The Helium Dilution Method for measuring FRC

Refer to the figure below for illustration.

The 3-Breath method is a clever way to measure exactly how much volume is in a person's Functional Residual Capacity . Remember that FRC cannot be directly measured via spirometry .

Here's how the Helium Dilution method works:

• First, obtain a volume of Y liters of Helium.
• Next, insert this into the breathing apparatus illustrated below, and measure the original concentration of Helium in the system. Call this concentration h1.
• You can use these figures to calculate the volume of the apparatus. Call this volume Vspirom.
  Vspirom = Y/h1.

• As the patient inhales and exhales, gas goes in and out of the apparatus. This gas mixture is recycled. However, to prevent buildup of carbon dioxide (CO2), CO2 is removed with every exhalation.
• As CO2 is removed, an equivalent volume of oxygen (O2) must be added to maintain a constant volume within the apparatus.
• Helium does not get significantly absorbed into the bloodstream, so the amount of helium circulating in between the apparatus and the lungs remains constant.
• After some time, measure the concentration of Helium again. Call this concentration h2.
• Since we know the original volume of the apparatus (Vspirom), and the original concentration of Helium (h1), and the final concentration of Helium (h2), we can now find the final volume of the system, which will be equal to Vspirom plus the volume in the subject's FRC.

This method directly measures FRC. Note that once you find either RV or FRC, you can calculate the other. Refer to the figure below for illustration of this idea:

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