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Body plethysmography is a method of obtaining the absolute volume of air within one's lungs. It is used in situations where several repeated trials are required or where the patient is unable to perform the multibreath tests.

The patient is enclosed in an airtight chamber often referred to as a body box; a pneumotachometer is used to measure airflow while a mouth pressure transducer with a shutter measures the alveolar pressure. The most common measurements made using body plethysmographs are thoracic gas volume (VTG) and airway resistance (RAW). This test is used mainly in the Pulmonary Function Testing laboratories.

Using body plethysmography, doctors can examine the lungs' resistance to airflow, distinguish between restrictive and obstructive lung diseases, determine the response to bronchodilators, and determine bronchial hyperreactivity in response to metacholine, histamine, or isocapnic hyperventilation.

There are two types of plethysmographs: flow and pressure. In flow plethysmography, airway resistance is measured by two maneuvers. The patient first pants while the mouth shutter is open to allow flow changes to be measured. Then, the mouth shutter closes at the patient's end expiratory or FRC level and the patient continues panting while maintaining an open glottis. This provides a measure of the driving pressure used to move air into the lungs.

Pressure plethysmographs are usually measured at the end-expiratory level and are then equal to FRC. The patient sits in the box, which has the pressure transducer in the wall of the device, and breathes through a mouthpiece connected to a device that contains an electronic shutter and a differential pressure pneumotachometer. The mouth pressure and box pressure changes that are measured during tidal breathing and panting maneuvers which are performed during the test by the patient at the end of expiration are sent to a microprocessor unit that calculates thoracic gas volume.