Helium dilution technique

The helium dilution technique is the way of measuring the functional residual capacity of the lungs (the volume left in the lungs after normal expiration).

This technique is a closed-circuit system where a spirometer is filled with a mixture of helium (He) and oxygen. The amount of He in the spirometer is known at the beginning of the test (concentration × volume = amount). The patient is then asked to breathe (normal breaths) in the mixture starting from FRC (functional residual capacity), which is the gas volume in the lung after a normal breath out. The spirometer measures helium concentration. The helium spreads into the lungs of the patient, and settles at a new concentration (C2). Because there is no leak of substances in the system, the amount of helium remains constant during the test, and the FRC is calculated by using the following equation:

${\displaystyle C_{1}V_{1}=C_{2}V_{2}}$

${\displaystyle C_{1}V_{1}=C_{2}(V_{1}+FRC)}$

${\displaystyle FRC={\frac {C1V1}{C2}}-V1}$

V2 = total gas volume (FRC + volume of spirometer)

V1 = volume of gas in spirometer

C1 = initial (known) helium concentration

C2 = final helium concentration (measured by the spirometer)