Gas distribution tests are specialized pulmonary function tests designed to evaluate how evenly air is distributed throughout the lungs during breathing. While basic spirometry provides valuable information about airflow and lung volumes, it does not reveal how well inspired gas reaches different regions of the lungs.
Gas distribution testing fills this gap by identifying ventilation abnormalities that may occur early in obstructive lung disease.
For respiratory therapists, these tests offer critical insights into small airway function, ventilation efficiency, and disease progression, making them an important component of comprehensive pulmonary assessment.
What Are Gas Distribution Tests?
Gas distribution tests assess how uniformly inhaled air spreads throughout the lungs. Instead of focusing solely on how much air a patient can inhale or exhale, these tests examine where the air goes once it enters the respiratory system. Uneven gas distribution often indicates airflow obstruction, air trapping, or poor ventilation of certain lung regions.
These tests are particularly sensitive to abnormalities in the small airways, which may be affected long before changes appear on spirometry. Because of this, gas distribution testing is useful for detecting early lung disease, evaluating disease severity, and monitoring response to therapy.
Why Gas Distribution Matters
In healthy lungs, inspired air is distributed evenly to all ventilated alveoli. This uniform distribution allows for efficient gas exchange and optimal oxygenation. In disease states, however, narrowed airways, mucus plugging, inflammation, or structural changes can cause air to preferentially flow to some regions while bypassing others.
Poor gas distribution contributes to ventilation–perfusion mismatch, hypoxemia, increased work of breathing, and reduced exercise tolerance. Identifying these patterns helps clinicians understand why a patient may be symptomatic even when traditional lung volumes appear near normal.
Types of Gas Distribution Tests
There are three primary methods used to assess gas distribution and lung volumes:
- Helium dilution
- Nitrogen washout
- Body plethysmography
Note: Each method uses a different technique to measure lung volumes and has unique strengths and limitations.
Helium Dilution
The helium dilution method is a pulmonary function test commonly used to measure a patient’s functional residual capacity (FRC), which is the volume of air remaining in the lungs after a normal exhalation.
This test uses a spirometer filled with a known volume of air containing a small concentration of helium. Helium is an inert gas that does not cross the alveolar-capillary membrane, making it ideal for measuring lung volumes. The patient breathes through a mouthpiece connected to a closed rebreathing system.
As the patient breathes, helium mixes with the air already present in the lungs. Over time, the helium concentration equilibrates between the spirometer and the lungs. By measuring the change in helium concentration before and after equilibration, clinicians can calculate the functional residual capacity.
Once FRC is known, other lung volumes can be derived, including residual volume (RV) and total lung capacity (TLC). However, a key limitation of helium dilution is that it only measures air in ventilated lung regions. Areas affected by severe obstruction or air trapping may not be included, potentially leading to underestimation of lung volumes in certain disease states.
Nitrogen Washout
The nitrogen washout method is another technique used to determine functional residual capacity. This test relies on the fact that atmospheric air contains approximately 79% nitrogen.
During the procedure, the patient breathes 100% oxygen through a mouthpiece. As the patient continues to breathe oxygen, nitrogen is gradually washed out of the lungs and exhaled. The volume of exhaled nitrogen is collected and measured over time.
By analyzing the total amount of nitrogen exhaled and knowing the initial nitrogen concentration in the lungs, clinicians can calculate the functional residual capacity. Like helium dilution, nitrogen washout allows for the calculation of residual volume and total lung capacity.
Similar to helium dilution, nitrogen washout may underestimate lung volumes in patients with severe airflow obstruction. Poorly ventilated lung regions may retain nitrogen, preventing complete washout and leading to inaccurate results.
Body Plethysmography
Body plethysmography is considered the most accurate method for measuring lung volumes, particularly in patients with obstructive lung disease. Unlike gas dilution techniques, plethysmography measures all the air within the thorax, including air trapped behind closed or narrowed airways.
During the test, the patient sits inside a sealed, airtight chamber known as a body box. The patient breathes through a mouthpiece while performing gentle panting maneuvers against a closed shutter. Changes in pressure within the chamber and the airway are measured and analyzed.
