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A spirometer is a device which is used to measure the flow of breath in and out of the lungs. This device is used in a medical testing procedure known as spirometry, in which a patient breathes in and out of a tube connected to the spirometer so that lung function can be assessed. Many people who are receiving treatment for pulmonary conditions have interacted with a spirometer at some point in their lives.
Several different methods can be used to construct a spirometer. The goal is to create a device which is very sensitive to changes in air pressure, so that it can measure the subtleties of airflow to and from the lungs. Spirometers produce a graphical output, with the airflow along one axis, and the time along another. This allows a doctor to see not only how much air the patient is breathing in and out, but also the period of time involved. A peak flow value can also be obtained by looking for the high point on the graph.
The original spirometer design was a cylinder suspended in water which could be moved up and down as the patient breathed. As the cylinder moved, it pushed a pen attached to graph paper, creating a graph which could be read after the test was over. A variety of other pressure-sensitive devices can be used for spirometry today, although the water-based method is still used in some hospitals.
Working with a spirometer takes some practice. When patients first use the device, they are often given a test run so that they know what the experience is like before data is collected. At least three cycles of breathing are completed on the spirometer to obtain a range of values which can be averaged to reflect the patient's condition. If the numbers differ wildly, it may be a sign that one or more of the tests was flawed, and needs to be repeated.
Spirometry is heavily dependent on patient cooperation, and it is important to remember that it does not measure total lung volume, but only the amount of air which can be pushed in and out of the lungs. Someone with severe bronchial obstruction might have a very high lung volume, but this would not be reflected in a spirometry test, since only a limited amount of air could be forced through the bronchial tubes. Other tests can be used to test lung volume, and a whole battery of pulmonary function tests or PFTs can be used to assess lung function as a whole.