Seca 515 - Manual (Page 45)

Medical basis • 45
English
Functional principle of bioelectric
impedance analysis
Bioelectric impedance analysis (BIA) is a method for rapid, simple, and
non-invasive assessment of body composition. Only one measurement
procedure is required on an individual device.
In the BIA method, the human body is regarded as an electrical conductor
in an alternating current circuit and its alternating current resistance
(impedance) is measured.
The following properties of an electrical conductor have an impact on its
impedance:
When it comes to the human body, age, gender, bodily fitness, and
ethnicity have an impact on the impedance.
In the event that a low alternating current is conducted to the arms and
legs via electrodes and the voltage drop is measured via a second
electrode pair, the following components of body impedance can be
calculated:
If the alternating current is applied with different frequencies, individual
parameters can be specifically determined. In doing so, the proportion of
extracellular water can be determined directly, for example, if lower
frequencies of between 2 and 5 kHz are used. Alternating current with
these frequencies is hardly able to penetrate cell walls. Cell walls and
intracellular water therefore have a very minor impact on the impedance.
The body composition of the patient can be calculated and then assessed
using the measured variables in combination with the patient's weight,
height, age and gender. The pre-requisite in this case consists of formulas
calculated according to scientific criteria.
Pioneering achievement: seca
formulas
A number of formulas are already available form a number of sources, with
the help of which the body composition can be calculated based on the R,
X
c
, weight, height, age, and gender of a patient. The formulas are based
on results of what are referred to as validation studies of reference
methods of gold standards, such as the Air Displacement
Plethysmography (ADP) or the Dual-Energy X-ray Absorptiometry (DEXA).
However, the comparability and accuracy of these formulas must be
regarded with a critical eye as the validation studies were performed with
differing reference methods and, in all respects, heterogeneous reference
populations. In addition, the study results cannot necessarily be
transferred to other manufacturers’ devices for technical reasons.
Equivalent circuit diagram:
Measurement of total body impedance
seca 515/514 :
Measurement of total body impedance
Property Correspondence in humans
Length Body size
Cross-section e.g. waist circumference
Material Body water, cell tissue
Impedance components Cause
Resistance (R), ohmic
resistance
Body water is a good electrical conductor
Reactance (X
c
), capacitive
resistance
Cell tissue acts as a capacitor
Phase angle (Φ)
Phase shift between the maximum current
and maximum voltage by the capacitor
effect of the cell tissue