When evaluating lung function, some diagnostic tests go beyond the basic measurement of airflow. One of the most advanced and accurate methods is body plethysmography, a technique that allows clinicians to measure lung volumes that cannot be assessed with routine spirometry alone.
For respiratory therapists, understanding this test is critical because it provides a complete picture of a patient’s pulmonary status, helping guide diagnosis, treatment, and long-term care planning.
What is Body Plethysmography?
Body plethysmography is a pulmonary function test (PFT) that measures static lung volumes, including the total lung capacity (TLC), residual volume (RV), and functional residual capacity (FRC). Unlike spirometry, which primarily measures dynamic airflow, plethysmography captures volumes that remain in the lungs after a full exhalation, giving a more precise assessment of air trapping and lung mechanics.
During the test, the patient sits in an airtight chamber, often referred to as a “body box.” The patient breathes through a mouthpiece connected to a flow sensor while the chamber records changes in pressure. Based on Boyle’s Law, which relates pressure and volume, the system calculates lung volumes with high accuracy, even those that cannot be measured directly.
Why is Body Plethysmography Important?
For respiratory therapists and pulmonologists, this test is especially valuable because it provides information that other tests cannot. Its clinical significance includes:
- Diagnosis of Obstructive Lung Diseases: In conditions such as COPD and asthma, plethysmography reveals the extent of air trapping and hyperinflation. These measurements are crucial for assessing disease severity and progression.
- Evaluation of Restrictive Lung Diseases: In disorders such as pulmonary fibrosis or neuromuscular diseases, plethysmography helps confirm whether a reduced lung volume is due to restriction rather than obstruction.
- Pre- and Post-Treatment Assessment: By comparing results before and after therapeutic interventions (e.g., bronchodilators), respiratory therapists can determine the effectiveness of treatment strategies.
- Preoperative Evaluation: Patients undergoing major surgery, especially thoracic or abdominal procedures, often require a detailed assessment of lung function to identify potential risks.
Relevance to Respiratory Therapists
Respiratory therapists play a vital role in administering, interpreting, and educating patients about plethysmography:
- Administration: Ensuring proper patient positioning, maintaining a good seal around the mouthpiece, and guiding patients through the test maneuvers.
- Interpretation Support: While pulmonologists finalize the results, therapists provide key input by recognizing patterns consistent with obstructive versus restrictive physiology.
- Patient Education: Therapists help patients understand why the test is performed, how it contributes to their diagnosis, and how the results impact their care plan.
- Clinical Decision-Making: The results influence ventilatory support strategies, medication choices, and rehabilitation programs—all areas where therapists are directly involved.
What is Boyle’s Law?
Boyle’s Law is a fundamental principle in physics that describes the relationship between the pressure and volume of a gas. It states that: At a constant temperature, the pressure of a gas is inversely proportional to its volume.
In other words, when the volume of a gas decreases, its pressure increases, and when the volume increases, its pressure decreases, provided the temperature remains unchanged.
Mathematically, it is expressed as:
P × V = constant
where
- P is pressure
- V is volume
This law is highly relevant in respiratory care. For example, during breathing, changes in thoracic volume lead to changes in pressure, allowing air to move in and out of the lungs. It’s also the principle behind body plethysmography, where lung volumes are calculated by measuring pressure changes inside a sealed chamber.
Plethysmography vs. Gas Dilution Techniques
Plethysmography provides a unique advantage over other pulmonary function test methods because it measures the entire volume of the chest. In contrast, gas dilution techniques such as helium dilution and nitrogen washout only measure the portion of the lung that is in communication with the outside air.
This means that areas of the lung that are poorly ventilated or isolated from airflow may not be captured by dilution methods. As a result, plethysmography is often considered a more accurate way to determine the functional residual capacity (FRC), particularly in patients with conditions like bullous lung disease or emphysema.
In these cases, parts of the lung may remain trapped with air and not communicate effectively with inhaled gases. Plethysmography accounts for these trapped air spaces, giving clinicians a more precise picture of lung volumes and disease severity.
Limitations of Body Plethysmography
Although highly accurate, body plethysmography does have some limitations. It requires specialized equipment, which may not be available in all settings, and it can be intimidating for patients who feel claustrophobic. Additionally, the accuracy of results depends heavily on proper technique and patient cooperation.
