Gas Distribution Tests Lung Volumes and Airway Resistance

Gas Distribution Tests: Lung Volumes and Airway Resistance

by | Updated: Mar 9, 2023

Gas distribution tests are pulmonary function tests (PFT) that can obtain a patient’s lung volume measurements.

This involves the amount of air that is inhaled and exhaled at various points during the breathing cycle. It also includes the volume that remains in the lungs after a complete exhalation.

In this guide, we’ll cover the different types of gas distribution tests, including helium dilution, nitrogen washout, and body plethysmography. We’ll also explain the difference between lung volumes, capacities, and airway resistance.

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What is a Lung Volume?

Lung volume refers to the amount of air in the lungs at a specific time during the breathing cycle. There are four types of lung volumes, including:

  1. Tidal Volume (VT)
  2. Inspiratory Reserve Volume (IRV)
  3. Expiratory Reserve Volume (ERV)
  4. 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. And residual volume is the amount of air remaining in the lungs after a maximal exhalation.

PFT Lung Box

What is a Lung Capacity?

A lung capacity is a combination of two or more lung volumes. There are four types of lung capacities, including:

  1. Vital Capacity (VC)
  2. Inspiratory Capacity (IC)
  3. Functional Residual Capacity (FRC)
  4. Total Lung Capacity (TLC)

Vital capacity is the maximal amount of air that can be exhaled after a maximal expiratory effort. Inspiratory capacity is the maximal amount 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. Total lung capacity is the sum of all the lung volumes.

What are Gas Distribution Tests?

Gas distribution tests are used to measure a patient’s absolute lung volumes during a breathing cycle. The three primary types include:

  1. Helium Dilution
  2. Nitrogen Washout
  3. Body Plethysmography

These gas distribution tests also provide a way to calculate a patient’s residual volume and total lung capacity by measuring their functional residual capacity.

Helium Dilution

The helium dilution technique is a pulmonary function test that measures a patient’s functional residual capacity (FRC).

It involves the use of a spirometer that is filled with a known volume of air. Then helium is introduced into the lungs as the patient breathes through a valve connected to a rebreathing system.

The lungs act as a mixing chamber where the helium mixes with the air that is already present. The change in concentration of the helium is used to measure the FRC. Then you will be able to calculate the residual volume and total lung capacity (TLC).

Nitrogen Washout

The nitrogen washout technique is a pulmonary function test that is also used to measure a patient’s functional residual capacity (FRC).

In general, atmospheric air contains approximately 79% nitrogen. The test involves washing nitrogen out of the lungs by having the patient breathe in 100% oxygen and the total expired volume is measured.

The FRC is calculated using the initial nitrogen concentration, amount washed out, and the final concentration. Then you can calculate the RV and TLC.

Body Plethysmography

Body plethysmography is a pulmonary function test that measures the functional residual capacity (FRC) and intrathoracic gas volumes.

During the test, the patient enters a large box with a tight seal and rigid walls. It generates pressure, which causes the volume to shift while the expiratory flow rate is recorded.

The changes in pressure and airway resistance allow the FRC to be measured, which is used to calculate the RV and TLC.

What is Airway Resistance?

Airway resistance (Raw) is the measurement of the opposition to airflow that occurs as air moves through the airways of the lungs. It can be 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.

Gas Distribution Tests Practice Questions:

1. 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 to the atmosphere.

2. What is cellular respiration?
Biochemical reactions using oxygen to produce energy (i.e. glycolysis, link, Krebs, ETC).

3. What is internal respiration?
Getting the gases in the blood to the tissues of the body and the gases in the flow of the body back into the blood.

4. What is external respiration?
Getting gas in and out of your lungs (i.e. breathing).

5. What does compliance mean when talking about the lungs?
Simply put, it is a way to show how stretchy the lungs are.

 6. Where does gas exchange take place in the lungs?
Respiratory bronchioles (NOT terminal bronchioles or anything more proximal), alveolar ducts,
alveolar sacs, and alveoli.

7. What happens to the gas you breathe in as it travels down to the alveoli?
Humidified and warmed by the walls of the airways.

8. Why is this important for the alveoli?
So, the gas won’t dry them out.

9. How do the airways protect us from invasive foreign materials?
Hairs in nose act as a filter. Trachea and main bronchi have sticky mucus to trap any particulate matter that does get this far. The autonomic and then
swallowed. Alveolar macrophages.

10. What is dead space?
The volume of gas in the lungs that does not take part in gas exchange, which in a normal healthy individual is the volume of gas in the conducting airways.

11. What happens to the cross-sectional area of the individual airways as branching increases?
Each individual airway has a smaller diameter.