Using Boyle’s law, these pressure changes allow clinicians to calculate lung volumes, including residual volume, functional residual capacity, and total lung capacity. Because plethysmography measures thoracic gas volume rather than ventilated gas alone, it provides a more complete picture of lung inflation and air trapping.
This test is particularly useful in diagnosing and monitoring conditions such as chronic obstructive pulmonary disease (COPD), asthma, and other disorders characterized by airflow limitation and hyperinflation.
What Is a Lung Volume?
A lung volume refers to the amount of air present in the lungs at a specific point during the breathing cycle. There are four primary lung volumes, each representing a distinct phase of breathing.
Tidal volume (VT) is the amount of air inhaled or exhaled during a normal, relaxed breath. Inspiratory reserve volume (IRV) is the additional amount of air that can be inhaled after a normal inspiration. Expiratory reserve volume (ERV) is the extra amount of air that can be exhaled after a normal exhalation.
Residual volume (RV) is the amount of air remaining in the lungs after a maximal exhalation. This volume cannot be measured directly with spirometry and requires gas distribution testing. Residual volume plays an important role in keeping the alveoli open and preventing lung collapse between breaths.
What Is a Lung Capacity?
A lung capacity is a combination of two or more lung volumes. These measurements provide a broader view of overall lung function and ventilation potential. There are four primary lung capacities.
Vital capacity (VC) is the maximum amount of air that can be forcefully exhaled after a maximal inhalation. Inspiratory capacity (IC) is the maximum volume of air that can be inhaled following a normal exhalation.
Functional residual capacity (FRC) is the volume of air remaining in the lungs after a normal exhalation. It is the sum of expiratory reserve volume and residual volume. FRC represents the lung’s resting volume and plays a key role in maintaining stable oxygen and carbon dioxide levels between breaths.
Total lung capacity (TLC) is the sum of all lung volumes and represents the maximum amount of air the lungs can hold after a maximal inhalation. Changes in TLC help distinguish between restrictive and obstructive lung diseases.
What Is Airway Resistance?
Airway resistance (Raw) refers to the opposition to airflow within the respiratory tract as air moves in and out of the lungs. It reflects how much pressure is required to generate airflow through the airways and is influenced by airway diameter, lung volume, and airflow patterns.
Increased airway resistance means that more effort is required to breathe, which is commonly seen in obstructive lung diseases. Narrowed airways, inflammation, mucus accumulation, or bronchospasm can all contribute to elevated resistance.
Airway resistance is calculated using the following formula:
Raw = (PIP – Plateau Pressure) / Flow
This represents the difference between alveolar pressure and mouth pressure divided by the airflow at the mouth. Because this measurement requires precise pressure and flow data, airway resistance is typically assessed using body plethysmography.
Measuring airway resistance provides valuable clinical insight, particularly in evaluating bronchoconstriction, monitoring response to bronchodilators, and assessing disease severity in conditions such as asthma and COPD.
Gas Distribution Tests vs. Spirometry
Spirometry measures airflow and lung volumes but does not evaluate how air is distributed within the lungs. Gas distribution tests provide complementary information by revealing ventilation abnormalities that spirometry may miss.
Key differences include:
- Spirometry focuses on airflow mechanics
- Gas distribution tests assess ventilation uniformity
- Gas distribution is more sensitive to small airway disease
Note: For respiratory therapists, understanding both allows for a more complete assessment of pulmonary function.
Clinical Conditions Associated With Abnormal Gas Distribution
Gas distribution tests are commonly abnormal in:
- Chronic obstructive pulmonary disease
- Asthma
- Cystic fibrosis
- Bronchiolitis
- Pulmonary fibrosis (early stages)
- Obesity-related respiratory dysfunction
- Aging lungs
Note: Early abnormalities often appear in gas distribution tests before spirometry changes, making them valuable for early detection.
Relevance to Respiratory Therapists
Gas distribution testing plays a critical role in respiratory care, and respiratory therapists are often responsible for test administration, quality control, and interpretation support.
