= Thoracic gas volume
{title2=$V_{\rm TG}$}
\b[Thoracic gas volume], $V_{\rm TG}$, is the compressible intrathoracic gas volume assessed by <body plethysmography> at the instant of airway occlusion. It is not the volume of the entire <thorax>, which also contains tissues and fluid, and it does not identify a respiratory phase by itself.<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}
The phase matters when assigning a named capacity. In a constructed record, suppose the plethysmographic gas estimate is 3.2 L at an occlusion 0.4 L above the preceding quiet end-expiratory volume. If the measured increment and gas conditions are consistent, the corresponding <functional residual capacity> is $3.2-0.4=2.8$ L. Calling the unadjusted 3.2 L an FRC measurement would change the reference state without changing the observation.
The original experiment linked gas volume to pressure and inferred volume changes through <Boyle's law>.<A rapid plethysmographic method for measuring thoracic gas volume by Arthur B. DuBois and others>[Pages 322–324, Method and derivation]{ref} In the small-change model,
$$
V_{\rm TG}\simeq-P\frac{\Delta V}{\Delta P}.
$$
Thus a pressure-amplitude error can mimic extra gas volume. For an original illustration, let the actual gas volume be 2.8 L but the mouth signal represent only $7/8$ of the relevant alveolar pressure change. With all other terms unchanged, the inferred volume is $2.8/(7/8)=3.2$ L.
A second model can instead contain 2.8 L readily communicating with a tracer plus 0.4 L of compressible gas not reached during the tracer test. It can yield the same apparent 3.2 versus 2.8 L comparison. The numerical difference alone does not prove which model applies.
This is why comparison with <helium dilution measurement of lung volume> or <nitrogen washout> needs both compartment definitions and measurement assumptions. The technical standard describes the different accessible gas pools and potential pressure-equilibration errors.<Standardisation of the measurement of lung volumes, 2023 update by Nirav R. Bhakta and others>[Measurement of FRC, method comparison; Table 1]{ref} <Thoracic gas volume> is a specific physical measurement, not a direct label for trapped gas or a substitute for every lung-volume definition.
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