Pulmonary function tests (PFTs) are noninvasive tests that show how well the lungs are working. The tests measure lung volume, capacity, rates of flow, and gas exchange. This information can help your healthcare provider diagnose and decide the treatment of certain lung disorders.
Pulmonary function tests (PFTs) allow physicians to evaluate the respiratory function of their patients. They are reproducible and accurate. Ultimately, the results of the PFTs are affected by the effort of the patient. PFTs do not provide a specific diagnosis, but together with the history, physical exam, and laboratory data help clinicians reach a diagnosis. PFTs also allow physicians to quantify the severity of the pulmonary disease, follow it up over time, and assess its response to treatment.[rx]
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 airway resistance. This causes a decreased flow of air.
- Restrictive. This is when the lung tissue and/or chest muscles can’t expand enough. This creates problems with airflow, mostly due to lower lung volumes.
PFT can be done with 2 methods. These 2 methods may be used together and perform different tests, depending on the information that your healthcare provider is looking for:
- Spirometry. A spirometer is a device with a mouthpiece hooked up to a small electronic machine.
- Plethysmography. You sit or stand inside an air-tight box that looks like a short, square telephone booth to do the tests.
- 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 the 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 that 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.
- Spirometry measures the rate of airflow and estimates lung size. For this test, you will breathe multiple times, with regular and maximal effort, through a tube that is connected to a computer. Some people feel lightheaded or tired from the required breathing effort.
- Lung volume tests are the most accurate way to measure how much air your lungs can hold. The procedure is similar to spirometry, except that you will be in a small room with clear walls. Some people feel lightheaded or tired from the required breathing effort.
- Lung diffusion capacity assesses how well oxygen gets into the blood from the air you breathe. For this test, you will breathe in and out through a tube for several minutes without having to breathe intensely. You also may need to have blood drawn to measure the level of hemoglobin in your blood.
- Pulse oximetry estimates oxygen levels in your blood. For this test, a probe will be placed on your finger or another skin surface such as your ear. It causes no pain and has few or no risks.
- Arterial blood gas tests directly measure the levels of gases, such as oxygen and carbon dioxide, in your blood. Arterial blood gas tests are usually performed in a hospital, but may be done in a doctor’s office. For this test, blood will be taken from an artery, usually in the wrist where your pulse is measured. You may feel brief pain when the needle is inserted or when a tube attached to the needle fills with blood. It is possible to have bleeding or infection where the needle was inserted.
- Fractional exhaled nitric oxide tests measure how much nitric oxide is in the air that you exhale. For this test, you will breathe out into a tube that is connected to the portable device. It requires steady but not intense breathing. It has few or no risks.
Normal values for PFTs vary from person to person. The amount of air inhaled and exhaled in your test results are compared to the average for someone of the same age, height, sex, and race. Results are also compared to any of your previous test results. If you have abnormal PFT measurements or if your results have changed, you may need other tests.
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. They are also routinely done in certain types of work environments to ensure employee health (such as graphite factories and coal mines). 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 lungs
- 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.
Spirometry is a physiological test that measures the ability to inhale and exhale air in relation to time. Spirometry is a screening test of general respiratory health. The main results of spirometry are forced vital capacity (FVC) and forced expiratory volume (FEV). The procedure of spirometry has 3 phases: 1) maximal inspiration; 2) a “blast” of exhalation; 3) continued complete exhalation to the end of the test. There are within-maneuver acceptability and between-maneuver reproducibility criteria for spirometry (Table 2).
Vital capacity (VC) is the volume of gas expelled from full inspiration to residual volume. The FVC is similar, but the patient is exhaling at maximal speed and effort.
The FEV is the forced expiratory volume in t seconds from a position of full inspiration. Forced expiratory volume in the first second (FEV)) is used to classify the severity of obstructive lung diseases. The reversibility testing is administered using a bronchodilator (short-acting beta 2-agonist or anticholinergic agent). An increase in either FEV or FVC of 3^12% and 3^200 mL is considered a positive bronchodilator response. A lack of a response does not predict lack of response to bronchodilators. The patient should hold their bronchodilators before the reversibility testing.
The maximal flow-volume curves are a great asset to detect mild airflow obstruction. There is an inspiratory and expiratory loop. [rx]
The key measurement of lung volumes is functional reserve capacity (FRC). Once FRC has been measured, all other volumes can be calculated. FRC is the volume of the amount of gas in the lungs at the end of expiration during tidal breathing. FRC is the sum of expiratory reserve volume (ERV) and residual volume (RV). ERV is the volume of gas maximally exhaled after end-inspiratory tidal breathing. RV is the volume of gas in the airways after a maximal exhalation.
In addition to RV and ERV, there is tidal volume (TV) and inspiratory reserve volume (IRV). IRV is the volume of gas that can be maximally inhaled from the end-inspiratory tidal breathing. TV is the volume of gas inhaled or exhaled with each breath at rest.
