pletysmography

Plethysmography is a non-invasive medical technique used to measure changes in the volume of an organ, or of the whole body, or of a limb. The word comes from the Greek words "plethysmos" meaning "increase" and "metron" meaning "measure". Various types of plethysmography exist, each designed to measure volume changes in specific areas or conditions.

The most common type is impedance plethysmography, which uses electrical impedance to detect changes in blood volume. As blood, which is a conductor of electricity, flows into an area, the impedance decreases, indicating an increase in volume. This method is frequently used to assess deep vein thrombosis (DVT) in the legs, by measuring changes in venous volume.

Another form is strain gauge plethysmography. This technique employs a flexible band containing wires that stretches as the limb or organ expands. The change in resistance of the wires is proportional to the change in diameter, and thus volume. It is also used for vascular studies, particularly in assessing blood flow to the limbs.

Photo-plethysmography uses light to measure blood volume changes. A light emitter and detector are placed on the skin. The amount of light reflected or transmitted back to the detector varies with the blood volume in the underlying tissue. This principle is utilized in pulse oximeters, which measure blood oxygen saturation and pulse rate.

Respiratory plethysmography measures changes in the volume of the chest and abdomen during breathing. It can be used to assess lung function and breathing patterns, particularly in diagnosing sleep apnea or other respiratory disorders. These devices typically involve bands worn around the chest and abdomen. Overall, plethysmography provides valuable quantitative data on physiological volume changes.