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Functional principle of
bioimpedance analysis
Equivalent circuit diagram:
measurement of total body impedance
:
seca 515/514
measurement of total body impedance
Pioneering achievement: seca
46 •
Bioimpedance analysis is a method for rapid, simple and non-invasive
assessment of body composition. Only one measuring procedure on a single
device is required.
In a bioimpedance analysis, the human body is regarded as an electrical con-
ductor 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:
Property
Length
Cross-section
Material
When it comes to the human body, age, gender, physical fitness and ethnicity
also have an impact on impedance.
In the event that a low alternating current is conducted into the body via elec-
trodes on the arms and legs and the voltage drop is measured via a second
electrode pair in each case, the components of body impedance below can
be determined.
Impedance component
Resistance (R), ohmic
resistance
Reactance (X
resistance
Phase angle (φ)
If the alternating current is applied at different frequencies, individual parame-
ters can be specifically determined. In this way, for example, the proportion of
extracellular water can be determined directly if low frequencies of between 2
and 5 kHz are used. Alternating current at these frequencies is hardly able to
penetrate cell walls. Cell walls and intracellular water therefore have a very
minor impact on impedance.
Using the measured parameters in combination with the weight, height, age,
and gender of a patient, the body composition of the patient can be deter-
mined and then assessed. The pre-requisite in this case are formulas calcu-
lated according to scientific criteria.
A number of formulas are already available from a number of sources which
formulas
can be used to determine body composition on the basis of R, X
height, age and gender of a patient. The formulas are based on results of
what are referred to as validation studies compared to reference methods of
the gold standard, such as air displacement plethysmography (ADP) or dual
energy X-ray absorptiometry (DEXA), for example.
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.
Correspondence in humans
Height
E.g. waist circumference
Body water, cell tissue
Body water is a good electrical conductor
), capacitive
c
Cell tissue acts as a capacitor
Phase shift between the maximum current
and maximum voltage as a result of the
capacitor effect of the cell tissue
Cause
, weight,
c
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