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 that is performed to measure the change in volume in an area of the body. how much air is in the lungs when you take a deep breath and how much is left after you exhale

Most commonly done as a pulmonary function test

Plethysmography may also be done on the limbs, to look for blood clots in the arms or legs, or the penis, to measure blood flow.

 In this article, we will discuss primarily plethysmography done to evaluate the lungs.

see how much air your lungs can hold.

Unlike spirometry, plethysmography provides your doctor with information about the absolute volume of air in your lungs.

What Does the Test Measure?

Plethysmography gives your doctor two measurements that can help her understand how well your lungs are functioning:

  • Functional residual volume- This number indicates 
  • 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 amount of air 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 chest after you've taken the deepest breath you possibly can.
  • Airway resistance - Airway resistance measures the pressure in the tiny air sacs of the lungs - alveoli - that's need to keep a flow rate in the lungs going of 1L/second.

    calculated using the principle of Boyle's law - The equation of this law of chemistry is P1V1=P2V2.  In other words, the pressure multiplied by the volume in setting number one is the same as the pressure times the volume in setting number two.  To obey this formula, when the pressure increases on one side, the volume must decrease.

     By measuring volume before and after, doctors can predict the pressure in the lungs can find the change in volume of air in the chest.Since an example is worth a thousand words..   pressure and volume before inhaling   as long as the temperature remains the same, you can use the volume of air in the chest to determine the pressure and vice versa.

    Reasons to Order a pley

    respiratory symptoms

    Lung Plethysmography Procedure

    During a plethysmography, you will be asked to sit in a small, airtight room that looks a bit like a telephone booth. You will be given a mouthpiece to breathe through, and a technician will instruct you on how to breathe at different times.  through and against the mouthpiece  put clips on your nose  breath normally, pant, breathe against the mouthpiece several ways  breathe normally, then pant for several breaths, then take a deep breath in and let it all out

    The procedure is usually tolerated quite well, though some people may feel claustrophobic in the room, or become lightheaded during the procedure.

    People who use oxygen usually do not use their oxygen during this test.

    Overall, the active part of this test only takes about 15 minutes.


    This test may be ordered for several reasons

    • To help with the diagnosis of restrictive or obstructive lung diseases
    • To monitor your condition -whether it is getting worse, improving, or staying the same
    • To assess response to treatment  ( for example, do your numbers improve after using an inhaler?
    • To evaluate your lungs to see if you would tolerate lung cancer surgery.  In other words, can you live reasonably well without a lung or a part of a lung?

    Plethysmography Results - What Do They Mean?

    Your doctor will read your results, which will help her..  The results are not read as a just a number, but rather ... take into account factors which can affect your numbers such as your age, your sex, your height and weight, and even your ethnicity.

    obstructive vs restrictive lung disease

    .  In general, lung conditions which are obstructive cause  whereas restrictive lung disease   Learn more about obstructive vs restrictive lung diseases.

    can think about how this works.  Functional residual capacity is determined by where in the spectrum forces fall - the balance of the  between the elastic forces of the lungs and the chest wall.

    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


    What Do the Results Mean?

    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.

    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 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

       in enzyme increased functional er == not all breath is exhaled due to lack of elasticity                                                                            

    Decreased Functional Residual Capacity (FRC)

    means that there is a decreased amount of air space present in the lungs.  This, in turn, can have several causes.  restrictive

    can be less elastic either externally 0 or internally - for example, muscle weakness in the chest due to a stroke, or scoliosis or decreased elasticity of the lungs themselves due to a chronic lung condition.

    may also be decreased if you have had a lung or part of a lung removed for lung cancer.


    Restrictive lung diseases often cause a decreased functional residual capacity.  Plethysmography helps measure the compliance of the lungs - how elastic they are.  some conditions stiff Conditions which may result in a decreased FRC include:

    Cautions and Risks

    Lung plethysmography is, in general, a very well tolerated procedure.  Some people may become lightheaded when   closed over the mouthpiece.  Those who experience claustrophobia may ... as would in a phone booth.

    can give clues as to what is going on in the lungs.

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


    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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