The measurement of Functional Residual Capacity (FRC) is an essential parameter to evaluate static lung volumes and identify obstructive or restrictive disorders. Among the available techniques, body plethysmography — particularly the cabin method — remains the ATS/ERS (American Thoracic Society / European Respiratory Society) gold standard for lung volume assessment.
In recent years, new technologies have emerged that claim to measure FRC through non-cabin plethysmography, yet available studies show significant limitations:
- Tests were conducted only in healthy subjects.
- These systems do not directly measure lung volumes — they estimate them through algorithms.
- The ATS/ERS has not yet validated non-cabin plethysmography as a clinically reliable method.
This article analyzes why whole body plethysmography continues to be the reference method for accurate and reproducible lung volume measurement. It also discusses why, although the ATS/ERS acknowledges other alternatives — such as nitrogen washout and helium dilution techniques — body plethysmography remains the gold standard for precise and repeatable results.
By América Torres
What Body Plethysmography Measures and Its Relationship with FRC
Body plethysmography measures Functional Residual Capacity (FRC) — equivalent to Thoracic Gas Volume (TGV) at the end of a normal expiration. This test allows the confirmation and quantification of restriction, hyperinflation, and air trapping.
Parameters Measured: FRC, Airway Resistance, and Lung Volumes
Whole body plethysmography precisely measures changes in thoracic gas volume (TGV). Direct measurement of thoracic volume, instead of flow integration at the mouth, provides a more accurate characterization of pulmonary mechanical parameters in terms of lung volume.¹
Lung volume subdivisions include:
- Functional Residual Capacity (FRC). Gas volume present in the lungs at the end of a tidal expiration.
- Expiratory reserve volume (VRE). The volume of gas that can be exhaled maximally from the end-expiratory level during tidal breathing (i.e., from the FRC).
- Inspiratory Capacity (IC). Maximum gas volume inspired from FRC.
- Inspiratory Reserve Volume (IRV). Maximum gas volume inhaled beyond the tidal inspiration.
- Residual volume (RV). The volume of gas that remains in the lungs after peak expiration, regardless of the lung volume from which exhalation was initiated.
TGV determined by plethysmography is the reference standard for measuring absolute lung volumes, as it includes non-ventilated air spaces.¹
Why Body Plethysmography Remains the Gold Standard
Both plethysmographic resistance and forced oscillatory resistance are influenced by the subject’s spontaneous breathing pattern. Clinicians therefore need complementary measurements to accurately define the physiopathological degree of airflow obstruction.
Plethysmographic assessment of resistance requires two independent measurements:
- Specific airway resistance (sRaw)
- Thoracic gas volume (TGV)
Evaluating resistance in relation to lung volume provides a clearer understanding of small airway obstruction. Hence, the ATS/ERS recognizes whole body plethysmography as the clinical gold standard for airflow resistance measurement.¹
What Is Functional Residual Capacity (FRC) and Why Is It Important?
Functional Residual Capacity (FRC) is a key component in lung volume measurement. Currently, the ATS/ERS validates only four methods for its determination:²
- Whole body plethysmography
- Gas dilution techniques
- Washout methods (such as nitrogen washout)
- Chest radiography
Achieving a stable FRC during tidal breathing ensures accurate measurements of:
- Functional Residual Capacity (FRC)
- Expiratory Reserve Volume (ERV)
- Inspiratory Capacity (IC)
- Inspiratory Reserve Volume (IRV)
This condition guarantees precision and reproducibility in static lung volume assessments.
FRC Measurement by Whole Body Plethysmography
The most reliable method to obtain FRC is through body plethysmography. According to ATS/ERS, to validate the accuracy of this measurement, it is necessary to simulate isothermal pulmonary conditions, as described in the 2023 technical standardization document for lung volume measurement.²
During calculations, volumes must be adjusted to ambient temperature and humidity — not to BTPS (Body Temperature and Pressure Saturated) conditions. This level of control can only be achieved within a sealed body plethysmograph, where environmental pressure and humidity remain stable and monitored, unlike open systems.²
Limitations of Non-Cabin Methods for FRC Measurement
Although whole body plethysmography is the gold standard for lung volume measurement, it is not always available. Space constraints or budget limitations may lead laboratories to explore alternatives.
This situation has led to the introduction of non-cabin plethysmography systems, designed to reduce investment and optimize space. However, these methods still lack sufficient scientific evidence to support the same level of accuracy and reliability achieved by whole body plethysmography.
According to current ATS/ERS guidelines, validated FRC measurement techniques remain limited to body plethysmography, gas dilution, nitrogen washout, and radiographic methods.
Algorithm-Based Estimations Instead of Direct Measurement
Existing literature on non-cabin plethysmography remains limited and presents potential biases. One such study states:
“Unlike plethysmography, determination of TLC by the MiniBox is based on inductive statistics and pattern recognition algorithms that are used to infer parameters of a mathematical model automatically. The present study demonstrated that this approach can be applied to a highly heterogeneous population of participants, yielding accurate and reproducible determination of TLC as compared with the current gold standard plethysmography technique”.
In practical terms, this means that the device does not directly measure lung volumes, but rather estimates them using algorithms. While this approach can provide reproducible results under certain conditions, it is not a substitute for the direct measurement of absolute lung volumes provided by body plethysmography, which is considered the international reference standard.
