Pulmonary Function Tests
Pulmonary function tests are a group of tests that measure how well the lungs take in and release air and how well they move gases such as oxygen from the atmosphere into the body’s circulation.
How the Test is Performed
Spirometry measures airflow. By measuring how much air you exhale, and how quickly, spirometry can evaluate a broad range of lung diseases. In a spirometry test, while you are sitting, you breathe into a mouthpiece that is connected to an instrument called a spirometer. The spirometer records the amount and the rate of air that you breathe in and out over a period of time. When standing, some numbers might be slightly different. The most important issue is to perform the test always while at the same position.
For some of the test measurements, you can breathe normally and quietly. Other tests require forced inhalation or exhalation after a deep breath. Sometimes you will be asked to inhale the substance or a medicine to see how it changes your test results.
Lung volume measurement can be done in two ways:
- The most accurate way is to sit in a sealed, clear box that looks like a telephone booth (body plethysmograph) while breathing in and out into a mouthpiece. Changes in pressure inside the box help determine the lung volume.
- Lung volume can also be measured when you breathe nitrogen or helium gas through a tube for a certain period of time. The concentration of the gas in a chamber attached to the tube is measured to estimate the lung volume.
To measure diffusion capacity, you breathe a harmless gas, called a tracer gas, for a very short time, often for only one breath. The concentration of the gas in the air you breathe out is measured. The difference in the amount of gas inhaled and exhaled measures how effectively gas travels from the lungs into the blood. This test allows the doctor to estimate how well the lungs move oxygen from the air into the bloodstream.
There are 2 types of disorders that cause problems with air moving in and out of the lungs:
- Obstructive. This is when air has trouble flowing out of the lungs due to resistance. This causes a decreased flow of air.
- Restrictive. This is when the chest muscles can’t expand enough. This creates problems with air flow.
- Tidal volume (VT). This is the amount of air inhaled or exhaled during normal breathing.
- Minute volume (MV). This is the total amount of air exhaled per minute.
- Vital capacity (VC). This is the total volume of air that can be exhaled after inhaling as much as you can.
- Functional residual capacity (FRC). This is the amount of air left in lungs after exhaling normally.
- Residual volume. This is the amount of air left in the lungs after exhaling as much as you can.
- Total lung capacity. This is the total volume of the lungs when filled with as much air as possible.
- Forced vital capacity (FVC). This is the amount of air exhaled forcefully and quickly after inhaling as much as you can.
- Forced expiratory volume (FEV). This is the amount of air expired during the first, second, and third seconds of the FVC test.
- Forced expiratory flow (FEF). This is the average rate of flow during the middle half of the FVC test.
- Peak expiratory flow rate (PEFR). This is the fastest rate that you can force air out of your lungs.
How Do I Prepare for Pulmonary Function Tests?
If you’re on medications that open your airways, such as those used for asthma or bronchitis, your doctor may ask you to stop taking them before the test. If it isn’t clear whether or not you should take your medication, make sure to ask your doctor. Pain medications may also affect the results of the test. You should tell your doctor about any over-the-counter and prescription pain medications you’re taking.
It’s important that you don’t eat a large meal before testing. A full stomach can prevent your lungs from inhaling fully. You should also avoid food and drinks that contain caffeine, such as chocolate, coffee, and tea, before your test. Caffeine can cause your airways to open. You should also avoid smoking and strenuous exercise before the test.
Be sure to wear loose-fitting clothing to the test. Tighter clothing may restrict your breathing. You should also avoid wearing jewelry that might affect your breathing. If you wear dentures, wear them to the test to ensure that your mouth can fit tightly around the mouthpiece used for the test.
Why might I need pulmonary function tests?
There are many different reasons why pulmonary function tests (PFTs) may be done. They are sometimes done in healthy people as part of a routine physical. Or you may have PFTs if your healthcare provider needs help to diagnose you with a health problem such as:
- Respiratory infections
- Trouble breathing from injury to the chest or a recent surgery
- Chronic lung conditions, such as asthma, bronchiectasis, emphysema, or chronic bronchitis
- Asbestosis, a lung disease caused by inhaling asbestos fibers
- Restrictive airway problems from scoliosis, tumors, or inflammation or scarring of the chest wall
- Sarcoidosis, a disease that causes lumps of inflammatory cells around organs such as the liver, lungs, and spleen
- Scleroderma, a disease that causes thickening and hardening of connective tissue
PFTs may be used to check lung function before surgery or other procedures in patients who have lung or heart problems, who are smokers, or who have other health conditions. Another use of PFTs is to assess treatment for asthma, emphysema, and other chronic lung problems. Your healthcare provider may also have other reasons to advise PFTs.
What are the risks of pulmonary function tests?
Because pulmonary function testing is not an invasive procedure, it is safe and quick for most people. But the person must be able to follow clear, simple directions.
All procedures have some risks. The risks of this procedure may include:
- Dizziness during the tests
- Feeling short of breath
- Asthma attack brought on by deep inhalation
In some cases, a person shouldn’t have PFTs. Reasons for this can include:
- Recent eye surgery, because of increased pressure inside the eyes during the procedure
- Recent belly (abdominal) or chest surgery
- Chest pain, recent heart attack, or an unstable heart condition
- A bulging blood vessel (aneurysm) in the chest, belly, or brain
- Active tuberculosis (TB) or respiratory infection, such as a cold or the flu
Your risks may vary depending on your general health and other factors. Ask your healthcare provider which risks apply most to you. Talk with him or her about any concerns you have.
What happens during pulmonary function tests?
You may have your procedure as an outpatient. This means you go home the same day. Or it may be done as part of a longer stay in the hospital. The way the procedure is done may vary. It depends on your condition and your healthcare provider’s methods. In most cases, the procedure will follow this process:
- Your height and weight will be recorded before the test. This is done so that your results can be accurately calculated.
- You’ll be asked to loosen tight clothing, jewelry, or other things that may cause a problem with the procedure.
- If you wear dentures, you will need to wear them during the procedure.
- You’ll need to empty your bladder before the procedure.
- You’ll sit in a chair. A soft clip will be put on your nose. This is so all of your breathing is done through your mouth, not your nose.
- You’ll be given a sterile mouthpiece that is attached to a spirometer.
- You’ll form a tight seal over the mouthpiece with your mouth. You’ll be instructed to inhale and exhale in different ways.
- You will be watched carefully during the procedure for dizziness, trouble breathing, or other problems.
- You may be given a bronchodilator after certain tests. The tests will then be repeated several minutes later, after the bronchodilator has taken effect.
What happens after pulmonary function tests?
If you have a history of lung or breathing problems, you may be tired after the tests. You will be given a chance to rest afterwards. Your healthcare provider will talk with you about your test results.