Contact persons

• Univ.-Prof. Dr.-Ing. Dr. med. Dr. h. c. Steffen Leonhardt

Project description

Bioimpedance spectroscopy (BIS) is a non-invasive technique used to measure the complex, frequency dependent impedance within a spectrum from typically 1 kHz to 1 mHz to determine, for example, body composition.

Methods

BIS is based on the well-known 4-point measurement technique. A small, harmless alternating current is injected between two electrodes and the resulting potential drop is measured between another pair of electrodes. This way it is possible to eliminate the influence of the skin-electrode contact impedance and measure only the body’s impedance.

Impedance from body tissue

In general, body tissue impedance is frequency dependent. This can be described by modelling tissue as a suspension of cells in a conductive fluid: the extracellular space. The cells are modelled as the intracellular space surrounded by a lipid bilayer cell membrane. Electrically, this is represented with the Cole model (see Figure 1, right). The current path is therefore frequency dependent i.e. at low frequencies, the current flows primarily through the extra cellular space while at higher frequencies the current flows through both the extra- and the intracellular space. A characteristic semicircular impedance curve results from this behavior, as depicted in figure 2.

Application fields of bioimpedance spectroscopy

BIS is usually applied when the composition of body tissue is to be analyzed. An everyday life example is the body-fat scale. Current research areas are, for example, in the field of lung monitoring. It is envisioned that the analysis of BIS-data may help in the diagnosis of diseases such as pulmonary edema or pneumonia. Such a non-invasive detection technique would be desirable, especially for critically-ill patients.