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What is Plethysmography?


Updated October 30, 2012

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.


Plethysmography is a lung test that is performed to see how much air your lungs can hold. It's also known as a pulmonary function test.

What Does the Test Measure?

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

  • Total capacity - How much air your lungs can hold after a deep breath, and

  • Residual volume (or functional reserve capacity) - How much air is left in your lungs after you have exhaled as much as possible.

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.

The Procedure

During a plethysmography, you will be asked to sit in a small 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.

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

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

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