The fit to fly test or the hypoxic challenge will show if you need additional oxygen when you fly.
The air we normally breathe is made up of about 21% oxygen. However, the amount of oxygen in the air gets lower the higher up we go, e.g. up a mountain or on a plane. To make sure that there is enough oxygen for the people on board an aeroplane to breathe in, aeroplanes are designed to keep the oxygen levels inside the plane at the right level artificially (pressurised).
However, oxygen levels are only kept at this level up to 8,000ft in the air. Above this, the amount of oxygen in the air drops to about 15%. This leads to lower levels of oxygen in your blood. If you do not have a lung condition, the drop in oxygen is not enough that you would feel the difference. If you have a lung condition, your oxygen levels may already be low, or your lungs may not be able to work properly to keep the amount of oxygen in your blood at a safe level. Lower levels of oxygen in your blood may make you feel unwell or could even be harmful to you during or after your flight. Additional oxygen keeps the oxygen levels in your blood at a comfortable level and keeps you safe.
The hypoxic challenge test, also known as a fit to fly test, recreates the oxygen levels on an aeroplane and measures how your body responds. The test involves breathing in air containing less oxygen than normal and seeing how it affects you in a controlled environment. Results of the test will show if you need additional oxygen when you fly.
First, your doctor will need to take some blood from you to check what your oxygen levels are like normally (a blood gas test).
If your oxygen levels are already low at this stage, your doctor will recommend that you take oxygen on your flight, and you will not need to take the fit to fly test.
If your oxygen levels are OK, your doctor may still think you could be at risk and ask you to take the hypoxic challenge test, also known as a fit to fly test.
The test involves breathing air containing 15% oxygen through a face mask or mouthpiece for up to 20 minutes. A healthcare professional will stay with you during the test to monitor your response.
During the test, your oxygen levels will be measured using a device called an oximeter, which shines a small light through your finger, ear or forehead to measure your oxygen levels. If it shows that your oxygen levels have fallen below the recommended amount, your doctor will stop the test and check your oxygen levels with another blood sample.
If the test continues for the full 20 minutes, your doctor will take another blood sample at the end of the test to check your oxygen levels.
Your doctor will usually tell you how long your test results are valid for. As long as your condition stays the same, the results should stay valid.
If your health condition gets worse (i.e. exacerbations, lower oxygen saturation levels at rest), you will need to take the test again to check your oxygen needs when you next fly.
Oxygen flow rate can be either 2 or 4 litres per minute.
In some hospitals, your doctor will decide which flow rate is best based on the results of the test and your general health.
In other hospitals, you will do an extra test called a titration study. In this test, you will be asked to repeat the fit to fly test on 2 litres per minute of oxygen. If your oxygen levels recover, this will be the flow rate you will need. If your level of oxygen does not return to acceptable levels, then your doctor will recommend that you use 4 litres per minute.
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