Body Plethysmography Practice Questions
1. What does body plethysmography primarily measure?
Thoracic gas volume (TGV), total lung capacity (TLC), airway resistance, and changes in volume within the lungs
2. Which physical law is body plethysmography based on?
Boyle’s Law
3. What does Boyle’s Law state?
Pressure and volume vary inversely, provided the temperature remains constant
4. What are the main components of a body plethysmograph?
Body box, pneumotachometer, and mouthpiece with a pressure transducer and shutter valve
5. What is the function of the body box in a plethysmograph?
It is airtight and measures pressure and volume changes during breathing maneuvers to calculate lung volumes
6. What is the purpose of the pneumotachometer in plethysmography?
To measure airflow at the mouth
7. What is the role of the shutter in the mouthpiece during plethysmography testing?
It briefly occludes airflow to measure pressure changes and calculate alveolar pressure
8. What happens during the Vtg (thoracic gas volume) test?
The patient pants against a closed shutter, and pressure and volume changes are recorded
9. How is thoracic gas volume (Vtg) calculated using Boyle’s Law?
By using the formula P1 × V1 = P2 × V2, assuming temperature remains constant
10. What is typically measured alongside Vtg during plethysmography?
Functional Residual Capacity (FRC), Residual Volume (RV), and airway resistance
11. What is the significance of measuring FRC via plethysmography?
It provides the most accurate measurement of FRC, especially in patients with obstructive lung disease
12. What lung condition is indicated by a large pressure difference between mouth and alveoli during testing?
Asthma
13. How is airway resistance calculated during plethysmography?
By dividing the pressure difference (ΔP) by the flow (ΔP/Flow)
14. What are the units of measurement for airway resistance?
cmH₂O/L/sec
15. What is the normal range for airway resistance?
0.6 to 2.4 cmH₂O/L/sec
16. What causes resistance in the airways?
Friction between gas molecules and the walls of the airways
17. What is total thoracic gas volume?
The entire volume of gas in the thoracic cavity, including areas distal to airway obstructions
18. What is the primary purpose of the pressure transducer in the mouthpiece?
To measure pressure at the mouth for calculating ΔP
19. How is flow measured in plethysmography?
By the pneumotachometer located at the mouthpiece
20. What does the patient do during a plethysmography test?
Breathes through a mouthpiece in an enclosed chamber while performing specific breathing maneuvers
21. Why is low-frequency panting used during the FRC portion of the test?
To ensure minimal difference between mouth and alveolar pressure for accurate pressure recordings
22. How is the volume of gas surrounding the patient inside the box determined?
By subtracting the patient’s body volume from the total box volume
23. How can the patient’s body volume be determined during plethysmography?
By calculating the displacement of air inside the sealed body box
24. How is airway obstruction simulated during the test?
By closing the shutter valve at the mouthpiece while the patient continues to pant
25. What are some challenges of body plethysmography testing?
Claustrophobia, difficulty performing panting maneuvers, and overestimation of volumes in pneumothorax
26. What is a potential issue with plethysmography in patients with pneumothorax?
It may overestimate lung volumes due to trapped gas in the pleural space
27. What extra-pulmonary condition can be evaluated using plethysmography?
Pneumothorax or air trapping in obstructive lung diseases
28. What plethysmographic measurement correlates well with radiographic lung volume studies?
Total Lung Capacity (TLC)
29. What can be calculated when radiologic estimates of TLC and FRC are available?
Residual Volume (RV)
30. How are radiologic estimations of lung volume typically obtained?
From standard posteroanterior and lateral chest X-rays
31. What is the standard source-to-film distance used for chest X-rays in lung volume estimation?
72 inches (183 cm)
32. What are the two primary radiographic methods used to estimate lung volumes?
Ellipsoid method and planimetry method
33. What is the ellipsoid method in radiologic lung volume estimation?
A technique that assesses the elliptical shape of the lungs and calculates volume based on geometrical segments like cylinders
34. What is the planimetry method in radiologic lung volume estimation?
A method that uses regression equations derived from surface area measurements of lung fields based on plethysmographic data
