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    91 Functional residual capacity (FRC)
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    91 Functional residual capacity (FRC)

    Functional residual capacity: lung volume after normal expiration, measurement, hyperinflation and restriction.

    Functional residual capacity (FRC) is the volume of air left in the lungs at the end of a normal, relaxed exhalation. It is the sum of the expiratory reserve volume and the residual volume. At FRC the inward elastic recoil of the lungs and the outward recoil of the chest wall are in balance, so no muscle effort is needed. FRC acts as an oxygen reservoir between breaths and keeps small airways and alveoli open. It cannot be measured by simple spirometry, because it includes the residual volume, which cannot be exhaled.

    Lung volumesHyperinflationRestriction
    Typical adult≈ 2–3 L
    FormulaFRC = ERV + RV
    Measured bybody plethysmography, gas dilution or washout
    Changes withposture, obesity, lung disease
    Spirogram of lung volumes: tidal breathing, a maximal inspiration and a maximal expiration, with tidal volume, inspiratory and expiratory reserve volumes, residual volume, vital capacity, functional residual capacity and total lung capacity Lung volumes and capacities (typical adult) TV IRV ERV RV VC FRC TLC RV, FRC and TLC cannot be measured by simple spirometry; they need body plethysmography or gas dilution.
    Figure 1. Lung volumes on a spirogram. TV tidal volume, IRV/ERV inspiratory/expiratory reserve volume, RV residual volume, VC vital capacity, FRC functional residual capacity, TLC total lung capacity. Typical adult values: TV ≈ 0.5 L, RV ≈ 1.2 L, FRC ≈ 2.4 L, VC ≈ 4.8 L, TLC ≈ 6 L.

    Measurement

    FRC is measured by body plethysmography (which also measures gas trapped behind closed airways), by helium dilution or by nitrogen washout. In obstructive disease, gas dilution methods underestimate FRC because they reach only ventilated lung regions. FRC falls when lying down, under anaesthesia and with obesity.

    Clinical significance

    Low FRC

    Pulmonary fibrosis and other restrictive diseases, obesity, pregnancy, abdominal distension, lying flat, general anaesthesia, atelectasis. Low FRC reduces the oxygen reserve, so oxygen saturation falls quickly during breath-holding or apnoea.

    High FRC (hyperinflation)

    COPD and emphysema, severe asthma attacks: airflow obstruction and loss of lung elasticity trap air. Hyperinflation flattens the diaphragm, increases the work of breathing and causes breathlessness, especially on exertion.

    When to see a doctor

    Chronic breathlessness, cough or wheezing, especially in smokers, should be investigated with spirometry and, if needed, lung volume measurement.

    References

    1. Wanger J, Clausen JL, Coates A, et al. Standardisation of the measurement of lung volumes. Eur Respir J. 2005;26(3):511–522. doi:10.1183/09031936.05.00035005
    2. Stanojevic S, Kaminsky DA, Miller MR, et al. ERS/ATS technical standard on interpretive strategies for routine lung function tests. Eur Respir J. 2022;60(1):2101499. doi:10.1183/13993003.01499-2021

    This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

    Published on 2 May 2024