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The functional residual capacity (FRC) of the lungs can be found by diluting the gases in a rebreathing circuit with helium. The rebreathing circuit includes a spirometer, a CO2 and water absorber and a fresh gas supply of oxygen at about 300 ml/min. A known quantity of helium is injected into the circuit and after a few breaths when helium has mixed with gases in all parts of the circuit and lungs, the concentration of helium is measured in a sample of the gas. The total volume of the closed circuit can then be easily calculated. The system is used in the lung function laboratory.
The helium analyser is usually based on the thermal conductivity of the gas. Since the sensor is not specific to helium a correction must be applied using a knowledge of the other gases in the mixture. Alternatively, the FRC may be calculated using a radioactive gas sample.
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