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.

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.

On request, larger plethysmographs can also be manufactured for other animals, such as rabbits, dogs, pigs, or primates.

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