This test measures the TGV (thoracic gas volume) and the Raw (airway resistance).
In particular, the plethismograph relates pressure variations inside the Body Box with volume variations of th e air within
the lungs, by means of the Boyle’s law. It states that, under isothermal conditions, the product of volume and pressure of
a gas is constant.
During a panting maneuver (respiratory frequency about 60 breaths per minute), the apnea valve is closed and patient is
asked to perform short and rapid breaths. While the patient try to breathe, he/she expands the gas within the lungs and
causes the pulmonary volume to increase. Consequently, the body box pressure increases, since its volume decreases.
When the patient exhales, the opposite occurs.
In healthy individuals, there is a little difference between the FRC measured with nitrogen wash -out technique (or other
dilution techniques) and the TGV measured by means of a Body Box. This is not true for patients with particular
pulmonary pathologies: for these patients the TGV is often much higher than the FRC.
Parameters measured during the tests include the following: