Spirometry measures how fast you can breathe out. It also measures how much
total air you breathe out. In this test, you breathe into a mouthpiece on a
device called a spirometer. At the same time, a machine makes a tracing of
the rate at which the air leaves the lungs. Diseases of airflow obstruction
and of lung stiffening give characteristic tracings with spirometry.
The test measures:
- The amount of air that can be expelled following a deep breath, called
forced vital capacity (FVC)
- The amount of air that can be forcibly exhaled in one second, called forced
expiratory volume in one second (FEV1)
These measurements are the most useful numbers that spirometry can give a
doctor. The doctor can use the ratio of FEV1 to FVC to evaluate patients for
airflow obstruction. It is normally 75 – 85%, depending on the patient's
age. The ratio is reduced in obstructive diseases, while it is preserved or
even increased in restrictive disorders (reduced lung volume). A lower than
normal FEV1 is a sign of a lung disease. A falling FEV1 is a sign that a person's
lung disease is getting worse.
The "normal" values for FVC and FEV1 for a patient depend on their age, gender,
height, and race. Normal values are higher for younger than for older people,
higher for tall than for short individuals, higher for men than for women,
and higher for whites than blacks or Asians. Therefore, the numbers are presented
as percentages of the average expected in someone of the same age, height,
sex, and race whose airways are not blocked. This is called percent predicted.
Any number smaller than 85% of predicted is considered abnormal.
If these numbers are lower than they should be, the patient is referred for
additional lung function tests to find out why. These tests may include checking
the patient's response to bronchodilators, absolute lung volumes, and blood
levels of oxygen and carbon dioxide, which tell how well gas exchange is occurring.
Other important measures of lung function are arterial blood gas tensions (PaO2
and PaCO2) and the diffusing capacity of the lung for carbon monoxide (DLCO).
Some doctors recommend having spirometry before age 25 to get baseline numbers
to compare against later test results. However, if you are a smoker, are occupationally
exposed to irritants, or have symptoms of cough, wheeze, or shortness of breath,
you should be checked with a spirometer every 3 - 5 years, or more often if
recommended by your doctor.
Abnormal spirometry numbers at any age mean that you are at risk for early
lung disease and even potentially fatal lung cancer, heart disease, or stroke.
You should immediately stop smoking, if you still do, and talk to your doctor
about other steps you may need to take depending on why you have abnormal spirometry
Created by the National Heart, Lung, and Blood Institute. Modified by A.D.A.M.,
Review Date: 5/16/2007
Reviewed By: Alan Greene, M.D., F.A.A.P., Department of Pediatrics, Packard Children's Hospital, Stanford University School of Medicine; Chief Medical Officer, A.D.A.M., Inc.
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