Which lung function test uses a sealed box to measure lung volumes?

• A. Body Plethysmography
• B. Spirometry
• C. Helium washout
• D. Nitrogen washout

Answer: (A)

Body plethysmography uses a sealed box. In this test the patient sits in an airtight chamber and breathes through a mouthpiece while a shutter inside the box is intermittently closed. The box pressure changes as the patient pant-breaths, and these pressure changes are analyzed with the patient’s mouth-flow data to calculate the volume of gas in the lungs (thoracic gas volume). From this, total lung capacity and residual volume can be derived. This method is especially useful for detecting air trapping and accurately measuring lung volumes when airway obstruction is present, something spirometry alone can’t do because spirometry only gauges airflow and volumes during forced maneuvers, not absolute lung volumes.

By comparison, spirometry relies on a flow sensor and a mouthpiece without any box and mainly measures airflow-related volumes like FEV1 and FVC. Helium or nitrogen washout methods estimate lung volumes by gas dilution in a rebreathing circuit rather than a sealed chamber—they track changes in inert gas concentrations as the individual breathes, which allows calculation of volumes but does not use a sealed box.