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.
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.
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 is 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.
- 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.
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
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 60 percent to 79 percent predicted = Mild obstruction
- FEV1 40 percent to 59 percent predicted = Moderate obstruction
- FEV1 less than 40 percent predicted = Severe obstruction
For further information on Sword Medical's range of Spirometer's, please click here to contact our Respiratory Specialist