Spirometry and Lung Volumes

Spirometry measures how much air a person can move and how fast during a maximal breathing maneuver, while lung-volume measurement captures the static compartments of air the lungs hold. Together they form the foundation of pulmonary function testing, distinguishing obstructive from restrictive patterns of impairment.

Definition

Spirometry is the measurement of inspired and expired air volume and flow as a function of time during forced breathing maneuvers, yielding the forced vital capacity (FVC), forced expiratory volume in one second (FEV1), and their ratio; lung-volume measurement quantifies the static volumes - total lung capacity, residual volume, and functional residual capacity - that spirometry alone cannot capture.

Scope

This topic covers the forced expiratory maneuver and its key indices (FVC, FEV1, and the FEV1/FVC ratio), the static lung volumes and capacities (total lung capacity, residual volume, functional residual capacity) and the methods used to measure them, and the interpretation of these values against reference equations. It is a methodological reference, not clinical guidance.

Core questions

• What do FEV1, FVC, and the FEV1/FVC ratio represent physiologically?
• How are static lung volumes measured when residual volume cannot be exhaled?
• How are spirometric values interpreted as obstructive or restrictive?
• How do reference equations and the lower limit of normal define abnormal results?

Key concepts

• Forced vital capacity (FVC)
• Forced expiratory volume in 1 second (FEV1)
• FEV1/FVC ratio
• Total lung capacity (TLC) and residual volume (RV)
• Functional residual capacity (FRC)
• Body plethysmography and gas-dilution methods
• Flow-volume loop
• Lower limit of normal and reference equations

Mechanisms

During a forced expiration, expiratory flow becomes effort-independent once airways are dynamically compressed, so the maximal flow-volume relationship reflects the mechanical properties of the airways and lung recoil rather than effort. FEV1 and FVC summarize this maneuver; a reduced FEV1/FVC ratio indicates airflow obstruction, while proportionally reduced volumes with a preserved ratio suggest restriction. Because residual volume cannot be exhaled, static volumes are measured indirectly - by body plethysmography (using Boyle's law and pressure-volume changes in a sealed box) or by inert-gas dilution and washout. Total lung capacity is required to confirm a true restrictive defect, since spirometry alone can only suggest it (Miller 2005; Wanger 2005; Stanojevic 2022).

Clinical relevance

Spirometry is the reference test for classifying ventilatory impairment as obstructive or restrictive and for grading its severity against predicted values. Lung-volume measurement confirms restriction and characterizes hyperinflation and air trapping. Reading these results in context - against appropriate reference equations and quality criteria - is part of evidence appraisal in respiratory medicine. This entry describes measurement and interpretation in general terms and is not a basis for individual diagnosis or treatment.

Evidence & guidelines

Spirometry technique and acceptability are governed by the joint ATS/ERS standardization statements, originally the 2005 statement (Miller 2005) and the 2019 update (Graham 2019), with companion standards for lung-volume measurement (Wanger 2005). Interpretation follows the 2022 ATS/ERS interpretive-strategies standard (Stanojevic 2022), using Global Lung Function Initiative reference equations (Quanjer 2012) to define predicted values and the lower limit of normal.

History

John Hutchinson introduced the spirometer and the concept of vital capacity in the mid-nineteenth century. The forced expiratory volume and the flow-volume curve were developed in the mid-twentieth century, and from 1979 onward the American Thoracic Society and later the European Respiratory Society issued successive standardization statements (notably 2005 and 2019) that made spirometry reproducible across laboratories, complemented by global reference equations in 2012.

Debates

Fixed ratio versus lower limit of normal for defining obstruction

Using a fixed FEV1/FVC cutoff is simple but misclassifies older and younger people; statistically derived lower limits of normal from reference equations are favored in current standards, though the choice still affects prevalence estimates.

Key figures

• Martin R. Miller
• Brian L. Graham
• Peter H. Quanjer
• Sanja Stanojevic

Seminal works

• miller-2005
• graham-2019
• wanger-2005
• quanjer-2012

Frequently asked questions

What is the FEV1/FVC ratio used for?

It is the primary spirometric index of airflow obstruction: a ratio below the lower limit of normal indicates that air leaves the lungs too slowly relative to the total volume exhaled, the hallmark of an obstructive pattern.

Why can't spirometry alone diagnose a restrictive defect?

Spirometry measures only the air that can be exhaled, not the residual volume left in the lungs; a true restrictive defect is confirmed by measuring total lung capacity with plethysmography or gas-dilution methods.