What is a lung function test?
A pulmonary function test is, as the name suggests, an examination that checks the function of the lungs and other airways. Various test procedures are available for this purpose:
- Spirometry (also called “Lufu” for “lung function”)
- Spiroergometry (examination of lung function under physical stress)
- Determination of diffusion capacity (an examination of gas exchange)
- Bodyplethysmography / whole-body plethysmography (based on volume determination)
- blood gas analysis (determination of the oxygen and carbon dioxide content in the blood)
- medicinal test procedures (targeted influencing of respiratory function by active substances)
Self-tests for home use:
In addition to peak flow measurement, there are some simple tests for home use that you can use to roughly assess your lung function yourself. Read more about this in the article Lung test at home.
Lung function test: Values and their meaning
The following values can be recorded with the various measurement methods in the pulmonary function test:
- Total lung capacity: volume of air in the lungs after the patient has inhaled as deeply as possible.
- Residual volume: Volume remaining in the lungs and airways after vigorous exhalation.
- Breath volume (also tidal volume): Amount of air the patient inhales with a normal breath.
- Inspiratory reserve volume: amount of air that the patient can breathe in additionally after a normal inspiration.
- Expiratory reserve volume: volume of air that the patient can additionally exhale after normal exhalation
- Peak expiratory flow (PEF): maximum strength of airflow during forced expiration.
- One-second capacity (FEV1): Breathing volume that the patient can exhale within the first second after inhalation with full force
- Tiffenau index: ratio of one-second capacity to vital capacity
- Mean expiratory flow (MEF): average strength of respiratory flow when a certain defined percentage of vital capacity is still in the lungs
Pulmonary function test – Evaluation: Table of standard values
The following table lists standard values for lung function. If the measured values (when measured repeatedly) deviate from these standard values, this indicates a lung function disorder, often also a specific lung disease.
|
Parameter |
common abbreviation |
normal value |
|
Total lung capacity |
TC, TLC |
6 to 6.5 liters |
|
Vital capacity |
VC |
4.5 to 5 liters |
|
Residual volume |
RV |
1 to 1.5 liters |
|
Breath volume |
VT |
0.5 liters |
|
Inspiratory reserve volume |
IRV |
|
|
Expiratory reserve volume |
ERV |
1.5 liters |
|
Functional residual capacity |
FRC |
2.5 to 3 liters |
|
Peak Expiratory Flow |
PEF |
>90% of the age/sex-specific normal value |
|
One-second capacity |
FEV1 |
>90% of the age/sex-specific normal value |
|
Tiffenau index |
FEV1 : VC |
>70% |
|
Mean expiratory flow |
MEF |
>90% of the age/sex-specific normal value |
When to perform a pulmonary function test?
For example, the doctor can use it to detect narrowed airways (obstruction). This occurs primarily in the common diseases asthma and COPD. In those affected, the lung function evaluation shows a reduction in the one-second capacity and the Tiffenau index. If the residual volume is increased, this may indicate emphysema, often a late consequence of obstructive airway disease.
- Pulmonary fibrosis
- Pleural effusion: accumulation of fluid in the pleural space (= space between the lung and pleura)
- Scars or adhesions in the lung tissue or pleural space
- Malformations in the thoracic skeleton
The decreased distensibility of the lungs in such diseases shows up in the pulmonary function test with a decrease in vital capacity and total lung capacity.
What do you do in a pulmonary function test?
Spirometry
Standard and thus usually the beginning of every diagnostic process is spirometry, during which the patient is asked to breathe sometimes more strongly, sometimes as usual through the mouthpiece. The examination can be combined with drug-related test procedures (such as the bronchospasmylosis test).
To find out exactly how spirometry works and what conclusions can be drawn from the measured values, read the article Spirometry.
Spiroergometry
You can read exactly what the patient has to do during spiroergometry and what the risks are in the article Spiroergometry.
Another exercise test besides spiroergometry is the 6-minute walk test. In this test, the doctor measures the (level) distance the patient can cover while walking as fast as possible for six minutes – patients with lung disease usually go much less far than healthy people. During the test, the patient’s pulse, blood pressure and oxygen saturation are sometimes also measured.
A more sensitive and accurate measurement of various respiratory variables is bodyplethysmography. Here, the patient sits in a sealed chamber, similar to a telephone booth. While he breathes into a mouthpiece on the one hand, similar to spirometry, the doctor measures the pressure changes inside the chamber in parallel.
To find out exactly how the examination works and what advantages it has over other lung function tests, read the article Bodyplethysmography.
Using bodyplethysmopgraphy equipment (see above), the physician can also measure the diffusion capacity of the lungs. This indicates how well the lungs can exchange respiratory gases. To measure diffusing capacity, the patient breathes in air with a safe amount of carbon monoxide (CO). This allows the physician to determine how well the lungs take in oxygen from the air breathed and release carbon dioxide. For more details, read the article Bodyplethysmography.
Blood gas analysis
With the help of blood gas values, the doctor can monitor the lungs and the heart. You can read exactly what the results of a blood gas analysis mean in the article Blood gas values.
Peak flow measurement
Patients with lung disease have the option of measuring their respiratory function at home using a handy, simple peak flow meter.
To find out which values are recorded during peak flow measurement and what the patient has to bear in mind, read the article Peak flow measurement.
What are the risks of a lung function test?
There are no certain dangers associated with the test procedures. However, after a lung function measurement has been performed several times, you may experience a cough or dizziness. However, this subsides after a short time.
What do I need to do after a pulmonary function test?
Immediately after the pulmonary function test, you should resume a normal breathing rhythm. Try to breathe in and out calmly and evenly. If you experience a slight cough or dry mouth, you should drink a little. Your doctor will discuss the results and further procedure with you following the lung function test.
