Overview of Functional Residual Capacity (FRC)

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Functional residual capacity (FRC) is the volume of air left in your lungs after a normal, passive exhalation (breathing out). 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.

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 expire. These tests require your cooperation, and you will be asked to follow certain instructions as you breathe in and out.

  • 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 would use a spirometer that contains helium. As you breathe in and out, 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.

FRC and Lung Function

You regularly breathe in and out at a rate of 10 to 20 breaths per minute. You can't breathe all of the air out of your lungs. After a normal breath, the FRC is the amount of air remaining in your lungs.

You can breathe out even more air than usual if you maximally exhale with as much effort as possible (expiratory reserve volume, ERV).

Even after you breathe out as much air as you can, your lungs will still contain some air. That volume is called residual volume (RV). FRC includes your ERV and your RV.

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. And 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.

FRC is a reflection of how elastic your lungs are. Your ERC can vary based on changes in your lung elasticity. 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. This test can also help in the diagnosis of lung disease—specifically when differentiating between restrictive and obstructive 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 be helpful in assessing whether you have lung disease and whether your lung disease is progressing.

  • 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.

Putting FRC Into Perspective

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

As you normally breathe in and out, the amount of air that you breathe in and out with each breathe is called the tidal volume (TV).

With extra effort, you can breathe in more air than usual, and the total amount of air that you could take into your lungs with maximal effort is called vital capacity (IC). The amount of air that you can breathe out after maximally breathing in is called your forced vital capacity (FVC), and it is a larger volume than your TV.

When you have a respiratory condition, your doctors may measure your TV, IC, and FVC with a simple spirometry test.

The total amount of air that your lungs can hold is called total lung capacity (TLC). All of these values, in addition to FRC, can all be altered by lung diseases.

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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