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# Helium Dilution Test This document describes the Helium Dilution Test, a method to measure lung volumes, specifically functional residual capacity (FRC) and residual volume (RV). **Key Points:** * **Calibration:** Helium is used because it is virtually insoluble in blood. Before the test, the...

# Helium Dilution Test This document describes the Helium Dilution Test, a method to measure lung volumes, specifically functional residual capacity (FRC) and residual volume (RV). **Key Points:** * **Calibration:** Helium is used because it is virtually insoluble in blood. Before the test, the spirometer is calibrated with a known amount of helium. * **Equilibration:** The subject breathes helium-containing air until the concentration of helium in the spirometer and the lungs equilibrates. The helium in the lungs stays at the same concentration, as helium is virtually insoluble. * **Calculation:** The initial amount of helium in the spirometer is compared to the amount of helium after equilibration. Differences indicate how much helium was absorbed by the lungs, which is used to calculate FRC. * **Insolubility:** Helium is virtually insoluble in blood, enabling accurate measurement of lung volumes. * **Methods:** Traditional spirometry does not directly measure residual volume (RV) or functional residual capacity (FRC) Helium dilution provides a way to measure these values. * **Equipment:** A spirometer is used in this procedure.