gasdilution

Gas dilution refers to a set of techniques that estimate gas volumes or ventilation characteristics by introducing a known quantity of an inert tracer gas into a closed system or a subject and measuring how the gas concentration changes. The approach relies on the conservation of mass and assumes adequate mixing of gases within the system being studied.

In respiratory physiology, gas dilution techniques are used to measure lung volumes such as functional residual capacity (FRC) and sometimes total lung capacity (TLC). Two common approaches are used. Closed-circuit helium dilution involves the subject breathing from a closed chamber containing a known concentration of helium. After several breaths, the final helium concentration is measured, and the lung volume is inferred from the degree of dilution. Open-circuit nitrogen washout uses a period of breathing 100% oxygen, during which exhaled nitrogen is monitored until its concentration falls to baseline, allowing calculation of the gas volume contained in the lungs. These methods are noninvasive and relatively quick but rely on assumptions about complete mixing and stable gas exchange.

Outside medicine, gas-dilution methods are employed to assess ventilation and air exchange in rooms or buildings. Tracer gases such as sulfur hexafluoride (SF6) or CO2 are released or monitored in a space, and their concentration decay or steady-state dilution is used to estimate air change rates and ventilation effectiveness. Safety considerations focus on choosing inert, non-toxic tracer gases and ensuring that measurements do not pose risks to occupants.

Overall, gas-dilution techniques provide practical, noninvasive means to quantify gas volumes and ventilation, with accuracy dependent on proper mixing, stable conditions, and appropriate tracer selection.