What is the Difference Between Static and Dynamic Pulmonary Function Tests

The key difference between static and dynamic pulmonary function tests is that static pulmonary function tests measure the absolute lung volume, while dynamic pulmonary function tests measure the amount and time taken for exhalation during a forced maneuver.

Lung volumes are subdivided into static and dynamic lung volumes. Static lung volumes are measured based on the completeness of respiratory maneuvres. Dynamic lung volumes are measured through a fast-breathing process, and the air is forced in and out of the lungs. The levels of lung volume vary with age, gender, body mass, reflex factors, daily activity patterns, and posture. The airflow levels of inspiration are influenced by inspiratory muscles, elastic recoil of lungs, and elasticity of thorax and adjacent structures, while the levels of expiration are influenced by expiratory muscles, compression of small airways, and mechanical properties of lungs and thorax.

CONTENTS

1. Overview and Key Difference
2. What are Static Pulmonary Function Tests
3. What are Dynamic Pulmonary Function Tests
4. Similarities – Static and Dynamic Pulmonary Function Tests
5. Static vs Dynamic Pulmonary Function Tests in Tabular Form
6. Summary – Static vs Dynamic Pulmonary Function Tests

What are Static Pulmonary Function Tests?

Static pulmonary function test measures the static or absolute lung volume. Parameters such as total lung capacity (TLC) and residual volume (RV) play an important role in this regard. This function test is carried out in a pulmonary function laboratory using a spirometer. There are three different static tests: helium dilution test, nitrogen washout test, and plethysmography. During these tests, the nose is clipped, and breathing is carried out through the mouth through a flanged rubber mouthpiece. Helium dilution test and nitrogen washout test are performed through steady and quiet breathing. At the end of quiet breathing, the air is inhaled deeply and exhaled steadily with maximum air. In the nitrogen washout test, after finishing the exhalation, a valve is switched to the breathing circuit, and the breathing of air is measured again for about two minutes. The tests end when the breath is complete. Both tests are usually performed at least twice. However, oxygen is measured throughout the tests and is stopped if the oxygen level falls below 84%.

Plethysmography is carried out inside a clear plexiglass booth. The cheeks are pressed with the hands lightly throughout the breathing process. The test process is similar to the nitrogen washout process.

What are Dynamic Pulmonary Function Tests?

Dynamic pulmonary function test measures the volume of air exhaled during a forced maneuver. Further, it also measures the time taken for the exhalation process. The forced vital capacity (FVC) test and maximum voluntary ventilation (MVV) are important parameters of the test. Usually, the FVC is similar to the vital capacity of the lungs. Through the FVC test, forced expiratory volume in one second is measured, and it is the maximum volume of air that can be exhaled in a one-second time period. The normal measure of the FVC during one second is approximately 80%. The MVV test measures the largest volume of air that is inhaled and exhaled from the lungs within one minute. This value is usually reduced during a pulmonary disease due to high airway resistance in the respiratory tract.

The dynamic pulmonary test is mainly performed through spirometry. It utilizes a spirometer that measures the amount and the speed (time) of the exhaled air. There are different spirometers based on size and shape; however, all measure the same parameters.

What are the Similarities Between Static and Dynamic Pulmonary Function Tests?

• Static and dynamic pulmonary function tests are carried out to assess the exhaled air in the lungs.
• The tests mainly consider the exhalation process of respiration.
• Moreover, both measure lung volumes.
• Both help in diagnosing pulmonary diseases or abnormalities.

What is the Difference Between Static and Dynamic Pulmonary Function Tests?

Static pulmonary function tests measure the absolute lung volume, while dynamic pulmonary function tests measure the amount and time taken for the exhalation process during a forced maneuver. Thus, this is the key difference between static and dynamic pulmonary function tests. Helium dilution test, nitrogen washout test, and plethysmography are tests used to measure static pulmonary function, while forced vital capacity test and maximum voluntary ventilation test are used to measure dynamic pulmonary function test. Moreover, TLC and RV are important parameters in static pulmonary tests, whereas FVC and MVV are important parameters in dynamic pulmonary tests.

The below infographic presents the differences between static and dynamic pulmonary tests in tabular form for side-by-side comparison.

Summary – Static vs Dynamic Pulmonary Function Tests

Lung volumes are subdivided into static and dynamic lung volumes. Static pulmonary function tests measure the absolute lung volume, while dynamic pulmonary function tests measure the amount and time taken for the exhalation process during a forced maneuver. Static pulmonary function tests are measured by a spirometer, where only the exhaled volume is measured. There are three types of tests: helium dilution test, nitrogen washout test, and plethysmography. Dynamic tests are measured through the time taken for the exhalation process of a certain volume of air. There are two types of tests: forced vital capacity test and maximum voluntary ventilation test. So, this summarizes the difference between static and dynamic pulmonary function tests.

Reference:

1. “Forced Ventilatory Volumes and Flows (Ventilatory Capacity).” Lung Function, pp. 130–142.
3. “Respiration Lab: Dynamic Lung Volumes.” Medicine. Mcgill.
3. “Static Lung Volumes.” Static Lung Volumes | ILD Collaborative.

Image Courtesy:

1. “Body plethysmograph box” By Stan3000 – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “DoingSpirometry” By Jmarchn – Own work (CC BY-SA 3.0) via Commons Wikimedia