Lung Volumes

The amount of air that your lungs can hold cannot be measured simply by blowing out. Instead, it must be measured indirectly by one of two methods:

Body Box (Plethysmograph) – In this procedure, you sit in a Plexiglas box about the size of a telephone booth and breathe in and out through a small mouthpiece. On the technologist’s prompt, you pant like a puppy for 2 or 3 seconds while the box is closed. With the help of data from the spirometry test (below), the instrument instantly calculates your lung volume.

The Nitrogen Washout Test uses simpler equipment, but gives the same results. By breathing pure oxygen through a mouthpiece, all the nitrogen in your lungs (normal air is 78% nitrogen) is replaced with oxygen. The system measures the amount of gas removed and calculates lung size from this data. The test takes no more than seven minutes.