Functional Residual Capacity Calculator

Variables and formula

The functional residual capacity is one of the four lung capacities determined through respiratory tests. The other parameters are: the vital capacity, inspiratory capacity and total lung capacity.

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

There are two pulmonary volumes, measured via spirometer (ERV) or estimated (RV), required to calculate FRC:

The formula used by the above calculator is:

Functional Residual Capacity = Expiratory Reserve Volume + Residual Volume

In short: FRC = ERV + RV

About functional residual capacity

The average value for FRC is around 2.4 L for men and 1.8 L for women. There are also tables with predicted values dependent on patient age, gender and height.

Abnormal FRC values are in most cases indicative of a respiratory condition and should prompt further investigations.

Increases in FRC are met in emphysema whilst decreases in FRC are usually met in obese patients due to the thoracic pressure of the fat tissue.

As a result of increased FRC, total lung capacity also increases.

Residual volume, the estimated component of the FRC, can be measured through:

■ Pulmonary plethysmography;

■ Radiographic planimetry;

■ Closed circuit helium dilution;

■ Nitrogen washout.

References

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

2. Quanjer PH, Tammeling GJ, Cotes JE, Pedersen OF, Peslin R, Yernault JC. 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. 1993; 16:5-40.

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