A plethysmograph is an instrument for measuring changes in volume within an organ or whole body (usually resulting from fluctuations in the amount of blood or air it contains).

[2] In a traditional plethysmograph (or "body box"), the test subject, or patient, is placed inside a sealed chamber the size of a small telephone booth with a single mouthpiece.

With cabinless plethysmography, the patient is seated next to a desktop testing device and inserts the mouthpiece into their mouth.

During this tidal breathing, a series of rapid interruptions occurs, with a shutter opening and closing, measuring pressure and volume.

While observing the so-called resistance loop (cabin pressure and flow), diseases can easily be recognized.

[5] Some plethysmograph devices are attached to arms, legs or other extremities and used to determine circulatory capacity.

In water plethysmography an extremity, e.g. an arm, is enclosed in a water-filled chamber where volume changes can be detected.

Air plethysmography uses a similar principle but based on an air-filled long cuff, which is more convenient but less accurate.

Another practical device is mercury-filled strain gauges used to continuously measure circumference of the extremity, e.g. at mid calf.

[6] An approximate female equivalent to penile plethysmography is vaginal photoplethysmography, which optically measures blood flow in the vagina.

Several techniques are used: Whole-body plethysmography is used to measure respiratory parameters in conscious unrestrained subjects, including quantification of bronchoconstriction.

[8] The double-chamber plethysmograph (dcp) measures respiratory parameters in a conscious restrained subject, including airway resistance and conductance.