Helium dilution technique

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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 and Oxygen. The amount of He in the spirometer is known at the beginning of the test ( Concentration x Volume = Amount). The patient is then asked to breath in the mixture starting from FRC (Functional Residual Capacity), also known as end expiration. Because there is no leak of substances in the system, the amount of He remains constant during the test, and the final concentration of He in the spirometer is measured. By using the following equation: C1V1 = C2V2

V2= Total Gas volume ( volume of lung + volume of spirometer). V1= Volume of gas in spirometer.