happens when the test is performed?
- 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
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 (see "Oxygen saturation test,"