Key responsibilities include:
- Proper patient instruction and coaching
- Ensuring consistent breathing patterns
- Identifying technical errors
- Recognizing abnormal patterns
- Communicating findings to the healthcare team
Note: Respiratory therapists who understand gas distribution principles can better correlate test results with patient symptoms, imaging findings, and treatment response.
Role in Disease Monitoring and Treatment Evaluation
Gas distribution tests are useful for:
- Detecting early disease progression
- Monitoring response to bronchodilators or anti-inflammatory therapy
- Evaluating airway clearance effectiveness
- Assessing postoperative lung function
Note: Improvement in gas distribution can indicate successful treatment even when spirometry changes are minimal.
Limitations of Gas Distribution Tests
While valuable, these tests have limitations:
- Require patient cooperation
- Sensitive to technique and breathing pattern
- Less widely available than spirometry
- Interpretation may be more complex
Note: Despite these challenges, their sensitivity to early airway disease makes them a powerful diagnostic tool.
Gas Distribution Tests Practice Questions
1. What can gas distribution tests measure?
Residual volume
2. A subject with an FEV1/FVC ratio of 37% performs a 7-minute N2 washout test. After 7 minutes, the alveolar N2 concentration is 5.7%. This is consistent with which of the following?
Chronic bronchitis
3. What correctly describes the measurement of FRC by the open-circuit method?
The test is continued until alveolar N2 is reduced to 1%, and some N2 is released from the blood and tissues.
4. To measure FRC in a body plethysmograph, how should the VTG should be?
It should be measured by closing the shutter at end-expiration.
5. How is alveolar pressure measured in a body plethysmograph?
By recording mouth pressure when the shutter is closed.
6. In addition to a spirometer, what pieces of equipment are needed to perform a closed-circuit FRC determination?
CO2 and H2O absorbers, an O2 source, and a He analyzer.
7. Which of the following is true when comparing dilutional lung volumes with body plethysmography in subjects who have severe obstruction?
Helium rebreathing underestimates the TLC.
8. What is the correct pant rate for measuring airway resistance or conductance?
90 to 120 breaths/min (i.e., 1.5 to 2.0 Hz)
9. What is the normal percentage of nitrogen for the nitrogen washout test?
Less than 1.5% within 3-4 minutes. If 2.5% or greater, there is an obstruction.
10. What is the formula for airway resistance?
Raw = (PIP – Plateau pressure) / Flow
11. Most of the airway resistance occurs where and at what percentage?
Most occurs in the upper airways; 40% of the total resistance occurs when breathing through the nose and 25% occurs when breathing through the mouth.
12. Where does the greatest resistance of airflow reside?
Medium-sized bronchi
13. What are the passive factors that contribute to airway resistance?
Airway resistance is inversely related to lung volume; therefore, as lung volume increases, airway resistance decreases because the bronchioles are becoming more dilated.
14. What are the methods for assessing airway resistance?
Spirometry, body plethysmography, and the isovolumic pressure-flow curve.
15. What is respiration?
The process by which oxygen from the atmosphere is delivered to cells of the body and enables them to produce energy by oxidative reactions. The by-product, carbon dioxide, is removed during exhalation.
16. What is cellular respiration?
A biochemical reaction that uses oxygen to produce energy.
17. What is a simple definition of lung compliance?
It shows how stretchy the lungs are.
18. Where does gas exchange take place in the lungs?
Alveoli
19. What happens to the air you breathe in as it travels to the alveoli?
It’s humidified and warmed by the walls of the airways.
20. What is dead space?
The volume of gas in the lungs that does not take part in gas exchange.
21. What is the most reproducible point in the breathing cycle?
FRC
22. What is the residual volume?
The volume left in the lungs after maximal exhalation that must be measured indirectly.
23. What are the methods to measure residual volume?
Helium dilution, nitrogen washout, and body plethysmography.
24. How thin are the walls between the alveoli and capillary?
0.5 μm
25. Roughly how many branches are there before you reach the alveoli?
24
26. What is the difference between a lung volume and capacity?
Volumes are directly measured; capacities are measured by adding the sum of one or more lung volumes.
