both the Extracellular Water (ECW) and Intracellular Water (ICW). The 200 kHz frequency can thus be used to calculate the Total Body Water (TBW).
By using these two different frequencies and by applying predictive equations, it is possible to estimate both ECW and TBW, and by deduction, ICW.
Low Frequency (5kHz) unable to pass through cells => Extracellular Water (ECW)
High Frequency (200kHz) pass through extra and intracellular components of cell => Total Body Water (TBW)
Using these specific frequencies will provide a more accurate estimate of ECW and ICW when compared with the stand alone frequency of 50 kHz.
The other benefit of using the 200 kHz and 5 kHz frequencies is that they can be used to calculate a unique indicator of health and fluid balance. This indicator is known as the ‘Prediction Marker’, which will be discussed shortly.
Impedance is Made Up of Resistance and Reactance
Electrical impedance is a measure of the opposition of a conductor to an alternating current. The impedance can be further broken down into two components, the Resistance (R) and the Reactance (Xc). Resistance is the opposition to the flow of an electric current, primarily related to the amount of water present in the tissues – lower amounts of water will increase the Resistance. Reactance represents the delay in the conduction of the current by cell membranes.
Within the body, healthy cells maintain an electromagnetic gradient, this allows various cellular functions to occur and is part of the complex communication network throughout the body. When a current is passed through the body, healthy cell membranes are able to temporarily accumulate a charge on each side of the cell membrane without letting it pass through. This is known as capacitance.
Reactance reflects the property of the cell membranes to act as capacitors and is associated with cell size and the integrity of the cell membranes. For example, unhealthy cell membranes will not be able to effectively accumulate a charge, leading to lower reactance. Low cell mass will also lead to a lower Reactance.
On the other hand, the Resistance is dependent on lean tissue mass and tissue hydration, so Resistance can provide information on fluid balance. By utilising this information, we can gain information about not only body composition, but aspects of cellular health.
If you like this article be sure to visit the Spinal Centre website at www.thespinalcentre.com.au and view more content by Dr. Hooper and the Spinal Rehabilitation Team.
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