12. What happens to the total cross-sectional area of each branching level?
It gets bigger, as the total cross-sectional area at the alveolar level is much higher than at the trachea because there are many more alveoli, despite each individual one being much smaller than the trachea in diameter.

13. What do goblet cells in the main airways secrete?
They secrete mucin, which is a precursor to mucus.

14. Why do the trachea and main bronchi have cartilage?
It serves as support, and to keep them open.

15. How do bronchioles stay open if they have no cartilage?
They have supportive, elastic connective tissue between them (a.k.a. lung parenchyma provides radial traction).

16. How thin are the walls between the alveoli and capillary?

17. What do macrophages do?
Scavenger cells that engulf and dissolves any foreign material.

18. What do type I pneumocytes do?
Gas exchange occurs across them, makes up 90% of SA of alveoli, tight junctions, and fused with capillary endothelium.

19. What do type II pneumocytes do?
Secretes surfactant, which is very important in lung function in decreasing the surface tension.

20. When are type II pneumocytes first made in an embryo?
At 24 weeks.

21. What happens if pneumocytes are not made?
Respiratory distress of the newborn.

22. Roughly how many branches are there before you get to the alveoli?

23. What’s the difference between a volume and a capacity?
Volumes are things that we directly measure, whereas capacities are sums of volumes that we don’t directly measure.

24. What is anatomical dead space?
Volume in the conducting airways.

25. What is physiological dead space?
The total volume of air that doesn’t take part in gas exchange. This includes anatomical dead space and any the volume of gas in any non-perfused alveoli (close to 0 in a healthy lung).

26. What is a normal tidal volume?
About 0.5L.

27. What is residual volume?
The volume of gas leftover in the lungs after a maximal expiration, which is about 1.2L.

28. How do we measure residual volume if we can’t breathe it out and use a spirometer?
We measure functional residual capacity and minus expiratory reserve volume from it. We
measure functional residual capacity using the Helium dilution method.

29. 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. The subject takes a maximum breath out (leaving only functional residual capacity in the lungs), at which point the concentration of helium in the bell is measured. V2 is then calculated using the equation V2 = (C1V1/C2) – V1, which in this scenario can be re-written as:  FRC = ([He]start x Vspirometer / [He]end) – Vspirometer.

30. 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.

31. Why do we use Helium?
Because it doesn’t dissolve in body tissues and so will stay in the lungs.

32. How can we estimate anatomical dead space in a patient?

33. Approximately what proportion of tidal volume is dead space gas?
About 1/3rd (150ml of the 500ml tidal volume).

34. How is the airway resistance defined?
It is defined as the pressure difference between the mouth and the alveoli (trans respiratory pressure) 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 patients?
COPD patients.

38. What does laminar gas flow refer to?
A gas flow that is streamlined.

39. Where 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 do you call the phenomenon of the product of airway and lung compliance?
Time Constant.

 44. How is time constant defined?
The time (in seconds) necessary to inflate a lung region to about 60 percent of its potential filling capacity.

45. When are alveoli said to have a long time constant?
When either an increased Raw or increased CL require more time to inflate.

46. When are alveoli said to have a short time constant?
When either a decreased Raw or decreased CL require less time to inflate.

47. What is the product of the time constants?
Dynamic compliance.

48. 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.

49. What airways are considered anatomic dead space?
Trachea, bronchi, bronchioles, and terminal bronchioles.

50. What airways are used for gas exchange?
Respiratory bronchioles, alveolar ducts, and alveolar sacs.

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51. What structures have cartilage and therefore don’t change in diameter?
Trachea and bronchi.

52. What airways can change in diameter?

53. Does resistance limit airflow in normal conditions?

54. Why doesn’t resistance limit airflow?
Lateral traction and transpulmonary pressure.

55. What is airway resistance a measure of?
The pressure difference between alveoli and the mouth divided by a flow rate.

56. What factors affect dynamic compliance?
Decrease dynamic compliance is seen with increased airway resistance in diseases like asthma, chronic bronchitis, and emphysema.

57. What is the equation for the work of breathing?
Work = Pressure X Volume.

58. What factors contribute to the elastic work of breathing?
Surface tension, elastic recoil of pulmonary parenchyma, and elastic recoil of muscles of respiration and rib cage.

59. What factors contribute to the resistive work of breathing?
Tissue or viscous resistance (20%) and airway resistance (80%).

60. What are the two types of airflow?
Laminar and turbulent.

61. What is the relationship of pressure and resistance to flow rate in laminar flow?
The flow rate is directly proportional to driving pressure and indirectly proportional to resistance (P=KV).

