Lung Function Tests

Lung function tests (also called pulmonary function tests, or PFTs) evaluate how well your lungs work. The tests determine how much air your lungs can hold, how quickly you can move air in and out of your lungs, and how well your lungs put oxygen into and remove carbon dioxide from your blood. The tests can diagnose lung diseases, measure the severity of lung problems, and check to see how well treatment for a lung disease is working.

Other tests such as residual volume, gas diffusion tests, body plethysmography, inhalation challenge tests, and exercise stress tests may also be done to determine lung function.

Spirometry is the first lung function test done. It measures how much and how quickly you can move air out of your lungs. For this test, you breathe into a mouthpiece attached to a recording device (spirometer). The information collected by the spirometer may be printed out on a chart called a spirogram.

The more common lung function values measured with spirometry are:

• Forced vital capacity (FVC). This measures the amount of air you can exhale with force after you inhale as deeply as possible.
• Forced expiratory volume (FEV). This measures the amount of air you can exhale with force in one breath. The amount of air you exhale may be measured at 1 second (FEV1), 2 seconds (FEV2), or 3 seconds (FEV3). FEV1 divided by FVC can also be determined.
• Forced expiratory flow 25% to 75%. This measures the air flow halfway through an exhale (FVC).
• Peak expiratory flow (PEF). This measures how quickly you can exhale. It is usually measured at the same time as your forced vital capacity (FVC).
• Maximum voluntary ventilation (MVV). This measures the greatest amount of air you can breathe in and out during one minute.
• Slow vital capacity (SVC). This measures the amount of air you can slowly exhale after you inhale as deeply as possible.
• Total lung capacity (TLC). This measures the amount of air in your lungs after you inhale as deeply as possible.
• Functional residual capacity (FRC). This measures the amount of air in your lungs at the end of a normal exhaled breath.
• Expiratory reserve volume (ERV). This measures the difference between the amount of air in your lungs after a normal exhale (FRC) and the amount after you exhale with force (RV).

Gas diffusion tests

Gas diffusion tests measure the amount of oxygen and other gases that cross the lungs' air sacs (alveoli) per minute. These tests evaluate how well gases are being absorbed into your blood from your lungs. Gas diffusion tests include:

• Arterial blood gases, which determine the amount of oxygen and carbon dioxide in your bloodstream.
• Carbon monoxide diffusing capacity (also called transfer factor, or TF), which measures how well your lungs transfer a small amount of carbon monoxide (CO) into the blood. Two different methods are used for this test. If the single-breath or breath-holding method is used, you will take a breath of air containing a very small amount of carbon monoxide from a container while measurements are taken. In the steady-state method, you will breathe air containing a very small amount of carbon monoxide from a container. The amount of carbon monoxide in the breath you exhale is then measured. Diffusing capacity provides an estimate of how well a gas is able to move from your lungs into your blood.

Body plethysmography

Body plethysmography may be used to measure:

• Total lung capacity (TLC), which is the total amount of air your lungs can hold. For this test, you sit inside an airtight booth called a plethysmograph and breathe through a mouthpiece while pressure and air flow measurements are collected.
• Residual volume (RV), which is the amount of air that remains in your lungs after you exhale as completely as possible. For this test, you sit inside the plethysmograph booth and breathe a known amount of a gas (either 100% oxygen or a certain amount of helium in air). The test measures how the concentration of the gases in the booth changes.

Inhalation challenge tests

Inhalation challenge tests are done to measure the response of your airways to substances (allergens) that may be causing asthma or wheezing. The tests also may determine the effect of chemicals such as histamine or methacholine on your airways. These tests are also called provocation studies.

During inhalation testing, increasing amounts of an allergen are inhaled through a nebulizer, a device that uses a face mask or mouthpiece to deliver the allergen in a fine mist (aerosol). Alternatively, increasing amounts of a substance (histamine or methacholine) may be inhaled through the nebulizer. Before and after inhaling the substance, spirometry readings are taken to evaluate lung function.

In rare cases, a bronchospasm can occur with inhalation challenge testing. You will be closely monitored during and after the test.

Exercise stress tests

Exercise stress tests evaluate the effect of exercise on lung function tests. Spirometry readings are done after exercise and then again at rest.

Lung function results are measured directly in some tests and are calculated in others. No single test can determine all of the lung function values, so more than one type of test may be done. Some of the tests may be repeated after you inhale medicine that enlarges your airways (bronchodilator).

Lung function tests are done to:

• Determine the cause of breathing problems.
• Diagnose certain lung diseases, such as asthma or chronic obstructive pulmonary disease (COPD).
• Evaluate a person's lung function before surgery.
• Monitor the lung function of a person who is regularly exposed to substances such as asbestos that can damage the lungs.
• Monitor the effectiveness of treatment for lung diseases.

Tell your doctor if you:

• Have had recent chest pains or a heart attack.
• Take medicine for a lung problem, such as asthma. You may need to stop taking some medicines before testing.
• Are allergic to any medicines.

