Why Residual Volume Cannot Be Measured by Spirometry
Spirometry can only measure volumes of air that move in and out through the mouth, but residual volume (RV) is the air that remains trapped in the lungs after maximal exhalation and therefore cannot be exhaled or detected by a spirometer. 1
The Fundamental Limitation
Spirometry measures dynamic lung volumes by recording air flow at the mouth during breathing maneuvers. The technique captures:
- Vital capacity (VC): the maximal volume exhaled from full inspiration to complete expiration 1
- Inspiratory capacity (IC): volume from end-tidal expiration to maximal inspiration 1
- Forced expiratory volumes and flows 1
However, RV represents the volume of gas remaining in the lungs after a maximal expiratory effort to the point where no more air can be voluntarily expelled 1. Since this air never exits the lungs and passes through the spirometer's mouthpiece, it is physically impossible for spirometry to directly measure it 2.
What Spirometry Actually Measures
The European Respiratory Society guidelines clearly delineate that during vital capacity maneuvers, subjects exhale "completely to RV" and then inhale to total lung capacity (TLC) 1. The spirometer records the volume change between these two points (the VC), but it cannot measure the absolute lung volume at either endpoint. The spirometer has no reference point to determine how much air remains in the lungs after maximal exhalation 1.
Alternative Methods Required
To measure RV and other static lung volumes (TLC, functional residual capacity), you must use techniques that can assess the total gas volume within the thorax:
- Body plethysmography: measures thoracic gas volume by detecting pressure-volume relationships during breathing against a closed shutter 1, 3
- Gas dilution techniques (helium dilution or nitrogen washout): calculate lung volumes based on dilution of an inert gas 4, 5
- Radiographic methods (CT scan with spirometric gating): directly visualize and calculate lung volumes 5
These methods are essential because RV measurement is clinically important for detecting air trapping, emphysema, and distinguishing obstructive from restrictive patterns 1, 6.
Clinical Implications
The inability to measure RV by spirometry means:
- You cannot calculate TLC from spirometry alone (TLC = VC + RV) 1, 2
- You cannot definitively diagnose restrictive lung disease with spirometry alone, as this requires demonstrating reduced TLC 1
- Air trapping and hyperinflation cannot be quantified without measuring RV 1, 6
- Isolated elevation in RV (which occurs in airway diseases like asthma, bronchiectasis, and emphysema) will be completely missed if only spirometry is performed 6
Common Pitfall
A reduced VC on spirometry does not prove restrictive lung disease—it is associated with low TLC only about half the time 1. The pattern may instead reflect submaximal effort, inability to exhale completely to RV due to weakness or coordination problems, or obstructive disease with air trapping 1. Therefore, when spirometry suggests restriction (reduced VC with normal or elevated FEV₁/VC ratio), you must proceed to body plethysmography or other lung volume measurement to confirm the diagnosis 1, 2.