Your doctor can order a lung plethysmography test to measure how much air
you can hold in your lungs. Lung plethysmography is also called pulmonary or body
plethysmography. It helps doctors assess the condition of people with lung
disease, which can occur with a decrease in total lung capacity (TLC). TLC is
the total volume of air in your chest after you’ve inhaled as deeply as
Although spirometry is the standard way to measure lung volumes, lung
plethysmography is more accurate. Measurements from this test are based on
Boyle’s Law, a scientific principle that describes the relationship between the
pressure and volume of a gas. This law says that if temperature remains the
same, you can use measurements of the volume of a gas to find out its pressure
and vice versa.
is the test performed?
A lung plethysmography test can take place in:
- pulmonary function laboratories
- cardiopulmonary laboratories
- pulmonology offices
A respiratory care technician usually performs it. They’ll monitor your condition
at all times.
You’ll sit or stand in a small, airtight chamber that is partially or
completely see-through and may resemble a phone booth. Babies may have a
special type of test that allows them to lay down. They’ll put clips on your
nose to shut off air to your nostrils. They’ll ask you to breathe or pant
against a mouthpiece when it’s both opened and closed. This will provide your
doctor with important measurements, including:
- the amount of air left in your lungs when you
breathe out normally, which is called functional residual capacity (FRC)
- how much air is left when you breathe out as
much as possible, or residual capacity (RC)
As your chest moves while you breathe or pant, it changes the pressure and
amount of air in the chamber. Your breathing also changes the pressure against
the mouthpiece. From these changes, your doctor can get an accurate measure of
TLC, FRC, and RC.
The mouthpiece may feel uncomfortable against your mouth. If you typically
struggle in tight spaces, sitting in the chamber might make you anxious.
However, you’ll be able to see outside the chamber at all times, and the test
usually takes three minutes to perform. They may include a tracer gas such as
carbon dioxide in the air you breathe during the test.
to prepare for the test
Tell your doctor about any medications you’re taking, especially those for
breathing problems, such as asthma. You may have to stop taking certain
medications temporarily before the test. You might also need to reschedule the
test if you have a cold or the flu.
You’ll be more comfortable if you wear loose clothes during the test. You
should also avoid:
- drinking alcohol at least four hours before the
- heavy meals at least two hours before the test
- smoking at least one hour before the test
- strenuous exercise at least 30 minutes before
All of these activities can affect your ability to breathe and may result in
inaccurate test results.
are the risks?
Risks associated with lung plethysmography include:
- shortness of breath
- anxiety for if you’re uncomfortable in tight spaces
- transmission of infection if the equipment, such as the
mouthpiece, isn’t properly cleaned before you use it
Too much carbon dioxide in your blood is called
“hypercapnia.” Too little oxygen in your blood is called “hypoxia.” Either can
occur if you’re in the chamber for a longer than typical amount of time. This
is the test performed?
Your doctor may order a lung plethysmography test to:
- help diagnose restrictive lung disease, which is a type
of disease that restricts lung expansion
- evaluate obstructive lung diseases, such as bullous
emphysema and cystic fibrosis
- follow the course of a disease and its response to
- measure your resistance to airflow
- measure your response to bronchodilator medications
- assess whether your lung capacity will be affected by
such treatments as methacholine, histamine, or isocapnic hyperventilation
You shouldn’t have lung plethysmography if you:
- are mentally confused
- lack muscle coordination, or you have poor muscle
control that causes jerky or unpredictable movements
- have a condition that prevents you from entering the
chamber or properly performing the required steps
- are claustrophobic, or you have a fear of being in small
- require continuous oxygen therapy
do the results mean?
Normal values depend on a combination of many factors, such as:
- ethnic background
A normal value for you may be different than a normal value for someone
else. Your doctor will assess whether or not your results are normal. If you
have a restrictive lung disease, your lung volumes will likely be lower than
expected. In some cases, however, unexpectedly high measurements can indicate a
condition that’s trapping air in the lungs, such as emphysema.
Abnormal results indicate that there’s a problem in your lungs. Your doctor
can’t use lung plethysmography to determine the cause of the problem, but they
can use it to narrow down the possibilities, such as a breakdown of the lung
structure, a problem with the chest wall and its muscles, or an inability of
the lungs to expand or contract.