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Spirogram

SMar 30 05

Alternative names
PFTs; Spirometry; Pulmonary function tests; Lung function tests

Definition

Pulmonary function tests are a broad range of tests that are usually done in a health care provider’s office or a specialized facility. They measure how well the lungs take in and exhale air and how efficiently they transfer oxygen into the blood.

Spirometry measures how well the lungs exhale. The information gathered during this test is useful in diagnosing certain types of lung disorders, but is most useful when assessing for obstructive lung diseases (especially asthma and chronic obstructive pulmonary disease, COPD).

Lung volume measurement detects restrictive lung diseases. In this set of diseases, a person cannot inhale a normal volume of air. Restrictive lung diseases may be caused by inflammation or scarring of the lung tissue (interstitial lung disease) or by abnormalities of the muscles or skeleton of the chest wall.

Testing the diffusion capacity (also called the DLCO) permits an estimate of how efficiently the lungs transfer oxygen from the air into the bloodstream.

How the test is performed


  • In a spirometry test, a person breathes into mouthpiece that is connected to an instrument called a spirometer. The spirometer records the amount and the rate of air that is breathed in and out over a specified time. Some of the test measurements are obtained by normal, quiet breathing, and other tests require forced inhalation or exhalation after a deep breath.
  • Lung volume measurement can be performed in two ways. The most accurate way is for a person to sit in a body plethysmograph, a sealed, transparent box that resembles a telephone booth, while breathing in and out against into a mouthpiece. Changes in pressure inside the box allow determination of the lung volume. Lung volume can also be measured when a person breathes nitrogen or helium gas through a tube for a specified period of time. The concentration of the gas in a chamber attached to the tube is measured, allowing estimation of the lung volume.
  • The diffusion capacity is measured when a person breathes carbon monoxide for a very short time, often one breath. The concentration of carbon monoxide in exhaled air is then measured. The difference in the amount of carbon monoxide inhaled and the amount exhaled allows estimation of how rapidly gas can travel from the lungs into the blood.

How to prepare for the test
Do not eat a heavy meal before the test. Do not smoke for 4 to 6 hours prior to the test. Specific instructions will be given if bronchodilators or inhaler medications should be withheld. Sometimes, medication may be inhaled prior to the test.

Children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For general information regarding how you can prepare your child, see the following topics:


  • Toddler test or procedure preparation (1 to 3 years)
  • Preschooler test or procedure preparation (3 to 6 years)
  • Schoolage test or procedure preparation (6 to 12 years)
  • Adolescent test or procedure preparation (12 to 18 years)

How the test will feel
Since the test involves some forced breathing and rapid breathing, some temporary shortness of breath or lightheadedness may be experienced. There is a tight fitting mouth piece to breathe through, and nose clips are applied.

Why the test is performed
The test is performed to diagnose certain types of lung disease (especially asthma, bronchitis, and emphysema), to determine the cause of shortness of breath, or to measure whether occupational exposure to contaminants affects lung function. It can also be used after the administration of medications to assess their effect, and to measure progress in disease treatment.

Normal Values
Normal values are based upon the age, height, ethnicity, and sex of the person being tested. Normal results are expressed as a percentage. A value is usually considered abnormal if it is less than 80% of the predicted value for that person.

What abnormal results mean
Abnormal results usually mean that a degree of chest or lung disease may be present.

What the risks are
The risk is minimal for most people. There is a small risk of collapsed lung in people with a certain type of lung disease. The test should not be given to a person who has experienced a recent heart attack, or who has certain other types of heart disease.

Special considerations
Cooperation from the patient performing the test is crucial in providing accurate results. A poor seal around the mouthpiece of the spirometer can give poor results that do not permit interpretation. Do not smoke before the test.

Johns Hopkins patient information

Last revised: December 8, 2007
by Armen E. Martirosyan, M.D.

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