Do not eat a heavy meal just before this test because a full stomach may prevent your lungs from fully expanding. You should not smoke or exercise strenuously for 6 hours before the test. On the day of the test, wear loose clothing that does not restrict your breathing in any way. You should also avoid food or drinks that contain caffeine because it can cause your airways to relax and allow more air than usual to pass through.

If you have dentures, wear them during the test to help you form a tight seal around the mouthpiece of the spirometer.

Lung function tests are usually done in special exam rooms that have all of the lung function measuring devices. The test is usually done by a specially trained respiratory therapist or technician. For most of the lung function tests, you will wear a nose clip to make sure that no air passes in or out of your nose during the test. You then will be asked to breathe into a mouthpiece connected to a recording device.

The exact procedure is different for each type of test. For example, you may be asked to inhale as deeply as possible and then to exhale as fast and as hard as possible. You also may be asked to breathe in and out as deeply and rapidly as possible for 15 seconds. Some tests may be repeated after you have inhaled a spray containing medicine that expands the airways in your lungs (bronchodilator). You may be asked to breathe a special mixture of gases, such as 100% oxygen, a mixture of helium and air, or a mixture of carbon monoxide and air. Sometimes a sample of blood may be taken from an artery in your wrist to measure blood gases.

If you have body plethysmography, you will be asked to sit inside a small enclosure similar to a telephone booth, with windows that allow you to see out. The booth measures small changes in pressure that occur as you breathe.

The accuracy of the tests depends on your ability to follow all of the instructions. The therapist may strongly encourage you to breathe deeply during some of the tests to get the best results.

The testing may take from 5 to 30 minutes, depending upon how many tests are done.

If you have an arterial blood gas test, you may feel some pain from the needle used to collect the blood. The other lung function tests are usually painless. Some of the tests may be tiring for people who have a lung disease.

You may cough or feel lightheaded after breathing in or out rapidly, but you will be given a chance to rest between tests. You may find it uncomfortable to wear the nose clip. Breathing through the mouthpiece for a long period of time may be uncomfortable.

If you have body plethysmography, you may feel uncomfortable in the airtight plethysmograph booth. However, the therapist will be nearby during the test to open the door if you feel too uncomfortable.

If you are given breathing medicine, it may cause you to shake or may increase your heart rate. If you feel any chest pain or discomfort, tell the therapist immediately.

Lung function tests present little or no risk to a healthy person. If you have a serious heart or lung condition, discuss your risks with your doctor.

Lung function tests (also called pulmonary function tests, or PFTs) evaluate how well your lungs work. The normal value ranges for lung function tests will be adjusted for your age, height, sex, and sometimes weight and race. Results are often expressed in terms of a percentage of the expected value. Most test results are available right away.

Normal

Test results are within the normal ranges for a person with healthy lungs.

Abnormal

Test results are outside of the normal range for a person with healthy lungs. This may mean that some kind of lung disease is present. There are two main types of lung disease that can be found with lung function tests: obstructive and restrictive.

Obstructive

In obstructive lung conditions, the airways are narrowed, usually causing an increase in the time it takes to empty the lungs. Obstructive lung disease can be caused by conditions such as emphysema, bronchitis, infection (which produces inflammation), and asthma.

FEV1 often increases after using medicine that expands the airways in people with reversible obstructive disease like asthma.

Restrictive

In restrictive lung conditions, there is a loss of lung tissue, a decrease in the lungs' ability to expand, or a decrease in the lungs' ability to transfer oxygen to the blood (or carbon dioxide out of the blood). Restrictive lung disease can be caused by conditions such as pneumonia, lung cancer, scleroderma, pulmonary fibrosis, sarcoidosis, or multiple sclerosis. Other restrictive conditions include some chest injuries, being very overweight (obesity), pregnancy, and loss of lung tissue due to surgery.

Reasons you may not be able to have the test or why the results may not be helpful include:

• Using medicine that expands the lungs' airways within 4 hours of the test.
• Using sedatives before the test.
• Eating food or drinks that contain caffeine before the test.
• Not being able to breathe normally because of pain.
• Pregnancy, obesity, or an enlarged stomach (after a large meal, for example).
• Not being able to follow instructions and give some effort to the tests.

• Spirometry is the most commonly used lung function test.
• If your spirometry tests are normal but your doctor thinks you may have asthma, additional tests may be done after you inhale a substance (methacholine or histamine) that narrows (constricts) your airways. This is called a bronchoprovocation test. It may be done while you sit in a small enclosure (plethysmograph) similar to a telephone booth. The amount of narrowing in your airways can help diagnose some conditions. This testing may take as long as 2 hours.
• Arterial blood gases (ABGs), which determine the amount of oxygen and carbon dioxide in your bloodstream, may be measured before, during, or after your lung function tests. For more information, see the medical test Arterial Blood Gases.
• Some lung function tests can be done at home. For more information, see the medical test Home Lung Function Test.

Other Works Consulted

• Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.

• Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.

• Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.