Definition & Overview
Pulmonary function testing, as the term suggests, is a diagnostic test used to evaluate the function of the lungs and to determine whether they are affected by certain diseases. This is an all-inclusive term that refers to the series of tests performed to measure different values, such as how much air the lungs can hold or how well the lungs move oxygen and filter out carbon dioxide from the blood.
Who Should Undergo & Expected Results
A pulmonary function testing is beneficial for those who are:
- Suspected of having lung problems
- Experiencing symptoms related to breathing (or having difficulty breathing)
- Suffering from existing lung condition and need to know how severe it is
- Undergoing treatment for a lung condition (to assess its effectiveness)
- About to undergo surgery and has to be checked for lung function
- Regularly exposed to substances that pose potential harm to the lungs
The results of a pulmonary function testing are expected to determine whether there is a lung disorder present and where it is located. The tests can also provide information regarding the cause of the disorder, as well as its nature, which is vital for doctors as they proceed with treatment.
How Does the Procedure Work?
The specific method of testing may differ, but lung function tests typically involve the following:
1. Spirometry - This is one of the most commonly used tests and is conducted by making the patient breathe into a mouthpiece that is attached to a spirometer, a special device designed to record the breathing pattern of the patient and convert it into printable information. The results are presented in chart form as a spirogram. A spirometry can measure the following values:
Forced vital capacity or FVC - the amount of air a person can forcefully exhale after maximum inhalation
Forced expiratory volume or FEV – the amount of air a person can forcefully exhale in a single breath
Forced expiratory flow or FEF – the average flow rate measured at the middle of an exhale process; this is usually measured at 25% and at 75%
Peak expiratory flow rate or PEFR – the maximum volume of air that is released when a person exhales to maximum capacity
Tidal volume or VT – the amount of air the lungs is able to inhale or exhale in a single normal breathing cycle
Minute volume or MV – also known as maximum voluntary ventilation or MVV, the total amount of air that is exhaled every minute
Vital capacity or VC - the volume of air the lungs is able to exhale after the person inhales as deeply as he can; this is also called SVC or slow vital capacity as the person is asked to exhale slowly
Functional residual capacity or FRC - the amount of air that is left in the lungs after a normal exhale
Residual volume – the amount of air in the lungs after complete exhalation
Expiratory reserve volume – the difference between the amount of air in the lungs after a normal exhale and an exhale with force
Total lung capacity - the total amount of air the lungs is able to hold or inhale
2. Gas diffusion tests – These measure the amount of different gases, including oxygen, that cross the alveoli or air sacs every minute to determine whether the body is absorbing gases from the blood well. There are two kinds of gas diffusion tests: the arterial blood gases test and the carbon monoxide diffusion capacity or DLCO.
3. Body plethysmography – These measure the total lung capacity and residual volume either as a standalone test or to provide supplementary information to a spirometry.
4. Inhalation challenge – Also known as provocation studies, these measure how the airways respond to common allergens, or substances that are known to typically cause wheezing or asthma. The test is conducted by having the patient inhale gradually while increasing the amount of a specific substance through a nebulizer. This is followed by a spirometry test to measure the values after the person inhales the said substance.
5. Exercise stress tests – These evaluate the effects that exercise has on a person’s lungs. Like inhalation challenge, an exercise stress test is both preceded and succeeded by a spirometry.
6. Multiple breath washout test – This is a specific test performed on patients suffering from cystic fibrosis . During the test, the patient is made to inhale air that contains tracer gas followed by regular air. The patient is then monitored to determine how much of the tracer gas he is able to exhale. This is measured using an LCI or lung clearance index, which results in higher values if the lungs are not able to exhale enough of the tracer gas. This means that there is a problem with the lungs.
Different tests measure different values, with the exception of a spirometry, which can measure most, but not all, indicators of lung health. Thus, in most cases, different tests are done together.
Possible Complications and Risks
As a diagnostic series of tests, pulmonary function testing poses only mild risk to the patient. Since the test results depend on the patient’s ability to breathe normally, with force, or deeply when being asked to do so, there is always a risk that the person might not do exactly as told, and this will yield inaccurate results. Thus, there are some preparations needed to prevent this. Patients are usually told to refrain from eating a heavy meal prior to the test so that the lungs can fully expand without a full stomach getting in the way. The same reason applies as to why patients are asked to wear only loose clothing while undergoing the test. Smoking and exercising should also be avoided 6 hours before the schedule set by the doctor. Likewise, drinking caffeinated beverages is also not allowed because caffeine can make the airways more relaxed than normal.
Patients with teeth issues may also have some difficulty forming a tight seal when using the spirometer’s mouthpiece. Those who are wearing dentures are advised to keep them on during the test.
There is also a risk of bronchospasm during an inhalation challenge test, but the patient will be under close monitoring by a lung specialist throughout the whole procedure.
Hegewald MJ, Crapo RO. Pulmonary function testing. In: Mason RJ, Broaddus VC, Martin TR, et al., eds. Murray and Nadel's Textbook of Respiratory Medicine. 5th ed. Philadelphia, PA: Elsevier Saunders; 2010:chap 24.
Reynolds HY. Respiratory structure and function: mechanisms and testing. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 85.