Nitrogen Washout

The nitrogen washout test gauges the functional residual capacity of your lungs and airflow. Specifically, your doctor is looking for the dead space in the small air sacs of the lungs, where air sits and is not taking part in the exchange of gases. This is usually caused by a non functioning capillary that is attached to the air sacs, which are not absorbing oxygen or expelling carbon dioxide. A large amount of dead space in the lungs will add to lowered efficiency and poor lung function.

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



You will first be asked to breathe through a machine that supplies you with nothing but pure oxygen. No other gases will be present in the air.

Then, you will exhale as long as possible through the machine that will then read the amount of nitrogen in your breath. Since nitrogen is not exchanged in the lungs, it sits there until you exhale. But if your breath is too shallow due to respiratory inflammation, it does not escape easily.

The air you normally breathe is roughly 78 percent nitrogen due to the nitrogen in the earth’s atmosphere. Therefore, if you breathe in just oxygen, you should exhale the residual nitrogen that has been sitting in your lungs. The amount of nitrogen over 78 percent, which is the assumed normal amount, will indicate how much residual gas sits in the lungs.

You may also be asked to breathe normally for a few minutes during which time you will continue to inhale pure oxygen. Typically, this measures how long it takes you to expel the nitrogen in your lungs. In a healthy person, this would take around 7 minutes, but a person suffering from a lung disease may need much longer.

The nitrogen washout test is just one of the many tests that are needed to judge the functioning of your lungs.


Risks involved



Nitrogen Washout is generally a safe test. You may feel short of breath or dizzy for a moment after you perform the test.

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

Patient Information

IMG 0799 (2)
Galway Clinic offers one of the widest ranges of services and procedures of any... Read More