Body Plethysmographs-To do a body plethysomograph, the person is enclosed in an airtight chamber often referred to as a body box. The most common measurements using the body plethysmographs are thoracic gas volume (VTG) and airway resistance (RAW). This test is used mainly in the Pulmonary Function Testing laboratories. There are two types of body plethysmographs that are used: flow plethysmographs and pressure plethysmographs. A pneumotachometer is used to measure the flow and a mouth pressure transducer with a shutter to measure the alveolar pressure.

Pressure Plethysmographs-This is usually measured at the end-expiratory level and is 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.

Flow Plethysomographs-Airway resistance is measured by two separate maneuvers. In the first maneuver, the patient pants while the mouth shutter is open to allow flow changes to be measured. In the second maneuver, 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.

Indications:

Used to meaure lung volumes to distinguish between restrictive and obstructive lung diseases.

Is a measurement of lung volumes when several repeated trials are required or when the person is unable to perform the multibreath tests.

Evaluates the resistance to airflow.

Determines the response to bronchodilators.

Determines bronchial hyperreactivity in response to metacholine, histamine, or isocapnic hyperventilation.

Used to follow the coarse of a disease and response to treatment.