Plethysmography vs. Pulmonary Volumes: Understanding the Distinction
Plethysmography is a measurement technique (a method), while pulmonary volumes are the actual physiological parameters (the measurements) that can be obtained using various techniques, including plethysmography. 1
What Plethysmography Is
Plethysmography refers specifically to whole-body plethysmography, which is a diagnostic tool—essentially a sealed chamber where the patient sits while breathing. 1 This technique:
- Measures lung volumes by detecting pressure changes inside a rigid, closed container as the patient breathes or makes respiratory efforts against an occluded airway 1
- Calculates functional residual capacity (FRC) and airway resistance (Raw) simultaneously by relating changes in alveolar pressure to changes in alveolar volume 1
- Works by having the patient make respiratory efforts against a briefly closed shutter, which compresses and rarefies thoracic gas, allowing calculation of total thoracic gas volume 1
What Pulmonary Volumes Are
Pulmonary volumes are the actual physiological measurements of air in the lungs at different phases of breathing. 2 These include:
- Total lung capacity (TLC): the maximum volume of air the lungs can hold 3, 4
- Functional residual capacity (FRC): the volume of air remaining in the lungs at the end of a normal, passive expiration, representing the equilibrium point where lung elastic recoil inward balances chest wall elastic recoil outward 2
- Residual volume (RV): the volume remaining after maximal expiration 4, 5
- Tidal volume: the volume of air moved during normal breathing 6
How They Relate
Plethysmography is one of several methods available to measure pulmonary volumes. 1 Other measurement techniques include:
- Helium dilution method: uses inert gas to calculate lung volumes, but may underestimate volumes in patients with air trapping 3, 7, 5
- Nitrogen washout technique: similar principle to helium dilution 7
- CT imaging: can quantify lung volumes radiographically 3, 4
- Pneumotachography with flow sensors: primarily for tidal volume measurement 6
Critical Clinical Distinction
The key advantage of plethysmography over gas dilution methods is that it measures ALL gas in the thorax, including trapped or poorly ventilated areas, making it the preferred reference standard for measuring static lung volumes, especially in obstructive lung disease. 1, 8
Important Caveats:
- Plethysmography systematically overestimates TLC compared to helium dilution or CT imaging in patients with airflow obstruction, with the greatest overestimation occurring when FEV₁ is <30% predicted 3
- The discrepancy between plethysmography and helium dilution measurements increases with COPD severity and correlates negatively with FEV₁% predicted 5
- Despite this overestimation, plethysmography remains the clinical standard because it captures trapped gas that other methods miss 4, 8
In summary: plethysmography is the "how" (the measurement device and technique), while pulmonary volumes are the "what" (the actual lung capacity parameters being measured). 1