body plethysmography
Body plethysmography seats you inside a sealed, airtight booth resembling a phone box and asks you to pant gently against a momentarily closed shutter. By watching how the pressure in the box and at your mouth change as your chest moves, it calculates the volume of air in your lungs — even air you cannot breathe out.
Its physics rests on a simple gas law: when you try to breathe against a blocked mouthpiece, your chest compresses or expands the gas in your lungs, and the matching pressure change in the closed box reveals exactly how much gas was there. This makes it the gold standard for measuring functional residual capacity, residual volume and total lung capacity.
The booth's great advantage over gas-dilution methods is that it measures all the air in the chest, including pockets trapped behind narrowed airways that dilution techniques miss. That makes it especially valuable in severe obstruction and emphysema, where it can also quantify airway resistance directly.
In an emphysema patient, gas-dilution methods underestimate lung volume, but body plethysmography reveals a markedly raised residual volume and total lung capacity from severe air trapping.
The body box captures trapped air that dilution methods miss.
The sealed booth can feel claustrophobic, and the panting manoeuvre takes coaching, but the test is harmless and involves no radiation.