There are 2 methods to measure lung volumes: body plethysmography and gas dilution methods (nitrogen washout and inert gas dilution). Gas dilution method uses an inert gas (poorly soluble in alveolar blood and lung tissues), either nitrogen or helium. The subject breathes a gas mixture until equilibrium is achieved. The volume and mixture of gas exhaled after the equilibrium have been achieved permit the calculation of FRC. In body plethysmography, the subject sits inside a body box and breathes against a shutter valve. FRC is calculated using Boyle Law (at a given temperature, the product of gas volume and pressure is constant). FRC calculated by body plethysmography is usually larger in subjects with obstructive lung disease and air trapping than FRC calculated using gas dilution methods. Body plethysmography is considered the gold standard for lung volumes measurement.
Capacities are the sum of 2 or more volumes (Figure 1). TLC is the gold standard for diagnosis of restrictive lung disease. TLC less than 5 percentile of predicted or less than 80% predicted are diagnostic of a restrictive ventilatory defect.[rx][rx]
Diffusion studies the diffusion of gases across the alveolar-capillary membranes. Its measurement uses carbon monoxide (CO) to calculate the pulmonary diffusion capacity. The most common method is the standard single-breath D. It is measured in milliliters per minute per mm Hg.
The factors affecting the D are volume and distribution of ventilation, mixing and diffusion, the composition of the gas, characteristics of the alveolar membrane and lung parenchyma, the volume of alveolar-capillary plasma, concentration, and binding properties of hemoglobin, and gas tensions in blood entering the alveolar capillaries. A detailed list of factors affecting DLCO is listed in Table 3.
The diffusion capacity depends on multiple factors, and its value should be adjusted. Specific adjustments should be made for hemoglobin, carboxyhemoglobin, and FiO for a correct interpretation. Adjustment for lung volumes is controversial, and further studies are needed.
The DLCO is interpreted in conjunction with spirometry and lung volumes. Table 3 shows the severity classification for DLCO. For example, high DLCO is associated with asthma, obesity, and intrapulmonary hemorrhage. Normal spirometry and lung volumes with low DLCO can be present in pulmonary vascular diseases, early ILD, or emphysema. An obstructive ventilatory defect with low DLCO suggests emphysema or lymphangiomyomatosis.[rx][rx]
Respiratory Muscle Pressures
The respiratory muscle strength is assessed with the maximal inspiratory pressure (MIP); also called negative inspiratory force (NIF) and maximal expiratory pressure (MEP). The MIP reveals the strength of the diaphragm and other inspiratory muscles; whereas, the MEP indicates the strength of the abdominal and other expiratory muscles.
MIP and MEP are measured three times, maximal value is reported. For adults 18 to 65 years old, MIP should be lower than -90 cmHO in men and -70 cmHO in women. In adults older than 65 years old, MIP should be less than -65 cmH2O in men and -45 cmH2O in women. MEP should be higher than 140 cmH2O in men and 90 cmH2O in women. MEP less than 60 cmH2O predicts a weak cough and difficulty clearing secretions.
Central and Upper Airway Obstruction
Central or upper airway obstruction (UAO) could occur in the extrathoracic (pharynx, larynx, and the extrathoracic part of the trachea) and intrathoracic airways (intrathoracic trachea and main bronchi). The FEV and/or FVC are not affected by this type of obstruction, but the peak expiratory flow (PEF) can be severely decreased. An FEV/PEF ratio of greater than 8 suggests central or UAO.
Three maximal and repeatable forced inspiratory and expiratory flow-volume curves are necessary. It is key that efforts, both inspiratory and expiratory are maximal.
The maximal inspiratory flow is largely decreased with an extrathoracic airway obstruction because there is a negative pressure inside the airways during inspiration. Inspiratory flows are not affected by intrathoracic lesions; the negative pleural pressure maintains the intrathoracic airways open. The maximal expiratory flow (peak flow) is decreased with intrathoracic and extrathoracic lesions.
The obstructions can be intrathoracic or extrathoracic and variable or fixed. In summary, fixed obstructions will have decreased inspiratory and expiratory flows; variable obstruction will depend on the location (intrathoracic or extrathoracic). The flow curve is not a sensitive test; the absence of the classic pattern does not rule out a central or UAO.[rx][rx][rx][rx]
There are multiple indications to obtain PFTs. Table 1 summarizes the most commons indications.
- PFTs can be physically demanding for patients, and it is recommended to wait one month after an acute coronary syndrome or myocardial infarction.
- Other relative contraindications are thoracic/abdominal surgery, brain/eye/ear/otolaryngological surgery, pneumothorax, ascending aortic aneurysm, hemoptysis, pulmonary embolism, severe hypertension (SBP greater than 200 mm Hg, DBP greater than 120 mm Hg).[rx]
- chronic bronchitis
- respiratory infections
- lung fibrosis
- bronchiectasis, a condition in which the airways in the lungs stretch and widen
- COPD, which used to be called emphysema
- asbestosis, a condition caused by exposure to asbestos
- sarcoidosis, an inflammation of your lungs, liver, lymph nodes, eyes, skin, or other tissues
- scleroderma, a disease that affects your connective tissue
- pulmonary tumor
- lung cancer
- weaknesses of the chest wall muscles
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
- Recent eye surgery, because of increased pressure inside the eyes during the procedure
- Recent belly 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.
