plethysmogram

A plethysmogram is the graphic record produced by plethysmography, a family of techniques for measuring changes in volume within a body part or organ. Plethysmography commonly tracks changes in blood or air volume caused by the pulsatile action of the heart or by respiratory movements. The resulting trace represents volume or impedance changes over time and is used to infer blood flow, vessel compliance, airway volumes, or limb perfusion, depending on the modality.

Common forms of plethysmography include photoplethysmography (PPG), impedance plethysmography, air or water displacement plethysmography, and venous occlusion plethysmography. Photoplethysmography uses optical sensors to detect blood volume changes in the microvasculature, producing a waveform that correlates with the pulse. Impedance plethysmography measures changes in electrical impedance due to limb blood volume fluctuations. Air displacement plethysmography is often used to assess body composition by measuring overall limb or body volume through changes in chamber air pressure. Body plethysmography is also used in pulmonary function testing to determine lung volumes and airway resistance.

Applications of plethysmography and the plethysmogram include vascular assessment for arterial insufficiency, venous reflux, and lymphedema; pulmonary evaluation of lung volumes and airway mechanics; and research into peripheral blood flow and tissue perfusion. Interpretation focuses on the pulsatile waveform’s timing and amplitude, with abnormal shapes or dampened responses suggesting perfusion or airway abnormalities. In respiratory plethysmography, changes in thoracic gas volume reflect alterations in lung volumes.

Limitations include sensitivity to movement, artifacts from ambient light in PPG, and calibration or modeling requirements that vary by modality.