What Is a Lung Plethysmography?

How is a Lung Plethysmography Done and What Does it Measure?

woman taking deep breath of air
What is lung plethysmography, how is it done, and what do the results mean?. Istockphoto.com/Stock Photo©AntonioGuillem

Plethysmography - Definition

Plethysmography is a lung test (pulmonary function test) performed to measure the compliance of the lungs by determining how much air your lungs can hold. (Lung plethysmography is different than impedance plethysmography, a test for blood clots in the legs.) It is useful along with other lung tests in that it is able to determine the absolute volume of air in your lungs.

How Does Plethysmography Work?

Plethsmography is based on one of the gas laws called Boyle's law. As long as temperature is constant, the pressure and volume of a gas are inversely proportional. This will make more sense when we discuss the procedure.

Indications - When is the Test Done?

A plethysmography may be ordered for several reasons. Some of these include:

  • In diagnosis to distinguish obstructive versus restrictive lung diseases.
  • To assess your response to treatment. For exaple, to see if your lung disease is getting worse, improving, or staying the same.
  • To evaluate your lungs to see if you would tolerate lung cancer surgery. 

How is a Lung Plethysmography Test Done? - Procedure

During a 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. Your technician will place clips on your nose and give you a mouthpiece to breathe through.

Some people feel claustrophobic when the procedure begins, but you can open the door or remove the mouthpiece at any time if you need (though this can lengthen the procedure.) The technician will then take you through various breathing patterns, having you breathe normally, then pant for several breaths, then take a deep breath in and blow it all out.

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 Does the Test Measure?

Plethysmography gives your doctor measurements that can help her understand how well your lungs are functioning. Most pulmonary function tests do not measure residual volume or the amount of air left in your lungs after you exhale as much air as you can. By determining this measurement, plethysmography helps your doctor calculate other numbers as well. Measurements that can be made using this test include:

  • Functional residual volume - Functional 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 have exhaled as much as possible (the expiratory reserve volume). plus the amount of air left in your lungs after you breathe out normally (the residual volume.)
  • Total lung capacity (TLC) - This is a measure of the total amount of air in your chest after you've taken the deepest breath you possibly can.

Understanding Plethysmography Results - What Do They Mean?

Depending on whether your results that your functional residual capacity is elevated, normal, or less than expected, your doctor can better evaluate what lung condition you may be suffering with.

Increased Functional Residual Capacity

Obstructive lung diseases often cause an increased FRC. In order to picture this, you can imagine how, with conditions such as emphysema, after each breath, the full volume is not exhaled. The elastic recoil is damaged so that extra air is left over. The extra air that is left over which can not be exhaled is added to the normal volume left over after breathing out. Conditions which may result in an increased FRC include:

  • Emphysema
  • Cystic fibrosis                                                                      

Decreased Functional Residual Capacity (FRC)

A decreased functional residual capacity means that there is a decreased amount of air space present in the lungs. This, in turn, can have several causes. The lungs may be "less elastic" either externally or internally, for example, from weakness of the chest muscles related to a stroke, or a decreased elasticity of the lungs themselves (decreased compliance) due to a chronic lung condition.This pattern may also be seen if you have had part of a lung removed for lung cancer. Conditions which may result in a decreased FRC include:

These numbers may be abnormal if your airways are narrowed or blocked in some way, if too much air is left in your lungs after you exhale (as in emphysema), or if your lungs are unable to expand completely.

Examples: Jay's doctor recommended he have a procedure called plethysmography before his surgery for lung cancer.

Sources:

Criee, C., Sorichter, S., Smith H. et al. Body plethysmography – Its principles and clinical use. Respiratory Medicine. 2011. 105(7):959-971.

U.S. National Library of Medicine. MedlinePlus. Plethysmography. Updated 11/22/14. https://www.nlm.nih.gov/medlineplus/ency/article/003771.htm

Zysman-Colman, Z., and L. Lands. Whole Body Plethysmography: Practical Considerations. Paediatric Respiratory Reviews. 2016. 19:39-41.

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