(LifeWire) - Doctors often order pulmonary function tests (PFTs) for people who have persistent asthma. These tests measure different aspects of how the lungs are functioning and give the doctor a better sense of the severity of the asthma and how well the treatment is working.
Also, when an asthma diagnosis is uncertain, PFTs can help differentiate asthma from other chronic lung disorders.
There are two main categories of lung disease: obstructive and restrictive. In obstructive diseases, such as asthma and emphysema, the flow of air in and out of the lungs is impaired. In restrictive diseases, such as pulmonary fibrosis, the lungs have lost size or elasticity and do not fill or expand properly. Both types of diseases can cause the same symptoms and therefore, doctors can order PFTs to help make a diagnosis.
What to Expect When Having a PFT
Most PFTs are performed in a pulmonary function laboratory. The person being tested will be asked to breathe into a special mouthpiece. Each measurement is done multiple times to make sure the results are consistent. These tests are very safe, although some people may experience some breathlessness or lightheadedness as a result of the deep breathing.
The doctor or the testing facility will give specific information as to what inhalers or other medications (if any) should be stopped prior to the tests. It is also advisable not to have a heavy meal or smoke in the few hours preceding the appointment.
Various PFTs Measure Different Lung Functions
There are many different kinds of PFTs that measure different aspects of lung function. The doctor will determine which tests are necessary based on a person's specific lung issues. The various PFTs fall into three main categories. The three types of PFTs and what they measure are:
- Spirometry: measures how much air is moving in and out of the lungs and at what speed
- Lung Volumes: measures the size and capacity of the lungs
- Diffusion Capacity: gives information about how well the lungs are extracting oxygen from the air
Each of these categories includes many different PFTs, all of which can yield diverse information about the lungs. Normal values for many of these tests will depend on the age, size and gender of the person being tested. Some of the most common tests are listed as follows:
- FVC (Forced Vital Capacity) - A measurement of the total air exhaled after a deep inhalation.
- FEV1 (Forced Expiratory Volume in 1 Second) - After a deep inhale, a measurement of the amount of air expelled in the first second of a forced exhalation.
- PEF (Peak Expiratory Flow) - Fastest rate that air can be exhaled (measured in liters per second).
- TV (Tidal Volume) - Amount or volume of air that is inhaled (or exhaled) in each normal breath.
- DLCO (Diffusing Capacity of the Lung for Carbon Monoxide) - Measurement of the ability of the lungs to transfer gases from the air to the blood, which is done by using small, harmless amounts of carbon monoxide.
- TLC (Total Lung Capacity) - Total amount of air in the lungs after a maximal inhalation.
- Pulse Oximetry - Noninvasive measurement of oxygen level in the blood.
- Arterial Blood Gas - Blood test measuring the level of oxygen in the blood.
What Doctors Can Learn From PFTs
Doctors will use the results of the PFTs to plan and refine treatment or to diagnose lung dysfunction.
Calculating the ratio between FEV1 and FVC is particularly helpful in deciding whether a person's lung disease is an obstructive or restrictive type. If the FEV1/FVC ratio is lower than expected, this indicates obstruction. If the FEV1/FVC ratio is normal but the FVC is decreased, this indicates restrictive disease. If both the FVC and the FEV1/FVC are reduced, this may mean that there are both restrictive and obstructive components to the problem.
The DLCO is also important in determining lung function. When the DLCO is diminished, this indicates that gas exchange is impaired. Conditions such as emphysema and blood clots in the lung can reduce the area of lung that is available for gas exchange, which therefore reduces DLCO. Pulmonary fibrosis and other diseases that cause a thickening of the lung tissue may also reduce DLCO. Although anemia may also diminish DLCO, because the blood itself has a lower oxygen-carrying capacity, it has nothing to do with lung functioning.
A Person With Asthma Can Have Normal PFTs
It is possible for a person to have asthma and also have completely normal PFTs. This means that in the absence of an acute attack, the person does not have any airway obstruction. One way to test for asthma in this situation is with something called bronchoprovocative testing. This testing involves having the person inhale an irritant (often methacholine), which is then measured for obstruction. If the person experiences asthma symptoms, the test is considered positive, and then medicines called bronchodilators are administered to reverse the effects.
Sometimes bronchodilators can be used to distinguish between asthma and other obstructive diseases, such as emphysema or chronic bronchitis. For patients with asthma, bronchodilators should be more effective in reversing obstruction.
Other Measurements of Lung Function
Flow-volume curves graph the amount of air exhaled over time. The appearance of these curves will differ depending on whether the pattern is one of obstructive, restrictive or normal lung function. These curves can be especially helpful in detecting severe obstruction resulting from a foreign body or tumors that compress the airway.
Plethysmography measures the TLC. Even after a person has exhaled completely, air remains in the lungs. By measuring this leftover air, the entire amount of air that the lungs can hold can be calculated. For this test, the person sits in an enclosed booth and breathes in and out as pressures are measured. The TLC can also be measured by having the person inhale a gas such as helium. The TLC is calculated by measuring the amount of helium that gets exhaled.
Another specialized test measures exhaled nitrous oxide. This gas is a marker of inflammation in the airway and may give the doctor some sense of how a person's asthma is being controlled.