An Overview of Functional Residual Capacity (FRC)

An important measurement of lung function

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Functional residual capacity (FRC) is the volume of air left in your lungs after a normal, passive exhalation. This test is used to evaluate your lung function, and you may need to have your FRC measured before lung surgery or if you have a lung disease such as emphysema or chronic obstructive pulmonary disease (COPD).

Calculation of your FRC is a bit complicated because this number reflects the air remaining inside your lungs, which cannot be directly measured. Tests that measure FRC include a lung plethysmograph or a helium dilution method.

What FRC Says About Lung Function

You regularly breathe in and out at a rate of 10 to 20 breaths per minute. Even though you exhale, air is still left in the lungs. The volume of remaining air after a normal breath is called the FRC.

As you breathe, your alveoli (tiny air sacs) always stay open. This is due to a number of factors. Surfactant, a sticky substance that lines the outside of your alveoli, helps pull them open. The elasticity of the lungs also keeps your alveoli partially inflated. Because your lungs remain partially open between breaths, it is easier for you to inhale. Less elasticity means more trouble breathing.

FRC is a reflection of how elastic your lungs are, and your medical team may use your FRC measurements to evaluate how well the treatment you are using for your lung disease is working or whether your condition is progressing.

Results can also help in the diagnosis of lung disease—specifically when differentiating between restrictive and obstructive lung disease.

FRC = ERV (the amount of extra air you can exhale if you do so forcefully) + RV (the amount of air remaining in the lungs no matter the extent of exhalation)

How FRC Is Measured

The tests used to measure FRC rely on indirect calculations to determine the volume of air in your lungs after you breathe out. These tests require your cooperation, and you will be asked to follow certain instructions as you inhale and exhale.

Lung Plethysmography

This procedure, also called pulmonary plethysmography or whole-body plethysmography, requires that you enter into a small chamber and breath into a mouthpiece. As you breathe in and out, the air pressure inside the chamber is measured. The calculation of your FRC is based on a mathematical formula that incorporates the amount of air that you breathe in and out, as well as the volume of air in your lungs.

Helium Dilution Method

This test uses a spirometer, which is a simple device that you can breathe into and out of. When you have a helium dilution test, you use a spirometer that contains helium. As you inhale and exhale, the concentration of helium remaining in the spirometer can be used to calculate your FRC.

Tests used to measure your FRC are safe, even if you have advanced lung disease.

Interpreting Your FRC Results

Normally, FRC values range between approximately 1800 ml to 2500 ml. But this number depends on several factors, such as age, weight, height, gender, and pregnancy. Major alterations in your FRC can indicate the presence or progression of lung disease.

  • Your FRC can decrease if your lung volume decreases. This can occur due to restrictive lung disease (such as pulmonary fibrosis) or conditions that prevent your lungs from adequately expanding. For example, FRC is reduced with liver or spleen enlargement, if fluid accumulates in the abdomen (ascites), or during normal pregnancy.
  • FRC can increase in the presence of severe airway obstruction, which impairs adequate exhalation. Emphysema, cystic fibrosis, and COPD can all cause this problem due to an effect on the lungs described as hyperinflation. With these conditions, your airways become unable to adequately deflate.

Additional Testing

When you have your FRC measured, you are likely to have other pulmonary function tests (PFTs) as well.

All of these values can all be altered by lung diseases:

  • Total lung capacity (TLC): The total amount of air that your lungs can hold
  • Tidal volume (TV): The amount of air that goes in and out of the lungs as you normally breathe
  • Vital capacity (VC): The total amount of air that you could take into your lungs with maximal effort
  • Forced vital capacity (FVC): The amount of air that you can breathe out after maximally breathing in

A Word From Verywell

If you have pulmonary disease, your doctor may prescribe respiratory therapy and physical therapy to help improve your breathing abilities. You can benefit from doing these exercises with a therapist, or you may learn how to do them on your own at home. As you advance in your therapies, you might need to have your FRC measured again to monitor your progress.

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