Compared to spirometry, whole-body plethysmography is the more exact measurement method. Using whole-body plethysmography (often referred to as "the large pulmonary function test" in our offices), we determine the airway resistance and so-called residual volume, i.e. the volume of air left in the lungs after completely breathing out.
This method has less to do with your participation. You sit in a closed glass chamber with a known volume and take part in several tests using a breathing tube. Your respiratory movements change the pressure in the chamber while a sensor measures the changes. The measured values correlate with the opposing change in pressure in your ribcage and pulmonary alveoli. Meanwhile, your respiratory flow is measured and recorded through the tube. When these values are displayed, we get a pressure-volume diagram that shows the so-called resistance loop. Each of the various pulmonary diseases can be recognized based on its distinct resistance loop.