Spirometry is the standard method for measuring most relative lung volumes; however, it isincapable of providing information about absolute volumes of air in the lung. Thus a differentapproach is required to measure residual volume, functional residual capacity, and total lungcapacity. Two of the most common methods of obtaining information about these volumes are gasdilution tests and body plethysmography.
In body plethysmography, the patient sits inside an airtight box, inhales or exhales to aparticular volume (usually FRC), and then a shutter drops across their breathing tube. Thesubject makes respiratory efforts against the closed shutter (this looks, and feels, likepanting), causing their chest volume to expand and decompressing the air in their lungs. Theincrease in their chest volume slightly reduces the box volume (the non-person volume of the box)and thus slightly increases the pressure in the box.
Using the data from the plethysmography requires use of Boyles Law.
To compute the original volume of air in thelungs, we first compute the change in volume of the chest.Using Boyle's Law (P1V1=P2V2, atconstant temperature), we set the initial pressure in the box times the initial volume of the box(both of which we know), equal to the pressure times volume of the box at the end of a chestexpansion (of which we know only the pressure).
We solve for the volume of the box during the respiratory effort. The difference between thisvolume and the initial volume of the box, is the change in volume of the box, which is the same asthe change in volume of the chest. Armed with this piece of information, we use Boyle's Lawagain, this time on the fixed amount of gas in the chest before and at the end of a respiratoryeffort. We set the initial volume of the chest (unknown) times the initial pressure at the mouth(known), equal to the inspiratory volume of the chest (the same unknown volume plus the change inthe volume of the chest, which we have just computed) times the pressure at the mouth during theinspiratory effort (known). Now we solve for the unknown volume, which will be the original volumeof gas present in the lungs when the shutter was closed. As mentioned before, the shutter isusually closed at the end of a normal exhalation, or at FRC.
Body plethysmography is particularly appropriate for patients who have air spaces within the lungthat do not communicate with the bronchial tree. In these individules, gas dilution methods ofmeasurement would give an erroneously low volume reading.