Gas distribution tests, a subset of pulmonary function testing (PFT), offer valuable insights into a patient’s lung volumes and capacities, how air is distributed during breathing, and the resistance it meets within the airways.
In the field of respiratory care, understanding how air moves within the lungs is crucial for diagnosing and monitoring various pulmonary conditions.
This article breaks down the types of gas distribution tests, the difference between lung volumes and capacities, and the significance of airway resistance in respiratory health.
What are Gas Distribution Tests?
Gas distribution tests are a subset of pulmonary function testing (PFT) that specifically measure and assess the volume and distribution of air within the lungs. These tests provide insights into how effectively the lungs can fill and empty, and they can detect abnormalities in the distribution of ventilation across different lung regions.
Types of Gas Distribution Tests
There are three primary types of gas distribution tests:
- Helium dilution
- Nitrogen washout
- Body plethysmography
Helium Dilution
The helium dilution method is a pulmonary function test designed to assess a patient’s functional residual capacity (FRC).
This technique involves the use of a spirometer filled with a predetermined volume of air. As the patient breathes, helium is delivered to their lungs through a valve linked to a rebreathing system.
Within the lungs, the helium blends with the pre-existing air, serving as a mixing chamber.
By evaluating the shift in helium concentration, one can accurately determine the patient’s functional residual capacity (FRC). From this data, it’s then possible to calculate both the residual volume and the total lung capacity (TLC).
Nitrogen Washout
The nitrogen washout method is another pulmonary function test used to determine a patient’s functional residual capacity (FRC).
Atmospheric air typically comprises about 79% nitrogen. During the test, the patient breathes in pure oxygen, effectively washing the nitrogen out of their lungs. The total volume of exhaled air is then measured.
By comparing the starting and ending nitrogen concentrations and factoring in the amount washed out, the FRC is determined.
Body Plethysmography
Body plethysmography is a pulmonary function test used to measure the total volume of air in the lungs, including the volume that doesn’t participate in gas exchange.
During the test, a person sits in a sealed chamber and breathes against a closed mouthpiece. Changes in pressure within the chamber allow for the calculation of lung volumes, including the residual volume, which is the air left in the lungs after a full exhalation.
This test is helpful in diagnosing and monitoring various lung conditions, such as chronic obstructive pulmonary disease (COPD) and asthma.
What is a Lung Volume?
A lung volume refers to the amount of air in the lungs at a specific time during the breathing cycle. There are four primary types of lung volumes:
- Tidal volume (VT)
- Inspiratory reserve volume (IRV)
- Expiratory reserve volume (ERV)
- Residual volume (RV)
Tidal volume is the amount of air inhaled or exhaled with each breath. Inspiratory reserve volume is the amount of air that can be inhaled beyond a normal inhalation.
Expiratory reserve volume is the amount of air that can be exhaled beyond a normal exhalation. Residual volume is the amount of air remaining in the lungs after a maximal exhalation.
What is a Lung Capacity?
A lung capacity is a combination of two or more lung volumes, and there are four primary types:
- Vital capacity (VC)
- Inspiratory capacity (IC)
- Functional residual capacity (FRC)
- Total lung capacity (TLC)
Vital capacity is the maximal amount of air that can be forcefully exhaled after a maximal inhalation. Inspiratory capacity is the maximum volume of air that can be inhaled after a normal exhalation.
Functional residual capacity is the volume of air remaining in the lungs after a normal exhalation. It’s the combined volume of the expiratory reserve volume and the residual volume.
Total lung capacity is the sum of all the lung volumes and represents the maximum volume of air that the lungs can hold. It is equivalent to the volume in the lungs following a maximal inhalation.
What is Airway Resistance?
Airway resistance (Raw) refers to the resistance of the respiratory tract to the flow of air as it moves in and out of the lungs during breathing. It’s a measure of how much effort is required to move air through the airways.
Airway resistance is calculated with the following formula:
Raw = (PIP – Plateau pressure) / Flow
Therefore, airway resistance is the difference between the patient’s alveolar pressure and mouth pressure, divided by the flow at the mouth.
Note: Measuring airway resistance requires the use of specialized pulmonary function tests (e.g., plethysmography), which give insights that can guide diagnosis and treatment decisions for various respiratory disorders.
Gas Distribution Test 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.
Access our quiz with sample TMC practice questions and detailed explanations to help you master pulmonary function testing (PFT).
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.
Final Thoughts
Gas distribution tests provide a comprehensive understanding of a patient’s pulmonary mechanics, including the resistance air encounters within the respiratory system.
Such insights are invaluable in diagnosing, treating, and monitoring respiratory conditions, ensuring optimal lung health and function.
As advancements in medical technology continue, these tests have become increasingly precise and accessible, leading to earlier detections and more effective treatments for various pulmonary disorders.
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
- Faarc, Mottram Carl Ba Rrt Rpft. Ruppel’s Manual of Pulmonary Function Testing. 11th ed., Mosby, 2017.
- Faarc, Kacmarek Robert PhD Rrt, et al. Egan’s Fundamentals of Respiratory Care. 12th ed., Mosby, 2020.
- Ponce MC, Sankari A, Sharma S. Pulmonary Function Tests. [Updated 2022 Sep 3]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023.
- LungVolume.jpg: The original uploader was Vihsadas at English Wikipedia. derivative work: rscottweekly, CC BY-SA 3.0, via Wikimedia Common
