Impedance-Plethysmography (IPG)

This method – often also named Rheography – allows to measure arterial and venous blood volume changes in nearly each body segment (arms, legs, head, …) non-invasively and continuously and is, therefore, suitable to be used for arterial and venous vascular diagnosis.

The IPG is based on the measurement of the electrical impedance (resistance) of a selected body segment. In comparison to other tissue, such as, muscle or bone, blood has a much lower impedance. Therefore, blood volume variations in a body segment correspond with measurable changes of the electrical impedance whereby an increase of the blood volume results in a lower impedance.

For the measurement of the electrical impedance usually 4 electrodes are applied to the body surface approximately in a line. The 2 outer electrodes (usually called current electrodes) are used to pass a very low and constant alternating current (1.5 mA, 86 kHz) through the body segment which is imperceptible to the patient and does not cause any physiological reaction. The 2 inner electrodes (usually called measuring electrodes) are placed between the 2 current electrodes and measure the voltage which is caused when the current flows through the body segment. This voltage corresponds with the impedance of the body segment which changes depending on venous and arterial blood volume variations. The positioning of the measuring electrodes defines the segment which is analysed which is between both electrodes. To guarantee a more homogeneous current dispersion in the measuring segment the current electrodes should be placed as fare as possible from the measuring electrodes (> 10 cm). The distance between measuring and current electrodes should be more than the distance between measuring electrodes.

The IPG signal consists of 3 components: The arterial pulse wave, changes of the venous blood volume and a very stable component - named basic impedance - which is defined by all other tissue in the analysed segment. The amplitudes of these signal components are very different and are illustrated in the diagram.

The arterial pulse waves of the IPG and their curve shapes are analysed for the diagnosis of peripheral arteries. In case of Venous-Occlusion Plethysmography (VOP) and Arterial-Occlusion-Plethysmography (AOP) venous blood volume changes are provoked and analysed which can be measured by IPG.