= Body plethysmography
\b[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>.<Standardisation of the measurement of lung volumes, 2023 update by Nirav R. Bhakta and others>[Measurement of FRC using body plethysmography, Introduction and theory]{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.<A rapid plethysmographic method for measuring thoracic gas volume by Arthur B. DuBois and others>[Pages 322–324, Method and Figure 1]{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> gives
$$
PV=(P+\Delta P)(V+\Delta V),\qquad
V=-\Delta V\frac{P+\Delta P}{\Delta P}.
$$
With 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.<Standardisation of the measurement of lung volumes, 2023 update by Nirav R. Bhakta and others>[Measurement of FRC; Table 1]{ref}
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