35. Why might radiologic methods be more accurate than gas dilution methods in certain patients?
They better estimate lung volumes in cases of severe air trapping
36. Besides plethysmography, what other techniques can be used to measure lung volumes and capacities?
Nitrogen washout and helium dilution
37. How does Boyle’s Law apply to plethysmographic lung volume measurements?
It relates pressure and volume changes during breathing to calculate thoracic gas volume using P1 × V1 = P2 × V2
38. What is the capacity being measured when using ∆P/∆V during closed-shutter panting?
Functional Residual Capacity (FRC), also known as thoracic gas volume (Vtg)
39. What is measured during open-shutter panting in body plethysmography?
Airway resistance (Raw)
40. What is measured during closed-shutter panting in body plethysmography?
Thoracic gas volume (Vtg) and specific airway conductance (sGaw)
41. During closed-shutter panting, what does mouth pressure represent?
Alveolar pressure, because no airflow means Pmouth = Palv
42. What are the acceptability criteria for FRCpleth measurements?
Closed-loop tracing, panting rate between 0.5–1 Hz, no off-screen drift, and at least three repeatable maneuvers agreeing within 5%
43. What are some advantages of plethysmography compared to other lung volume measurement methods?
Fast, accurate, includes trapped gas, and can be combined with spirometry
44. What does the FRCpleth/FRCdilution ratio indicate, and what is the normal value?
It’s an index of air trapping; a normal value is 1.0
45. What does an FRCpleth/FRCdilution ratio greater than 1.0 suggest?
Obstructive disease with air trapping not measurable by dilution methods
46. Is the FRCpleth measurement dependent or independent of patient effort?
It is effort-independent
47. How is air trapping reflected in RV, VC, and TLC values?
RV increases, VC decreases, TLC remains unchanged
48. How is hyperinflation reflected in RV, VC, and TLC values?
RV increases, VC remains stable, and TLC increases
49. What physical law forms the basis of body plethysmography?
Boyle’s Law
50. What does Boyle’s Law state in the context of respiratory measurements?
Pressure and volume vary inversely if temperature remains constant
51. What three key measurements are obtained from body plethysmography?
Thoracic gas volume (Vtg), functional residual capacity (FRC), and airway resistance (Raw)
52. Why is FRC measured by plethysmography typically higher than by gas dilution methods?
It accounts for all gas, including that trapped behind airway obstructions
53. What is the purpose of the shutter in plethysmographic testing?
To eliminate airflow and resistance effects, allowing accurate pressure measurements
54. What will a malfunctioning box pressure transducer affect during plethysmography testing?
It will affect thoracic gas volume (FRC) and airway resistance, but not lung compliance or spirometry volumes
55. Why does FRC measured by the body box differ from helium dilution or nitrogen washout methods?
Because the body box measures trapped gas, resulting in a higher FRC value
56. What is a key advantage of using the body box to measure FRC in patients with obstructive lung disease?
It provides a more accurate assessment by including trapped gas that other methods miss
57. What are some disadvantages of using a body plethysmograph?
Patient size limitations, claustrophobia, and difficulty panting effectively during testing
58. What is the purpose of the vent in the body box?
It allows heat to escape, preventing temperature-induced pressure changes that could affect readings
59. What three parameters must be calibrated in a body plethysmograph?
Mouth pressure, airflow (flow sensor), and box pressure
60. How is mouth pressure calibrated in plethysmography?
By verifying against a mercury or water column barometer
61. How are flow measurements calibrated in the body box?
Using a rotameter
62. How is box pressure calibrated in the body box?
With a sine wave rotary pump
63. Which lung volume measurement method is considered the most accurate for determining FRC?
Body plethysmography
64. What is thoracic gas volume (Vtg)?
It is the total volume of gas in the thorax, including trapped gas, measured at end-expiration
65. How is Vtg measured during body plethysmography?
By having the patient pant against a closed shutter at end-expiration using Boyle’s Law