Certain things can make PFTs less accurate. These include:
- The degree of patient cooperation and effort
- Use of medicines that open the airways (bronchodilators)
- Use of pain medicines
- Stomach bloating that affects the ability to take deep breaths
- Extreme tiredness or other conditions that affect a person’s ability to do the tests (such as a head cold)
How do I get ready for pulmonary function tests?
Your healthcare provider will explain the procedure to you. Ask him or her any questions you have. You may be asked to sign a consent form that gives permission to do the procedure. Read the form carefully. Ask questions if anything is not clear.
Tell your healthcare provider if you take any medicines. This includes prescriptions, over-the-counter medicines, vitamins, and herbal supplements.
Make sure to:
- Stop taking certain medicines before the procedure, if instructed by your healthcare provider
- Stop smoking before the test, if instructed by your healthcare provider. Ask your provider how many hours before the test you should stop smoking.
- Not eat a heavy meal before the test, if instructed by your healthcare provider
- Follow any other instructions your healthcare provider gives you
Your height and weight will be recorded before the test. This is done so that your results can be accurately calculated.
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:
- 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.
Below are the steps for the most common types of lung function tests.
For a spirometry test
- You’ll sit in a chair and a soft clip will be put on your nose. This is done so you’ll breathe through your mouth, rather than your nose.
- You’ll be given a mouthpiece that is attached to a machine called a spirometer.
- You’ll place your lips tightly around the mouthpiece, and breathe in and out as instructed by your provider.
- The spirometer will measure the amount and rate of airflow over a period of time.
For a lung volume (body plethysmography) test
- You’ll sit in a clear, airtight room that looks like a telephone booth.
- As with a spirometry test, you’ll wear a nose clip and place your lips around a mouthpiece connected to a machine.
- You’ll breathe in and breathe out as instructed by your provider.
- The pressure changes inside the room help measure lung volume.
For a gas diffusion test
- You’ll wear a mouthpiece connected to a machine.
- You will be asked to inhale (breathe in) a very small, non-dangerous amount of carbon monoxide or other types of gas.
- Measurements will either be taken as you breathe in or as you breathe out.
- The test can show how effective your lungs are in moving gases to your bloodstream.
For an exercise test, you will
- Ride a stationary bike or walk on a treadmill.
- You’ll be attached to monitors and machines that will measure blood oxygen, blood pressure, and heartbeat.
- This helps show how well your lungs perform during exercise.
Normal values are based on your age, height, ethnicity, and sex. Normal results are expressed as a percentage. A value is usually considered abnormal if it is approximately less than 80% of your predicted value.
Normal value ranges may vary slightly among different laboratories, based on slightly different ways to determine normal values. Talk to your provider about the meaning of your specific test results.
Different measurements that may be found on your report after pulmonary function tests include:
- Diffusion capacity to carbon monoxide (DLCO)
- Expiratory reserve volume (ERV)
- Forced vital capacity (FVC)
- Forced expiratory volume in 1 second (FEV1)
- Forced expiratory flow 25% to 75% (FEF25-75)
- Functional residual capacity (FRC)
- Maximum voluntary ventilation (MVV)
- Residual volume (RV)
- Peak expiratory flow (PEF)
- Slow vital capacity (SVC)
- Total lung capacity (TLC)
What Abnormal Results Mean
Abnormal results usually mean that you may have chest or lung disease.
Some lung diseases (such as emphysema, asthma, chronic bronchitis, and infections) can make the lungs contain too much air and take longer to empty. These lung diseases are called obstructive lung disorders.
Other lung diseases make the lungs scarred and smaller so that they contain too little air and are poor at transferring oxygen into the blood. Examples of these types of illnesses include:
- Extreme overweight
- Pulmonary fibrosis (scarring or thickening of the lung tissue)
- Sarcoidosis and scleroderma
Muscular weakness can also cause abnormal test results, even if the lungs are normal, that is, similar to the diseases that cause smaller lungs.
Normal and Critical Findings
Normal findings of spirometry are FEV/FVC ratio of greater than 0.70 and both FEV and FVC above 80% of the predicted value. If lung volumes are performed, TLC above 80% of the predictive value is normal. Diffusion capacity above 75% of the predicted value is considered normal as well.
PFTs are safe in general, and there are no complications. There is some potential harm from 4 key factors:
Maximal pressures generated in the thorax and their impact on abdominal and thoracic organs/tissues
Large swings in blood pressure causing stresses on tissues in the body
Expansion of the chest wall and lungs
Spread of infections (e.g., tuberculosis, hepatitis B, HIV)
Contraindications of PFTs are related to those 4 factors to prevent potential complications like acute coronary syndrome, rupture of aneurysms, and dehiscence of the surgical wound.[rx]