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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Happens During the Test?

This testing is done in a special laboratory. During the test, you are instructed to breathe in and out through a tube that is connected to various machines.

A test called spirometry measures how forcefully you are able to inhale and exhale when you are trying to take as large a breath as possible. The lab technicians encourage you to give this test your best effort, because you can make the test result abnormal just by not trying hard.

A separate test to measure your lung volume (size) is done in one of two ways. One way is to have you inhale a small carefully measured amount of a specific gas (such as helium) that is not absorbed into your bloodstream. This gas mixes with the air in your lungs before you breathe it out again. The air and helium that you breathe out is tested to see how much the helium was diluted by the air in your lungs, and a calculation can reveal how much air your lungs were holding in the first place.

The other way to measure lung volume is with a test called plethysmography. In this test, you sit inside an airtight cubicle that looks like a phone booth, and you breathe in and out through a pipe in the wall. The air pressure inside the box changes with your breathing because your chest expands and contracts while you breathe. This pressure change can be measured and used to calculate the amount of air you are breathing.

Your lungs' efficiency at delivering oxygen and other gases to your bloodstream is known as your diffusion capacity. To measure this, you breathe in a small quantity of carbon monoxide (too small a quantity to do you any harm), and the amount you breathe out is measured. Your ability to absorb carbon monoxide into the blood is representative of your ability to absorb other gases, such as oxygen.

Some patients have variations of these tests for example, with inhaler medicines given partway through a test to see if the results improve, or with a test being done during exercise. Some patients also have their oxygen level measured in the pulmonary function lab.

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