 Body plethysmography

Body plethysmography uses pressure and volume changes in an enclosing chamber to measure respiratory mechanics and compressible thoracic gas volume. For lung-volume measurement, a brief closed-airway effort relates gas compression inside the lungs to the chamber signal. The gas volume belongs to the instant of occlusion; it is not automatically residual lung volume.^[ref]

In the original 1956 experiment, Arthur B. DuBois and colleagues used an airtight chamber of approximately 600 L, a mouthpiece shutter, capacitance pressure transducers and a cathode-ray oscillograph. One signal represented mouth pressure and the other chamber pressure, converted to volume change through instrument calibration.^[ref] An open communication between mouth and alveolar gas during the closed-airway effort is essential to interpreting the mouth signal.

For an ideal isothermal gas amount, Boyle's law givesWith stipulated effective absolute gas pressure $P=100$ kPa, compression $\Delta V=-0.006$ L and increase $\Delta P=0.2$ kPa, the exact expression gives 3.006 L. Neglecting the product of small changes gives the first-order result $V\simeq -P\Delta V/\Delta P=3.0$ L. Here $\Delta V$ is the inferred lung-gas change, not an unconverted box-pressure reading.

If the measurement starts at quiet end-expiration, the interpreted lung-volume reference is functional residual capacity; linked inspiratory capacity and vital capacity maneuvers then provide other capacities. The thermal behavior of gas, instrument response and mouth-to-alveolar pressure equilibration remain physical assumptions to assess. A precise algebraic fit cannot repair a pressure signal that represents a different compartment.

Unlike tracer dilution, compression can probe gas that is not freely reached by the tracer during the test. This explains why body plethysmography and nitrogen washout can give different results without being duplicate names for the same observable.^[ref]