27. What is a normal tidal volume?
About 0.5 L
28. What is a normal residual volume?
1.2 L
29. Can you measure residual volume with a spirometer?
No
30. How can you measure residual volume?
Measure the functional residual capacity (FRC) first using helium dilution. Then subtract the expiratory reserve volume (ERV) to indirectly calculate the residual volume. from it.
31. What is the helium dilution method?
A certain amount of helium is placed in the bell of a spirometer. The subject then breathes normally for a while and then takes a maximum exhalation (leaving only functional residual capacity in the lungs), at which point the concentration of helium in the bell is measured.
32. Will C2 be higher or lower in a subject with a large FRC compared to a subject with a small FRC?
If FRC is high, then C2 will be lower because the He is being diluted in a larger total volume of V1+V2.
33. Why do we use helium?
Because it doesn’t dissolve in body tissues, so it will generally stay in the lungs.
34. How is airway resistance defined?
It is defined as the pressure difference between the mouth and the alveoli divided by the flow rate.
35. What does airway resistance refer to?
The pressure created by the gas flowing through the conducting tubes of the lungs.
36. What is normally the Raw in the tracheobronchial tree in adults?
About 0.5 to 1.5 cmH2O/L/sec
37. The Raw may vary in which types of patients?
Those with COPD
38. What does laminar gas flow refer to?
A gas flow that is streamlined.
39. When does laminar gas flow occur?
At low flow rates and at low-pressure gradients.
40. What does turbulent gas flow refer to?
It refers to gas molecules that move through a tube in a random manner.
41. When does turbulent gas flow occur?
At high flow rates and high-pressure gradients.
42. Where does tracheobronchial or transitional gas flow occur?
In areas where the airways branch.
43. What is the product of the time constants?
Dynamic compliance.
44. How is dynamic compliance defined?
It is defined as the change in the volume of the lungs divided by the change in the transpulmonary pressure during the time required for one breath.
45. What airways are considered anatomic dead space?
Trachea, bronchi, bronchioles, and terminal bronchioles.
46. What airways are used for gas exchange?
Respiratory bronchioles, alveolar ducts, and alveolar sacs.
47. What factors affect dynamic compliance?
Decreased dynamic compliance is seen with increased airway resistance in diseases like asthma, chronic bronchitis, and emphysema.
48. What is the equation for the work of breathing?
Work = Pressure x Volume
49. What factors contribute to the elastic work of breathing?
Surface tension, elastic recoil of pulmonary parenchyma, and elastic recoil of the muscles of respiration and rib cage.
50. What factors contribute to the resistive work of breathing?
Tissue or viscous resistance (20%) and airway resistance (80%).
51. What are the two types of airflow?
Laminar and turbulent.
52. What is the relationship between pressure and resistance to flow rate in laminar flow?
The flow rate is directly proportional to driving pressure and indirectly proportional to resistance.
53. What law does laminar flow follow?
Poiseuille’s law
54. What predicts the likelihood of flow becoming turbulent?
Reynold’s number > ~2000
55. How is Reynold’s number calculated?
Density x linear velocity x diameter / gas viscosity
56. What value indicates an airway obstruction?
FEV1/ FVC ratio < 80%.
57. Why does airflow increase at greater lung volumes?
Because greater lung volume causes decreased pressure in the lungs, which causes a greater pressure gradient between the atmospheric pressure and intrapulmonary pressure, which forces air into the lungs.
58. What is the normal value of the change in nitrogen from 750-1,250?
Less than 1.5% but up to 3% for adults, and up to 10% in patients with severe emphysema.
59. What is the closing capacity?
Closing capacity = RV + closing volume
60. When would you see an increase in closing volume and closing capacity?
Elderly patients, restrictive diseases, smokers, and CHF.
61. What lung volume does the nitrogen washout technique directly measure?
Functional residual capacity (FRC)
62. Why does nitrogen washout underestimate lung volumes in patients with severe COPD?
Because poorly ventilated lung regions trap nitrogen that cannot be washed out effectively.
63. What gas is used during the nitrogen washout test?
100% oxygen
64. What is the primary clinical use of gas distribution tests?
To evaluate lung volumes and detect air trapping or uneven ventilation.
