Photo-Plethysmography (PPG)

This method allows to measure arterial and venous blood volume changes in the skin surface and in small limbs (fingers and toes) of the body non-invasively and continuously and is, therefore, suitable to be used for peripheral vascular diagnosis.

The PPG is based on the determination of the optical properties of a selected skin area using non-visible infrared light. In comparison to other tissue blood has a much higher absorption of infrared light than other tissue. Therefore, blood volume variations correspond with measurable changes of the light absorption whereby an increase of blood volume lead to an increase of light absorption whereby there isn´t a difference between venous and arterial blood.

For the measurement of the optical properties infrared light is emitted into the skin by a LED. Depending on the blood volume in the skin more or less light is absorbed. Consequently, the reflected or transmitted light measured by a photo sensor corresponds directly with the variations of the blood volume in the selected skin area. It should be considered that the PPG has a limited penetration depth so that only surface vessels are measured which are very sensitive to skin temperature.

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

The arterial pulse waves of the PPG are analysed to evaluate arteries in fingers, toes and ear lobe. If the PPG is used to evaluate provoked changes of the venous blood volume in the lower legs to diagnose the function of the venous valves then this application is named Light-Reflection-Rheography / D-PPG