62. What is the equation for laminar flow?
Q = ??^4∆?/8Ƞ?.

63. How is flow resistance calculated in laminar flow?
R = 8Ƞ?/ ??^4.

64. What law does laminar flow follow?
Poiseuille’s Law.

65. What is the normal percentage of nitrogen for nitrogen washout test?
Less than 1.5% within 3-4 minutes. If 2.5% or greater, there is an obstruction.

66. What predicts the likelihood of flow becoming turbulent?
Reynold’s number (Re) > ~2000.

67. How is Reynold’s number calculated?
Density x linear velocity x diameter / gas viscosity.

68. Most of the airway resistance occurs where and at what percentage?
Most occurs in the upper airways. It is 40% of the total resistance when breathing through the nose and 25% when breathing through the mouth.

69. Where does the greatest resistance of airflow reside, and why?
In the medium size bronchi. Even though resistance is technically greatest in individual small airways, the total resistance from these small airways together is still very low b/c they are mostly parallel.

70. What are the active factors contributing to airway resistance?
The autonomic nervous system is parasympathetic (constriction) and sympathetic (dilation). Next, local response – increased local PCO2 or decreased local PO2 causes dilation; decreased local PCO2 causes constriction.

71. What are the passive factors contributing to airway resistance?
Airway resistance is inversely related to lung volume. As lung volume increases, airway resistance decreases, because bronchioles are becoming more dilated.

72. What are the methods for assessing airway resistance?
Spirometry, body plethysmograph, isovolumic pressure-flow curve.

73. What value indicates airway obstruction?
FEV in first second (FEV1)/ FVC < 80%.

74. What does effort independence indicate?
That resistance to airflow is increasing as intrapleural pressure increases.

75. Why is airflow greater at greater lung volumes?
Because greater lung volume = decreased pressure in the lungs which causes a greater pressure gradient between the atmospheric pressure and intrapulmonary pressure which forces air into the lungs. Intrapulmonary is usually 760 mm Hg, the same as atmospheric, but when intrapulmonary decreases (because of greater volume), the air wants to move from higher pressure (now atm) to lower pressure (in lungs).

76. What two things help you evaluate gas distribution?
Change in Nitrogen from 750-1250 (beginning of phase 3-alveolar plateau) and slope of Phase 3.

77. What is the normal value of the change in nitrogen from 750-1250?
Less than 1.5%. Up to 3% for adults. Up to 10% in severe emphysema.

78. What is measured at the slope of phase 3?
This is the point at which 30% of the vital capacity remains until the onset of phase IV. The normal value is 1%-1.5% N2/Liter of lung volume.

79. What is the closing capacity?
Closing capacity = RV + closing volume.

80. When would you see an increase in closing volume and closing capacity?
Elderly patients, restrictive diseases, smokers, and CHF.

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81. When does normal closing or phase IV begin?
80-90% of the expired VC or closing capacity of 30% of TLC.

82. What is suspect if the closing volume is greater than 20% of VC?
Small airways abnormality.

83. What is a ventilation-perfusion scan used for?
To assess for pulmonary embolism.

84. A V/Q scan uses what two things?
Radioactive xenon gas and radioactive albumin. Xenon is an insoluble gas that will accumulate and get stuck. The albumin will light up in the lungs because they get stuck in the capillaries of the lung.

85. What is the gold standard for diagnosing a pulmonary embolism?
Pulmonary angiography (this is invasive, so you want a V/Q scan first to give you an idea first).

86. Regions with poor ventilation will show up how on a V/Q scan?
Light during wash instep. This is when the patient breaths in the xenon gas.

87. Areas of even or good ventilation show up how on a V/Q scan?
Rapid washing in and rapid wash out. The scan will appear dark, which is good. The xenon or albumin are not lighting up.

88. Areas of trapped gas show up how on a V/Q scan?
Wash in very slowly and wash out very slowly.

89. What can Gas Distribution Tests measure?
The patient’s residual volume.

90. What is the formula for Airway Resistance?
Raw = (PIP – Plateau pressure) / Flow

Final Thoughts

Lung volumes and capacities provide valuable information about the size and function of a patient’s lungs. Gas distribution tests are used to obtain a measurement of the absolute lung volumes during a breathing cycle.

The three primary types include helium dilution, nitrogen washout, and body plethysmography. These tests also provide a way to calculate a patient’s residual volume and total lung capacity.

We have a detailed guide on the other types of pulmonary function testing that I think you’ll find helpful. Thanks for reading!

Written by:

John Landry, BS, RRT

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.


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