plethysmography

Plethysmography refers to a group of noninvasive methods for measuring changes in volume within an organ or the whole body. The technique relies on detecting the small changes in volume that accompany blood flow, respiration, or other fluid movements, using pressure changes in a closed space, optical signals, or electrical impedance.

Common forms include air plethysmography (often called air-displacement plethysmography), used to estimate body composition by measuring limb or whole-body volume changes; body plethysmography (closed chamber) used in pulmonary function testing to determine thoracic gas volume and airway resistance; impedance plethysmography, which measures changes in electrical impedance in a limb to infer blood flow and volume; photoplethysmography, which uses light to detect blood volume changes in the microvasculature and is widely used in pulse oximetry and heart-rate monitoring.

Applications include assessment of venous insufficiency and lymphedema, respiratory function testing, cardiovascular monitoring, and noninvasive vascular status assessment. Advantages include noninvasiveness, real-time data, and bedside applicability; limitations involve sensitivity to movement, temperature, and device calibration, as well as potential need for modeling to interpret indirect measurements. The evidence base varies by modality and clinical context.

Modern plethysmography encompasses optical, pneumatic, and electrical approaches and remains in clinical and research use for vascular, respiratory, and body-composition assessment.