From the Guidelines
In obstructive lung disease, the residual volume (RV) and functional residual capacity (FRC) will be elevated on body plethysmography, with the most significant finding being an increased ratio of residual volume to total lung capacity (RV/TLC ratio). These elevations occur due to air trapping caused by airway obstruction, which prevents complete exhalation. The RV/TLC ratio is considered the hallmark measurement of air trapping, as it reflects the degree of lung hyperinflation and airway obstruction. This occurs because in obstructive diseases like COPD, asthma, or bronchiectasis, narrowed airways collapse during exhalation, trapping air in the lungs and preventing normal emptying, as noted in a study published in the European Respiratory Journal 1.
Body plethysmography is particularly valuable for detecting this air trapping because it measures all gas in the thorax, including trapped air that spirometry might miss. Other plethysmography values that may be elevated include total lung capacity (TLC) and airway resistance (Raw), while specific airway conductance (sGaw) is typically decreased. These measurements help differentiate obstructive from restrictive lung diseases and quantify the severity of air trapping. According to a recent study published in 2025, the use of pre- and post-bronchodilator spirometry can help diagnose COPD and identify individuals with airflow obstruction 1.
Some key points to consider when interpreting body plethysmography results in obstructive lung disease include:
- The RV/TLC ratio is a sensitive marker of air trapping and lung hyperinflation
- Elevated RV and FRC values indicate air trapping and obstructive lung disease
- TLC may be elevated in obstructive lung disease due to lung hyperinflation
- Raw may be elevated in obstructive lung disease due to airway obstruction
- sGaw may be decreased in obstructive lung disease due to airway obstruction.
Overall, body plethysmography is a valuable tool for diagnosing and monitoring obstructive lung disease, and the RV/TLC ratio is a key measurement for assessing the severity of air trapping and lung hyperinflation.
From the Research
Body Plethysmography Values in Obstructive Disease
- In obstructive disease, body plethysmography values such as residual volume (RV) and total lung capacity (TLC) are commonly elevated 2.
- RV is particularly increased in airway obstruction, and TLC may be normal or increased in the late stages of chronic obstructive pulmonary disease (COPD) 2.
- Functional residual capacity (FRC) can also be increased in obstructive disease, and body plethysmography is considered more accurate than gas dilution methods in measuring FRC in patients with airway obstruction 3.
- The use of body plethysmography can detect COPD earlier, and parameters such as inspiratory capacity (IC), specific airway resistance (sGawtot), and specific airway conductance (sGaweff) are sensitive enough to detect differences between smokers without COPD and healthy controls 4.
- Body plethysmography allows for the assessment of functional residual capacity (FRC), specific airway resistance (sRaw), total lung capacity (TLC), and residual volume (RV), providing significant additional information compared to spirometry 5.
Comparison of Lung Volumes Measured by Body Plethysmography and Computed Tomography
- Computed tomography (CT) quantifications routinely underestimate TLC and overestimate RV in comparison to body plethysmography 6.
- The difference between CT-derived TLC and plethysmography TLC is larger in obstructive patients, with a mean difference of 781 mL 6.
- Spirometry-gating for CT acquisition can reduce the level of disagreement between CT and body plethysmography, and may be of assistance when patients are already undergoing CT 6.