Functional Residual Capacity Calculator

How does the functional residual capacity calculator work?

This health tool computes the sum between the expiratory reserve volume and the residual volume in order to retrieve the functional residual capacity. This is one of the four lung capacities, along the vital capacity, inspiratory capacity and total lung capacity.

It can be defined as the volume of air that remains in the lungs at the end of passive expiration (after a normal expiration).

The functional residual capacity calculator requires the following pulmonary volumes, extracted from the spirometer during the ventilator pulmonary function test:

■ Expiratory reserve volume – represents the volume of air that can be exhaled during a forced expiration and after the normal expiration. Average values are 1.2 L for males and 0.7 L for females.

■ Residual volume – represents the volume of air that remains in the lungs after a forced expiration and is more complex to determine, being measured indirectly. Average value is around 1.2 L for males and 1.1 L for females.

Functional residual capacity = Expiratory reserve volume + Residual volume

In short: FRC = ERV + RV

The average value for FRC is around 2.4 L for males and 1.8 L for females. Abnormal FRC are most likely indicative of the presence of some kind of respiratory condition. Elevated FRC is met in emphysema while abnormally low FRC values are met in obese patients due to the thoracic pressure of the fat tissue.

Residual volume can be measured through one of the following methods:

■ Radiographic planimetry;

■ Pulmonary plethysmography;

■ Closed circuit helium dilution;

■ Nitrogen washout.

There are also estimates available, computing RV from vital capacity: as 0.24 of VC for males and 0.28 for females.

Example of a calculation

Let’s take the following example:

■ Expiratory reserve volume (ERV) = 1.3 L;

■ Residual volume (RV) = 1.2.

Functional residual capacity (FRC) = ERV + RV = 1.3 L + 1.2 L = 2.5 L

References

1) Crapo RO, Morris AH, Gardner RM. (1981) Reference Spirometric Values using Techniques and Equipment that meet ATS recommendations. American Review of Respiratory Disease, Volume 123, pp.659-664.

2) Gommers D. (2014) Functional residual capacity and absolute lung volume. Curr Opin Crit Care; 20(3):347-51.

3) Quanjer PH, Tammeling GJ, Cotes JE, Pedersen OF, Peslin R, Yernault JC. (1993) Lung volumes and forced ventilatory flows. Report Working Party Standardization of Lung Function Tests, European Community for Steel and Coal. Official Statement of the European Respiratory Society. Eur Respir J Suppl; 16:5-40.