Nitrogen Washout - Functional Residual Capacity (FRC)

This test measures the Functional Residual Capacity (FRC). In people with normal lungs the FRC is equal to the Thoracic Gas Volume (TGV) measured in the body plethysmograph. The helium dilution method only measures the air in the lung that under goes gas exchange. The plethysmograph method measures all air in the lung, including any trapped air. Inspiratory and expiratory volumes must also be measured with the FRC. A technologist will instruct you on how to perform the test and coach and encourage you to do your best.

Performing the test

  1. You will be asked to come on the mouthpiece with noseclips on and to breathe normally

  2. You will be asked to slowly blow out all of your air until you are empty

  3. You will then return to normal breathing for a few minutes

  4. Then you will be asked to slowly blow out until empty and then to take a slow deep breath in until your lungs are completely filled

  5. You will then be asked to blow out slowly until you are completely empty

  6. You will then return to normal breathing for a few breaths before coming off the mouthpiece

  7. You will rest 5 to 10 minutes before repeating the test



This test will measure static lung volumes:
  • Functional Residual Capacity (FRC)
  • Inspiratory Capacity (IC)
  • Expiratory Residual Volume (ERV)
  • Slow Vital Capacity (SVC)
  • Total Lung Capacity (TLC)

After a short rest period, the test is repeated until there are two reproducible tests. This test may be done with or without a bronchodilator.


This information has been approved by George Zeman, RRT, CPFT, BS and Amy Olson, MD, MSPH (August 2013).