65. Why is FRC considered the most stable lung volume?
Because it represents the equilibrium point between lung recoil and chest wall expansion.
66. What does delayed nitrogen washout indicate?
Ventilation inhomogeneity or airway obstruction.
67. Which lung disease commonly causes prolonged nitrogen washout times?
Emphysema
68. Why is body plethysmography considered the gold standard for lung volume measurement?
It measures all intrathoracic gas, including trapped gas.
69. What does thoracic gas volume represent?
The total volume of gas within the thorax at a given lung volume.
70. How does body plethysmography measure lung volumes?
By applying Boyle’s law to pressure and volume changes during panting.
71. What law of physics is used in body plethysmography?
Boyle’s law
72. Why must the shutter be closed at end-expiration during plethysmography?
Because end-expiration corresponds to functional residual capacity.
73. What does an increased difference between plethysmographic and dilution lung volumes suggest?
Significant air trapping
74. Which lung disease shows the greatest discrepancy between helium dilution and plethysmography?
Severe obstructive lung disease
75. Why is helium unsuitable for measuring trapped gas?
Because helium does not reach poorly ventilated lung units.
76. What does airway conductance (Gaw) represent?
The inverse of airway resistance.
77. How does lung volume affect airway resistance?
Airway resistance decreases as lung volume increases.
78. Why is panting required during plethysmography?
To produce measurable pressure changes without large volume shifts.
79. What clinical condition increases closing capacity?
Chronic smoking
80. What does an elevated closing volume indicate?
Premature airway closure during expiration.
81. What does uneven gas distribution primarily indicate?
Obstructive airway disease causing ventilation inhomogeneity.
82. Why is body plethysmography more accurate than gas dilution methods in obstructive disease?
Because it measures both ventilated and non-ventilated (trapped) gas.
83. What happens to functional residual capacity (FRC) in emphysema?
It is increased due to air trapping.
84. What clinical condition typically causes a decreased FRC?
Restrictive lung disease
85. Why must patients avoid leaks during gas distribution testing?
Leaks cause inaccurate volume and pressure measurements.
86. What does an increased RV/TLC ratio suggest?
Significant air trapping
87. Why is nitrogen washout less reliable in severe airflow obstruction?
Because nitrogen cannot be fully eliminated from poorly ventilated regions.
88. What breathing pattern is required during helium dilution testing?
Relaxed tidal breathing
89. What happens to airway resistance during bronchoconstriction?
It increases due to narrowed airways.
90. Why does airway resistance decrease at higher lung volumes?
Radial traction pulls airways open.
91. What is the clinical significance of increased thoracic gas volume?
It indicates hyperinflation.
92. What does a prolonged nitrogen washout curve suggest about ventilation?
Ventilation is uneven or delayed.
93. Which lung condition commonly shows increased closing capacity?
Chronic obstructive pulmonary disease.
94. Why is closing volume important clinically?
It identifies early small-airway disease.
95. What does body plethysmography measure that spirometry cannot?
Residual volume
96. How does airway obstruction affect dynamic compliance?
It decreases dynamic compliance.
97. What type of airflow predominates in small distal airways?
Laminar flow
98. What happens to Reynolds number as airflow velocity increases?
It increases, promoting turbulent flow.
99. Why is Poiseuille’s law important in respiratory physiology?
It explains how airway radius affects airflow resistance.
100. What is the primary advantage of gas distribution tests in clinical practice?
They help differentiate obstructive from restrictive lung disease.
Final Thoughts
Gas distribution tests provide a deeper look into pulmonary function by revealing how effectively air reaches different regions of the lungs. For respiratory therapists, these tests bridge the gap between basic spirometry and more advanced physiologic assessment.
By identifying ventilation inhomogeneity, air trapping, and small airway dysfunction, gas distribution testing supports earlier diagnosis, more precise monitoring, and better-informed treatment decisions.
Mastery of these concepts enhances clinical competence and reinforces the respiratory therapist’s role as an essential expert in pulmonary diagnostics and patient-centered 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
- Ponce MC, Sankari A, Sharma S. Pulmonary Function Tests. [Updated 2023 Aug 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025.