Clinical Calculator: Lung Volume Multicalc
FRC is measured using gas dilution techniques or a plethysmograph (which is more accurate in patients who have airflow limitation and trapped gas).
Gas dilution techniques include
• Nitrogen washout
• Helium equilibration
With nitrogen washout, the patient exhales to FRC and then breathes from a spirometer containing 100% O 2 . The test ends when the exhaled nitrogen concentration is zero. The collected volume of exhaled nitrogen is equal to 81% of the initial FRC.
With helium equilibration, the patient exhales to FRC and then is connected to a closed system containing known volumes of helium and O 2 . Helium concentration is measured until it is the same on inhalation and exhalation, indicating it has equilibrated with the volume of gas in the lung, which can then be estimated from the change in helium concentration that has occurred.
Both of these techniques may underestimate FRC because they measure only the lung volume that communicates with the airways. In patients with severe airflow limitation, a considerable volume of trapped gas may communicate very poorly or not at all.