How do you calculate FRC

Functional residual capacity (FRC) is one of several parameters used in combination with others to determine the lung capacity. Patients with respiratory disorders tend to have a lower or higher FRC. This HealthHearty article defines this parameter and tells you how to calculate it.

Did You Know?

The FRC of an average-sized, 150-pound person is around 2400 ml. It is equivalent to breathing for 8 to 10 minutes in the oxygen that’s available.

We normally expect the amount of air inhaled and exhaled out of the lungs during breathing to be the same. However, this does not occur, as some amount of air is not exhaled and still remains in the lungs, which is referred to as functional residual capacity (FRC). In essence, FRC represents the volume of air that is left behind after expelling it out of the lungs at the end of respiration. Thus, even after exhalation, a small amount of air still lingers in the air sacs or the alveoli. FRC along with other parameters are useful to evaluate the lung capacity.

Parameters Explained

FRC is the volume of air that does not leave the lungs after expiration, and is often expressed as the total of expiratory reserve volume (ERV) plus residual volume (RV).
✦ After breathing out, you may further exhale air with your full capacity. This extra air that comes out of the lungs after completing exhalation is ERV.
✦ However, even after breathing out with your maximum capacity, a miniscule amount of air may continue to stay in the lungs, which is called RV.

Helium Dilution Technique

✦ The helium dilution technique is often used to calculate FRC of the lungs. In this method, a combination of oxygen and helium (He) is introduced into a spirometer and allowed to fill to its full capacity. The quantity of He present in the spirometer is calculated by the following formula:

Concentration x Volume = Amount

✦ The concentration (C1) of He is recorded before the start of the test. The test begins with the person inhaling and exhaling the mixture. Upon inhalation, helium gets into the lungs, and its concentration level (C2) is again recorded using the spirometer.

✦ As the test is carried out in a closed environment, the quantity of helium used remains unchanged throughout the test. Hence, the formula that can be used to compute FRC is as follows:

C1 x V1 = C2 x V2

Here, V2 is the total volume of helium gas used in the test. Now, this is equal to the volume of spirometer (V1) plus FRC, the volume of air that does not leave the air after exhalation. So, the equation changes to:

C1 x V1 = C2 x (V1 + FRC)

((C1 x V1)/C2) = V1 + FRC

FRC = ((C1 x V1)/C2) – V1

FRC in Lung Diseases

Patients diagnosed with COPD, asthma, chronic bronchitis, and emphysema tend to have a higher FRC. Due to the impaired lung capacity, a fair amount of air remains inside the lungs after exhalation, resulting in greater FRC. In these conditions, air passage is significantly constricted, causing a substantial amount of inhaled air to be trapped inside the lungs. On the other hand, obesity that changes the lung mechanics as well as alters airway resistance, however, leads to lower FRC.

In general, patients with an increase or decrease in FRC are likely to have an impaired lung function. Also, postural changes in healthy individuals alter the FRC. For instance, it increases when in a sitting position, whereas it decreases in a lying position. However, in highly obese people, FRC does not alter with body posture.

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