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What is spirometry?
Spirometry is the most basic and frequently performed test of pulmonary (lung) function. A device called a spirometer is used to measure how much air the lungs can hold and how well the respiratory system is able to move air into and out of the lungs. Because spirometry is based on a maximal forced exhalation, the accuracy of its results are highly dependent on the patient's understanding, cooperation, and best efforts.
Spirometry differs from peak flow readings in that spirometry records the entire forced breathing capacity against time, and peak flow records the largest breathing flow that can be sustained for 10 milliseconds. Both are often used in asthma care.
Why would I need a spirometry test?
This test is used to determine the cause of shortness of breath, to rule out any kind of obstructive disease that blocks breathing, or restrictive disease that limits the expansion and capacity of the lungs. Spirometry is most often used to diagnose and monitor lung problems, such as chronic bronchitis, emphysema, pulmonary fibrosis, chronic obstructive pulmonary disease (COPD), or asthma.
Spirometry is also used to monitor how well medications for respiratory problems are working and to evaluate breathing capability prior to surgery.
How is spirometry performed?
A spirometry test is done with a spirometer, which consists of a mouthpiece and disposable tubing connected to a machine that records the results and displays them on a graph.
To perform spirometry, a person inhales deeply, closes the mouth tightly around the tube and then exhales through the tubing while measurements are taken. Some test measurements are obtained by normal breathing, and other tests require rapid and forceful inhalation and/or exhalation. The volume of air inhaled or exhaled, and the length of time each breath takes are recorded and analyzed.
Nose clips are usually used to make sure air is only coming out of the mouth. Sometimes a test will be repeated to get the best and maximum effort. Often, the tests are repeated after a person takes a medication that opens the airways of the lungs (a bronchodilator). A spirometry test can take anywhere from five minutes to a half an hour, depending on the different types of breathing tests being measured.
Common parameters that spirometry measures are:
- Forced vital capacity (FVC) - The maximum volume of air, measured in liters that can be forcibly and rapidly exhaled.
- Forced expiratory volume (FEV1) - The volume of air expelled in the first second of a forced expiration.
What are normal spirometry results?
Normal spirometry results are based on the age, height, and gender of the person being tested and most are expressed as a percentage of a predicted value. Normal spirometry results include:
- Tidal volume - 5 to 7 milliliters per kilogram of body weight
- Expiratory reserve volume - 25 percent of vital capacity
- Inspiratory capacity - 75 percent of vital capacity
- Forced expiratory volume - 75 percent of vital capacity after one second, 94 percent after two seconds, and 97 percent after three seconds
What are abnormal spirometry results?
Spirometry results are expressed as a percentage, and are considered abnormal if less than 80 percent of the normal predicted value. An abnormal result usually indicates the presence of some degree of obstructive lung disease such as asthma, emphysema or chronic bronchitis, or restrictive lung disease such as pulmonary fibrosis. FEV1 values (percentage of predicted) can be used to classify the obstruction that may occur with asthma and other obstructive lung diseases like emphysema or chronic bronchitis:
- FEV1 65 percent to 79 percent predicted = Mild obstruction
- FEV1 40 percent to 59 percent predicted = Moderate obstruction
- FEV1 less than 40 percent predicted = Severe obstruction
Are there risks associated with spirometry?
The risks are minimal for most people. Because the test involves forced and rapid breathing, some people may experience temporary shortness of breath. Spirometry should not be done if a person suffers from chest pains, has had a recent heart attack, or has serious heart disease.
How should I prepare for spirometry?
- Do not eat a heavy meal before spirometry testing.
- Refrain from smoking for four to six hours before the test.
- Empty your bladder right before testing.
- Specific instructions are given if medications such as bronchodilators or inhalers need to be withheld before the test.
- Sometimes, medication may be inhaled prior to the test, to test how well an individual responds to medication.
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