Lung Plethysmography Test: Uses and Results

A type of pulmonary function test for measuring residual capacity

Plethysmography is a pulmonary function test that measures the health and function of the lungs by determining how much air the lungs can hold. It may be used along with other lung tests to help in the diagnosis of lung disease, to determine disease severity, to see whether treatment is working, or to evaluate the lungs before lung surgery.

Lung plethysmography is also called pulmonary plethysmography or body plethysmography and it differs from impedance plethysmography, which is a test for blood clots in the legs.

woman taking deep breath of air
Istockphoto.com / Stock Photo / AntonioGuillem

Uses

Lung plethysmography may be ordered for several reasons.

Some of these include:

How Does Plethysmography Work?

During a lung plethysmography test, you will sit in a small, enclosed, airtight chamber and will be asked to take deep breaths.

Plethysmography measures the total amount of air your lungs can hold. The principle behind it is based on one of the gas laws: as the pressure of a gas increases, the volume of the gas decreases, and vice versa.

As you exhale, the volume of air in your lungs can be calculated by recording the change in pressure of the chamber you are sitting in.

Procedure

When your healthcare professional recommends this test, they will explain the preparation, what will happen during the procedure, and when you can expect to receive your results.

Before the Test

You will be able to drive yourself to and from the test. You should avoid doing anything that could inhibit your ability to breathe comfortably during the test.

  • No special dietary guidelines are usually given prior to the test, but it's a good idea not to eat heavily to the point where it could interfere with your breathing.
  • Wear loose-fitting, comfortable clothes and avoid any attire that could limit your breathing, such as tightly fitted shirts or a tight belt.
  • You should not exercise heavily or smoke for several hours prior to the test.
  • Avoid environmental pollutants, such as indoor or outdoor air pollution prior to the test.
  • Perfumes or perfumed shampoos or other personal care products that could trigger an allergic reaction should be avoided before the procedure.

During the Test

During plethysmography, you will be asked to sit in a small, airtight room that looks a bit like a telephone booth. If you ordinarily use oxygen, you will not need to receive your oxygen during the test.

A technician will place clips on your nose and give you a mouthpiece to breathe through. Some people feel claustrophobic when the procedure begins, and you can open the door or remove the mouthpiece at any time if you need to, though this can lengthen the time it takes to complete your procedure.

The technician will guide you through various breathing patterns, asking you to breathe normally, then take a deep breath in, and blow it all out.

Risks and Side Effects

Most people tolerate the procedure very well, though some people may feel claustrophobic or become lightheaded during the procedure. Overall, the active part of this test takes around 15 minutes.

What the Test Measures

Plethysmography measures the amount of air that you exhale. It helps your healthcare professionals understand how well your lungs are functioning.

The results of this test help your practitioner calculate other pulmonary functions as well.

Measurements that can be made using this test include:

  • Residual volume: Residual volume is the amount of air left in your lungs after you exhale as much air as you can.
  • Functional residual capacity (FRC): Functional residual capacity (FRC) is a measure of how much air is left in your lungs after you breathe out normally.
  • Total lung capacity (TLC): This is a measure of the total amount of air in your lungs after you've taken the deepest breath you possibly can.

These results help determine whether you have problems with inhalation, exhalation, or both.

Plethysmography vs. Spirometry

Spirometry is another lung test that looks at lung volumes but doesn't determine residual volume.

Interpreting Results 


Thes result of lung plethysmography can be abnormal if your airways are narrowed or blocked, if too much air is left in your lungs after you exhale, or if your lungs are unable to expand completely. Lung diseases can be classified based on whether your functional residual capacity is increased, decreased, or normal.

The results can also vary based on age, sex, height, and weight.

Increased Functional Residual Capacity

Obstructive lung diseases often cause increased FRC—they damage the lungs in a way that prevents you from exhaling out as much air as you would with healthy lungs.

With conditions such as emphysema, the full volume of air is not exhaled from the lungs after each breath. The elastic recoil is damaged so that extra air is left over.

Conditions that may result in an increased FRC include:

  • Emphysema
  • Cystic fibrosis                                 

Decreased Functional Residual Capacity (FRC)

A decreased FRV means that there is a decreased amount of airspace present in the lungs. This can have several causes.

The lungs might not be able to expand normally, either due to external or internal factors. This can occur due to weakness of the chest muscles after a stroke, or a decreased elasticity of the lungs (decreased compliance) due to a chronic lung condition. This pattern may also be seen if you have had part of a lung removed for treatment of lung cancer.

Conditions that may result in decreased FRC include:

A Word From Verywell

When combined with other pulmonary function tests, lung plethysmography can help differentiate lung diseases or determine the response to treatment and much more. While it takes time to have these tests done, having an accurate assessment of your condition can help guide you and your doctor to the treatments that will work best for you personally.

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