66. Are residual volume (RV) and total lung capacity (TLC) measured or calculated?
They are calculated after measuring FRC and vital capacity
67. In what units is FRCpleth typically reported?
Liters, in BTPS (Body Temperature, Pressure, Saturated) conditions
68. What does the body box measure that gas dilution methods do not?
All gas in the lungs, including trapped gas behind obstructed airways
69. Is FRC measured directly or calculated in body plethysmography?
It is measured directly
70. Is expiratory reserve volume (ERV) measured or calculated?
Calculated by subtracting residual volume from FRC
71. What is the gold standard for diagnosing restrictive lung disease?
Body plethysmography
72. What are the indications for body plethysmography testing?
Diagnosing restrictive disease, distinguishing between obstructive/restrictive processes, evaluating Raw, bronchodilator response, methacholine challenge, and monitoring disease progression
73. What are contraindications for body box testing?
Claustrophobia, continuous oxygen therapy, mental confusion, muscular incoordination, or incompatible medical equipment
74. What does FRCpleth represent in plethysmographic testing?
The volume of gas in the lungs at FRC, including trapped gas, determined using Boyle’s Law during closed-shutter panting
75. According to Boyle’s Law, what happens to the volume of gas in a container if temperature remains constant?
The gas volume varies inversely with changes in pressure
76. What is the formula used to calculate thoracic gas volume (VTG)?
VTG = (Alveolar pressure × Change in box volume) / Change in alveolar pressure
77. What is the operating principle behind body plethysmography?
A patient panting against a closed shutter causes pressure changes that reflect thoracic gas volume, based on Boyle’s Law
78. During tidal breathing in the body box, what must the patient use?
A mouthpiece, with the nose clipped closed
79. How long should a patient sit in the body box to reach thermal equilibrium?
30 to 60 seconds
80. What type of sensors does the body box use to measure mouth and box pressure?
Pressure transducers in a constant-volume system
81. In body plethysmography, what does “Hz” represent?
The frequency of panting, or breaths per second
82. What is considered a quick and accurate method for measuring lung volumes?
FRCpleth (plethysmographic measurement of functional residual capacity)
83. What is one potential issue a patient may experience during a body box test?
Claustrophobia
84. What should the respiratory therapist do before testing begins in the body box?
Demonstrate the panting maneuver and have the patient practice it with the door open
85. Are FRCpleth values typically higher than those obtained from nitrogen washout or helium dilution?
Yes, especially in patients with air trapping
86. What steps are involved in measuring VTG using the body box?
Demonstrate panting, practice with door open, close door, establish FRC, begin panting, close shutter, obtain 3–5 trials, then perform SVC maneuver
87. How many acceptable panting trials are required for FRCpleth?
Three to five acceptable trials
88. How should patients be instructed to breathe during plethysmography testing?
Hands on cheeks, breathing normally at end-expiration (FRC)
89. What is thermal drift in plethysmography?
It is a temperature-induced artifact caused by not allowing the box to stabilize before testing
90. What is the frequency range for acceptable panting maneuvers in plethysmography?
Between 0.5 and 1.0 Hz
91. What are the acceptability criteria for FRCpleth measurements?
Closed loop without drift, correct frequency, no off-screen tracing, 3 repeatable trials within 5%, and average of those reported
92. According to ATS recommendations, what is the preferred sequence for measuring related lung volumes?
Measure ERV immediately after FRC, followed by a linked slow inspiratory VC maneuver
93. How is residual volume (RV) reported during lung volume testing?
RV = FRC – ERV
94. How is total lung capacity (TLC) reported during body plethysmography?
TLC = RV + the largest acceptable inspiratory vital capacity (VC)
95. What is the ATS-recommended method for lung volume computation?
The linked maneuver method (FRC, followed by ERV and IVC in sequence)
96. What does it indicate if the total lung capacity (TLC) is below the lower limit of normal (<80% predicted)?
It suggests the presence of restrictive lung disease
97. What condition should be suspected if the total lung capacity (TLC) is greater than 120% of predicted?
Hyperinflation
98. What does a reduced vital capacity (VC) with a normal or elevated FEV1/FVC ratio indicate?
A restrictive ventilatory defect
99. What does an increased RV/TLC ratio with a normal TLC suggest?
Air trapping
100. What does a high RV/TLC ratio along with an increased TLC indicate?
Hyperinflation due to gas trapping in obstructive lung disease
Final Thoughts
Body plethysmography is one of the most comprehensive pulmonary function tests available. For respiratory therapists, it provides critical insights into lung mechanics, helping distinguish between obstructive and restrictive conditions, guiding therapy, and improving patient outcomes.
By mastering this test, therapists strengthen their role as essential members of the healthcare team, ensuring that patients receive precise, evidence-based respiratory care.
Written by:
John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.
References
- Sharma P, Sankari A. Body Plethysmography. [Updated 2025 Jun 2]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025.