Lung plethysmography is a test used to measure how much air you can hold in your lungs. It helps to assess patients with lung diseases, which are often associated with a decrease in total lung capacity (TLC).It can measure a person’s TLC, which is the total volume of air in the chest after they have inhaled as deeply as possible. Plethysmographic measurements are based on Boyle’s Law, a principle that describes the relationship between the pressure and volume of a gas.

Test Duration

Approximately: 15 minutes

Test Preparation

You may generally continue with your normal routine. However, there are some points to note to improve comfort and ensure accurate results:

  • Refrain from smoking for as long as possible before the test
  • Avoid tight clothing
  • Avoid a heavy meal 2 hours before the test
  • Do not drink alcohol for at least 4 hours before the test
  • Avoid vigorous exercise for a few hours before the test
  • If you use an inhaler please bring it with you

Follow your doctor's instructions about whether you should avoid use of inhaled breathing medications or other medications before the test.

If it is your first time attending the Respiratory Department for a Lung Function Test do not take the following medication before the test:

  • Salbutamol (Ventolin, Salamol) or Brycanil for at least 4 hours
  • Atrovent or Combivent for at least 8 hours
  • Symbicort, Serevent, Seretide for 12 hours
  • Spiriva (Tiotropium) for 24 hours.

Please take all other drugs as prescribed.

Test Procedure

The Respiratory Physiologist will ask to measure your height and weight. That will be used to determine your normal predicted values.

You will be asked to sit in a small, airtight chamber and breathe or pant against a mouthpiece.

Clips will be put on your nose to shut off air to your nostrils.

You’ll be asked to breathe against the mouthpiece when it’s both opened and closed. This will provide your doctor with important information.

As your chest moves while you breathe or pant, it changes the pressure and amount of air in the chamber and against the mouthpiece. From these changes, your doctor can get an accurate measure of TLC, the amount of air in your lungs.

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 will be repeated at least 3 times for accurate and reproducible results to be obtained.

Risks involved

Lung plethysmography is generally a safe test. You may feel short of breath or dizzy for a moment after you perform the test. Also anxiety might occur for those who are uncomfortable in tight spaces.

Because the test requires some exertion, it isn't performed if you've had a recent heart attack or some other heart conditions. Rarely, the test triggers severe breathing problems.

If you have any of the following please call us to discuss:

  • Unstable angina
  • A recent pneumothorax (air trapped beneath the chest wall)
  • A recent heart attack or stroke
  • Recent eye or abdominal surgery
  • Coughed up blood recently and the cause is not known