Pulmonary Plethysmography Test: Uses and Results

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

Pulmonary plethysmography, also called lung plethysmography or body plethysmography, is different from impedance plethysmography, which is a test for blood clots in the legs.


Pulmonary plethysmography may be required for a number of reasons.

Some of these include:

  • Help differentiate the diagnosis of obstructive and restrictive lung disease
  • Assess your response to treatment and whether your lung disease is getting worse, improving or staying the same
  • Determining the severity of lung disease, such as chronic obstructive pulmonary disease (COPD)
  • Evaluate your lungs to see if you can tolerate lung cancer surgery

How does plethysmography work?

During the pulmonary plethysmography test, you will sit in a closed chamber and you 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 laws of gas: as the pressure of the gas increases, the volume of the gas decreases, and vice versa.

As you exhale, you can calculate the amount of air in your lungs by recording the change in pressure in the air chamber you are sitting on.


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

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before testing

You will be able to drive to and from the exam yourself. You should avoid doing anything that might prevent you from breathing comfortably during the test.

  • No special dietary guidelines are usually given before the test, but it is best not to overeat so as not to interfere with breathing.
  • Wear loose, comfortable clothing and avoid any clothing that restricts breathing, such as tight shirts or tight belts.
  • You should not exercise vigorously or smoke in the hours before the test.
  • Avoid environmental contaminants such as indoor or outdoor air pollution before testing.
  • Fragrance or shampoos, shampoos, or other personal care products that can trigger allergic reactions should be avoided before surgery.

During the test

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

The technician will place the clip over your nose and give you a mouthpiece for breathing. Some people feel claustrophobic at the beginning of the procedure, and you can always open the door or remove the mouthpiece if needed, but this may prolong the time it takes to complete the procedure.

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

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Risks and Side Effects

Most people tolerate the procedure well, although some may feel claustrophobic or dizzy during the procedure. Overall, the active portion of this test takes about 15 minutes.

What are the test measures

Plethysmography measures the amount of air you exhale. It can help your healthcare professional understand how well your lungs are functioning.

The results of this test can also help your doctor calculate other lung functions.

Measurements that can be made with this test include:

  • Residual volume: Residual volume is the amount of air left in your lungs after you exhale as much air as possible.
  • Functional residual capacity (FRC): Functional residual capacity (FRC) is a measure of the amount of air left in your lungs after you exhale normally.
  • Total vital capacity (TLC): This is a measure of the amount of air in your lungs after you breathe as deeply as possible.

These results can help determine if you have problems breathing in, breathing out, or both.

Plethysmography and spirometry

Spirometry is another lung test that checks lung volume but does not determine residual volume.

Interpret the results

Pulmonary plethysmography results may be abnormal if your airways are narrowed or blocked, if there is too much air in your lungs after you exhale, or if your lungs do not fully expand. Lung disease can be classified according to whether your functional residual capacity is increased, decreased or normal.

Results may also vary by age, gender, height and weight.

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Increase function remaining capacity

Obstructive pulmonary disease often causes an increase in FRC—they damage the lungs, preventing you from exhaling as much air as healthy lungs.

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

Conditions that may lead to an increase in FRC include:

  • emphysema
  • cystic fibrosis

Decreased functional residual capacity (FRC)

A lower FRV means that there is less airspace in the lungs. There may be several reasons for this.

The lungs may not expand properly due to external or internal factors. This can be due to muscle weakness in the chest after a stroke, or a reduction in lung elasticity (less compliance) due to chronic lung disease. This pattern can also occur if you have had part of your lung removed to treat lung cancer.

Circumstances that may lead to lower FRC include:

  • idiopathic pulmonary fibrosis
  • other types of pulmonary fibrosis
  • sarcoidosis
  • removal of a lung or part of a lung
  • obesity
  • strokes
  • Scoliosis

VigorTip words

When combined with other lung function tests, pulmonary plethysmography can help differentiate lung disease or determine response to treatment, among other things. While it takes time to complete these tests, an accurate assessment of your condition can help guide you and your doctor in the treatment that is best for you as an individual.