Pulmonary Function Testing – The How’s and Why’s

Do you have trouble breathing or have you noticed significant changes in everyday lung function? If so, it’s important to contact your health care provider. If your physician suspects lung function issues, they may order a Pulmonary Function Test (PFT) to help determine how well your lungs are working.

Two types of disorders can cause problems with the ability to move air in and out of the lungs. The first is an obstructive disorder, which is when air has trouble leaving the lungs due to resistance, causing a decrease in air flow. The second is a restrictive disorder, which is when the chest muscles cannot expand enough, causing problems with air flow into the lungs.

Pulmonary Function Testing can help diagnose asthma, bronchitis, emphysema, and other disorders. Respiratory infections, allergies, and respiratory airway problems can also be brought to light.

How the Tests Work

A complete PFT is comprised of three separate tests. For each test, you are fitted with a mouthpiece that is connected to the testing machine. You will also wear a nose clip to ensure that all airflow is measured through the mouth. The three components of a PFT are:

Flow Volume (FV) Loop Test

This test, also known as spirometry, measures how well your lungs work by measuring how much air you inhale, how much you exhale, and how quickly you exhale. A spirometer records the rate of air movement and the amount of air that you breathe in and out over a period of time.

The test begins with resting breathing. When prompted, you will inhale deeply, filling your lungs as much as possible. When you cannot inhale any more, you will breathe out, exhaling as much air as possible. When you cannot breathe out any further, you will complete the loop test by inhaling once more.

Spirometry is often performed before and after you use a bronchodilator, which is a substance such as albuterol sulfate that widens air passages by relaxing the bronchial smooth muscle. When the test is repeated after you’ve used a bronchodilator, it can determine if albuterol sulfate is effective in opening the bronchioles and relaxing the muscles in the lungs

Diffusion Capacity (DLCO) Test

The DLCO test measures how well your lungs allow oxygen and carbon dioxide to pass in and out of your blood, a process called diffusion. Diffusion is most efficient when the surface area for transferring oxygen and carbon dioxide in the lungs is high and blood is readily available to transport these gasses. Certain lung conditions will decrease the surface area or efficiency in which lungs transfer oxygen or carbon dioxide into your bloodstream.

With this test, you will breathe a tiny (and safe) amount of carbon dioxide to measure the diffusion capacity. You will begin with resting breathing, then the technician will ask you to take a deep breath in, a deep breath out, and a final deep breath in that you will hold for about 6 seconds. Over the course of this 6 seconds, diffusion takes place. When you finally exhale, the air is measured and analyzed.

Total Lung Capacity (TLC) Test

This third test determines your Total Lung Capacity (TLC). Even after you exhale as much as possible in the FV (spirometry) test there is still a measurable amount of air left in your lungs. The information gathered from this test is combined with data from the FV test. With obstructive diseases, measuring the FV and TLC can reveal air trapping in the lungs and hyperinflation (when air sacs become less elastic). In restrictive diseases, the TLC confirms true restriction and the degree of restriction.

This test is performed with an inert gas such as nitrogen. The patient breathes in nitrogen through the mouthpiece for a specified period of time. The inert gas concentration is then measured in the expired air, from which the residual volume is calculated.

If you have questions about your lung health, contact the doctors at Boulder Medical Center at (303) 440-3000.


By Susan Pucket, PA-C
Susan specializes in Otolaryngology (Ear, Nose and Throat) and Internal Medicine at Boulder Medical Center