You will sit in a small, airtight room known as a body box. You will breathe or pant against a mouthpiece. Clips will be put on your nose to shut off your nostrils. Depending on the information your doctor is looking for, the mouthpiece may be open at first, and then closed.
You will be breathing against the mouthpiece in both the open and closed positions - they give different information to the doctors. As your chest moves while you breathe or pant, it changes the pressure and amount of air in the room and against the mouthpiece. From these changes, the doctor can get an accurate measure of the amount of air in your lungs.
Depending on the purpose of the test, you may be given some medication before the test.
How to prepare for the test
Let your doctor know if you are taking any medications, especially ones for breathing problems. You may have to temporarily stop taking certain medications before the test.
Wear loose clothes that allow you to breathe comfortably.
Avoid smoking and heavy exercise for 6 hours before the test.
Avoid heavy meals before the test. They can affect your ability to take deep breaths.
How the test will feel
The test involves rapid and normal breathing, and should not be painful. However, you may feel short of breath or light-headed. You will be monitored at all times by a technician.
The mouthpiece may feel uncomfortable against your mouth.
If you have trouble in tight spaces, the box might make you anxious. However, it is clear and you can see outside at all times.
Why the test is performed
The test is done to see how much air you can hold in your lungs during rest. It can help your doctor determine if a lung problem is due to damage to the lung structure, or a loss of the lungs' ability to expand (get bigger as air flows in).
Normal values depend on your age, height, weight, ethnic background, and gender.
What abnormal results mean
Abnormal results point to a problem in the lungs. This problem can be due to a breakdown of the lung structure, a problem with the chest wall and its muscles, or a problem with the lungs being able to expand and contract.
Lung plethysmography will not find the cause of the problem. However, it will help the doctor narrow down the list of possible problems.
What the risks are
Shortness of breath
Although this test is the most precise way to measure how much air you can hold in your lungs, it is not often used because of its technical difficulties.
Hegewald MJ, Crapo RO. Pulmonary function testing. In: Mason RJ, Broaddus VC, Martin TR, et al, eds. Murray and Nadel’sTextbook of Respiratory Medicine. 5th ed. Philadelphia, Pa: Saunders Elsevier; 2010:chap 24.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; Denis Hadjiliadis, MD, Assistant Professor of Medicine, Division of Pulmonary, Allergy and Critical Care, University of Pennsylvania, Philadelphia, PA. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Health Solutions, Ebix, Inc.