Reference — Pulmonary Function Testing
Lung Volume Measurement Methods
How the lung volumes spirometry can’t reach are measured — body plethysmography, helium dilution, and nitrogen washout for FRC, RV, and TLC. Each section covers what the method captures, the principle it relies on, and where it falls short.
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
Spirometry measures the volumes a patient can move at the mouth, but it cannot measure the gas that stays in the lungs — residual volume (RV), and therefore functional residual capacity (FRC) and total lung capacity (TLC). Three methods measure these “trapped” volumes, and they do not always agree.
The Three Methods
| Method | Principle | Measures | Limitation |
|---|---|---|---|
| Body plethysmography | Boyle's law (P1V1 = P2V2): the patient pants against a closed shutter in a sealed box, and the pressure-volume changes give the thoracic gas volume | FRC (thoracic gas volume), then RV and TLC | Measures ALL intrathoracic gas, including trapped and non-communicating gas and bullae — so it can read higher than the dilution methods. |
| Helium dilution (closed circuit) | A known helium concentration is rebreathed until it equilibrates, and the dilution gives the lung volume | FRC, RV, TLC | Measures only gas that COMMUNICATES with open airways — it underestimates volume with air trapping, bullae, or severe obstruction. |
| Nitrogen washout (open circuit) | The patient breathes 100% O₂, and the nitrogen washed out of the lungs is collected; its volume back-calculates FRC | FRC, RV, TLC | Also misses non-communicating gas and can underestimate in severe obstruction. |
Clinical Notes
- In obstructive disease with air trapping, body plethysmography reads a HIGHER TLC and RV than helium dilution or nitrogen washout — the gap itself suggests trapped gas.
- The dilution and washout methods measure only communicating gas; plethysmography measures all intrathoracic gas (Boyle’s law).
- TLC is the value that confirms restriction (reduced) or hyperinflation (increased), which spirometry cannot provide.
- A single-breath helium dilution is also performed during the DLCO maneuver to estimate the alveolar volume (VA).
Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Lung volume measurement chapters.
- Wanger J, Clausen JL, Coates A, et al. Standardisation of the measurement of lung volumes. Eur Respir J. 2005;26(3):511-522.