It is worth mentioning that the study authors declared financial interests in the MiniBox manufacturing company, which should be considered when interpreting results.
Inconsistencies in FRC and TLC Using Non-Cabin Devices
The document “Comparison of the measurement of lung volumes with two different plethysmography techniques in a real-world environment”³ conducted at the National Institute of Respiratory Diseases (INER), Mexico (July–November 2023), compared traditional plethysmography (JAEGER® MasterScreen Body) with the MiniBox®.
The study was carried out at the INER (National Institute of Respiratory Diseases) in Mexico City, from July to November 2023.
- 91 healthy adults with normal spirometry, non-smokers, and without respiratory symptoms in the last four weeks participated.
- Participants had no persistent respiratory symptoms during the 12 months prior to the study.
- All tests were performed according to ATS/ERS standards and following the manufacturers’ instructions for both cabin and non-cabin plethysmography equipment.
- The tests were conducted by pulmonologists specialized in respiratory physiology.
- The equipment used included the JAEGER® MasterScreen Body and the MiniBox®. There was a 15-minute interval between tests.
- To assess the degree of concordance, the mean of differences, and the 95% limits of agreement between volume measurements, the concordance correlation coefficient (CCC) and Bland–Altman plot were used.
Results revealed inaccuracies in Total Lung Capacity (TLC) measurement using non-cabin devices, even in healthy adults with normal spirometry. The authors concluded that such discrepancies may impact diagnostic and prognostic accuracy, and emphasized that the 2025 ERS/ATS technical statement has not yet recommended these systems for clinical use until further validated research is available.
Whole Body Plethysmography: Reliable and Reproducible
The ATS/ERS recognizes body plethysmography as the most reliable and reproducible method for measuring absolute lung volumes. Its ability to maintain stable temperature, humidity, and pressure conditions ensures accurate and comparable results.
The SCHILLER-GANSHORN PowerCube Body+ system integrates the ATS/ERS-endorsed techniques for comprehensive lung volume assessment:
- Whole body plethysmography
- Pulmonary diffusion tests (DLCO)
Additionally, it includes spirometry, making the PowerCube Body+ a robust diagnostic tool that meets the latest ATS/ERS technical standards.
Experience the Precision of True Measurement. Request a free, no-obligation demonstration and discover how PowerCube Body+ delivers accurate and reproducible lung volume measurement.
Key Clinical Questions About Whole-Body Plethysmography and ATS/ERS Lung Volume Standards
What is body plethysmography and how does it measure Functional Residual Capacity (FRC)?
Whole-body plethysmography measures the volume of gas in the lungs by detecting pressure changes inside a sealed chamber. It calculates Functional Residual Capacity (FRC) from thoracic gas volume at the end of tidal expiration, providing accurate values even in patients with severe airway obstruction.
Why is whole-body plethysmography considered the gold standard for lung volume measurement?
According to ATS/ERS guidelines, whole-body plethysmography is the gold standard because it measures all intrathoracic gas, including trapped air, under controlled pressure and temperature conditions. This ensures highly reproducible and precise results compared to indirect dilution or washout methods.
What parameters can be obtained from a body plethysmography test?
Body plethysmography provides several parameters: Functional Residual Capacity (FRC), Residual Volume (RV), Total Lung Capacity (TLC), airway resistance (Raw), and specific airway conductance (sGaw). These values are essential for diagnosing and differentiating obstructive from restrictive ventilatory defects.
Which ATS/ERS 2023 guidelines apply to body plethysmography?
The 2023 ATS/ERS technical statement outlines standardized procedures for lung volume measurement, calibration, and data interpretation. It confirms whole-body plethysmography as the reference method for determining absolute lung volumes and airway resistance in clinical pulmonary function testing.
Why are non-cabin plethysmography systems not yet as reliable as whole-body plethysmography?
Non-cabin plethysmography devices estimate lung volumes using mathematical models and flow-pressure algorithms instead of direct measurement of thoracic gas volume. Because they cannot replicate the stable pressure and humidity conditions achieved inside a sealed body box, their results often underestimate total lung capacity (TLC) and Functional Residual Capacity (FRC). Current evidence and ATS/ERS 2023 guidelines indicate that these systems still lack sufficient clinical validation for routine diagnostic use.
REFERENCES
[1] M.D. Goldman H.J. Smith W.T. Ulmer. Whole-body plethysmography . ERS Publications. Page: 15-43 DOI: https://doi.org/10.1183/1025448x.00031002
[2] Bhakta NR, McGowan A, Ramsey KA, et al. European Respiratory Society/American Thoracic Society technical statement: standardisation of the measurement of lung volumes, 2023 update. Eur Respir J 2023; 62: 2201519 DOI: 10.1183/13993003.01519-2022
[3] Benítez-Pérez Rosaura Esperanza et. al. Comparison of the measurement of lung volumes with two different plethysmography techniques in a real-world environment. Instituto nacional de Enfermedades respiratorias (INER) Ismael Cosío Villegas. European Respiratory Journal 2024 64(suppl 68): PA2599; DOI: https://doi.org/10.1183/13993003.congress-2024.PA2599