 Thoracic gas volume

Thoracic gas volume, $V_{\mathrm{TG}}$, 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.^[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.^[ref] In the small-change model,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.^[ref] Thoracic gas volume is a specific physical measurement, not a direct label for trapped gas or a substitute for every lung-volume definition.