Photoplethysmography Measurements
venous test   PPG pulse wave analysis

Photoplethysmography is based on the determination of the optical properties of a selected skin area. For this purpose non-visible infrared light is emitted into the skin. More or less light is absorbed, depending on the blood volume in the skin. Consequently, the backscattered light corresponds with the variation of the blood volume. Blood volume changes can then be determined by measuring the reflected light and using the optical properties of tissue and blood.

You can see this effect with your own eyes: skin with less blood looks white, while skin with more blood looks darker. The photoplethysmograph does nearly the same thing but with much higher sensitivity and resolution.

Modern measuring methods, using sophisticated equipment, allow you to greatly improve the accuracy and reliability of photometric methods. Nevertheless, keep in mind that only relative measurements are possible so that only blood volume changes, not absolute levels, can be recorded.

The measured signal records venous blood volume changes as well as the arterial blood pulsation in the arterioles.

Light-reflection-rheography (LRR) also refers to the venous test. The name is derived from a special kind of sensor that has emitting and sensing parts side by side on the same side of the sensor. The sensor measures the reflected part of the emitted light. (Note that the other sensors described below, transmission sensors, can only measure pulse waves. Only the reflective sensors will work for the venous test.)

The venous reflux test - also called tip-toe-test or muscle pump test - is an exercise test to diagnose insufficiencies of the venous valves in the lower extremities. Venous insufficiency is a very common venous disease and can also be the result of thromboses (post-thrombotic syndrome).

To perform the test, the patient should be in a sitting position on a height-adjustable chair. The legs should be extended slightly (angle of 110°).

During an exercise test the patient performs dorsiflexions following a metronomic dictation to pump the blood out of the veins. To do this they must flex their foot upwards above the heel and then relax it. This procedure should be repeated about 10 times. During this exercise the blood in the veins is pumped toward the heart due to the contractions of the muscles.

After finishing the exercise test the patient must wait motionless until the recording of the refilling curve is completed (signaled by a double-tone from the machine). During this time the refilling of the blood is measured. If the venous valves work well and close properly no blood can flow back and the refilling occurs only via the arterial inflow. This refilling takes a longer time--approx. 30 up to 120 seconds.

If a venous insufficiency is present, blood can flow backwards and the refilling occurs much faster (faster than 25 seconds). A faster refilling generally indicates a higher degree of insufficiency.

This photoplethysmographic test is especially suitable for a first test of venous function. The test is so easy that it can even be used in drugstores for venous screening (comparable to blood pressure measurements). More detailed examinations can be make by specialized doctors using phlebodynamometry and vascular doppler.

NEW! The rheoscreen light (S) uses a patented method allowing even measurements through nylons and compression stockings. This enables the evaluation of the hemodynamic effects of individual, selected compression stockings.

Similar to impedance plethysmography, photoplethysmography can separate the arterial pulse wave from the venous blood volume changes.

1. Reflection probes

This type of probe can also be used for the venous test. The light emitting and sensitive parts are located side by side in one probe. The photosensors detect the light, which is backscattered from the tissue of the skin. Due to the body's anatomy, the PPG sensors can only detect the pulse waves in areas that contain many arterio-venous anastomoses such as the fingers, toes, earlobes, or some regions of the face. These are different locations than those used for the venous test with the same sensor described above.

2. Transmission probes

In these probes the photosensors are located on the opposite side as the light emitting parts. The tissue is located between them. This limits the field of application to locations where the light can penetrate all the way through the tissue (fingers, toes, earlobes). In contrast to the reflection probe, the main sources of pulsation also contain the large vessels making these sensors especially useful for peripheral blood pressure measurements.

The arterial photoplethysmography measures and evaluates the size and shape of the pulse waves, similar to the impedance plethysmography. The easy application on fingers and toes makes this method especially useful in:

• diagnosing arterial diseases in fingers and toes

  This is possible just by watching the curve shape.

• diagnosing functional disturbances of blood flow (Raynould syndrome, death finger, fibration syndrome)

  For diagnosing functional disturbances, for example, fingers are exposed to cold some minutes. Afterwards the time is measured until the pulse amplitude is normal again.

• diagnosing thoractic outlet syndrome

  In case of the thoractic outlet syndrome the arteria in the shoulder is occluded by some extensions of the shoulder, the arms or the head. By monitoring the pulse wave in a digit, the positions where the shoulder arterie are occluded may be found.

• measuring peripheral blood pressure even on digits

  Especially with transmission probes it is very easy to measure the peripheral systolic blood pressure. The PPG-probes are used as blood flow sensors (instead of the widespread but much more difficult to use doppler probes). A cuff is inflated to aoversystolic pressure and than deflated slowly. The systolic blood pressure is the cuff pressure where the PPG-probe just measured a pulse wave again. With small cuffs even a blood pressure measurement on digits is possible (see picture).