What is plethysmography?

The medical term plethysmography (from the Ancient Greek 'plethysmos' meaning enlargement' and 'graphein' meaning 'to write') describes a test of lung function which determines the total lung capacity (The volume of air in the lungs after a maximal inspiration) and other lung volumes such as the 'Functional Residual Capacity', which is the volume of air in the lungs at the end of a quiet resting expiration, and the residual volume which is the volume of air left in the lung after a maximal expiration.

The test is carried out using a device called a plethysmograph. It is also referred to as 'body plethysmography',' whole body plethysmography', 'lung plethysmography' or 'pulmonary plethysmography'.

What is plethysmography used for?

Plethysmography measures total lung volume and as such is useful in diagnosing restrictive lung diseases (such as pulmonary fibrosis or 'scarring of the lung') and obstructive lung diseases (such as cystic fibrosis and emphysema).

It is also used to check the response to certain medications and to monitor for lung complications during and after toxic treatments like chemotherapy. The test will not give a definitive diagnosis of a specific condition, but it can give medical professionals guidance as to what type of lung condition is present.

What is involved in a plethysmography?

There are many different tests that can be conducted during a plethysmography. During the test you will be asked to sit in a sealed box that look a little like a telephone box. There is a mouthpiece inside the box which you will need to breathe in and out of during the test.

After a few minutes to get settled, the operator will tell you how to breathe in and out of the mouthpiece while measurements are taken. A shutter inside the mouthpiece will open and close to allow various readings to be taken. Depending on the tests required, you may need to breathe in other (inert and harmless) gases as well as air. The whole test generally takes no longer than 4-5 minutes.

How should I prepare for a plethysmography?

Let the doctor know if you are taking any medications, particularly if they are related to breathing difficulties, as you may need to stop taking these before the test. If you catch a cold or other illness that may prevent you breathing properly, you may need to rearrange the test for when you are better.

Do not wear any clothing that might stop you from breathing in and out fully and avoid eating a large meal within two hours of the test, or drinking alcohol (within four hours) or smoking (within an hour) of the test. You should also not do any strenuous exercise in the 30 minutes before the test.

Are there any risks associated with the test?

Very rarely, if you are in the chamber for a longer amount of time than is normal, you may feel lightheaded or get dizzy. Tell the operator immediately if you start to feel this way. If you are uncomfortable in small spaces (claustrophobic) it may be advisable not to undergo the test.