Hypoxia Altitude Simulation Test
What is it?
HAST is a special breathing test to determine whether you need supplemental oxygen when flying on an airplane. This test has not been standardized so there may be differences in the way this test is performed by different pulmonary function labs. This test is safe and there should be no lasting effects.
What to expect?
When you fly on an airplane the pressure in the cabin is usually regulated to the equivalent of 6000 to 8000 feet of altitude. For this test you will be breathing a gas mixture with approximately 14% to 15% oxygen which is the same you would be breathing at those altitudes.
You will be asked to sit in a chair and an oximeter probe will be placed on a finger or earlobe. If you use supplemental oxygen you will need to turn it off and remove your nasal cannula off. You will probably need to be off your supplemental oxygen for 10 minutes before the test is started and before a baseline oxygen saturation level is taken.
A nasal cannula will be placed on your nose and then a soft rubber mask will be placed over your face, covering your mouth and nose. This mask needs to fit very snugly against your face so a harness may be placed over your head to hold the mask in place. If you have a beard you may not be able to perform this test because the soft rubber mask will probably not fit tightly enough.
A breathing system will be attached to the mask and the HAST gas mixture will be turned on. Your oxygen saturation will be measured at regular intervals. If your oxygen saturation decreases below 90% then supplemental oxygen will be turned on and delivered through the nasal cannula while you continue to breath the HAST gas mixture. The oxygen flow rate will be increased in steps until your oxygen saturation remains at 90% or above.
You may be asked to walk a short distance or on a treadmill while wearing the mask and breathing the HAST gas mixture. If this occurs there may be further steps where the supplemental oxygen flow rate is adjusted.
The test will be over when your oxygen saturation remains at or above 90% for a certain period of time, probably between 5 and 10 minutes.
The soft rubber mask and nasal cannula should both be new and clean at the start of your testing session. The staff person performing your test should be wearing gloves or at a minimum should have performed hand hygiene before your testing session.
What is a normal measurement?
Everybody's oxygen saturation decreases while flying on an airplane but usually remains above 90%. Oxygen saturation levels below 90% will cause a strain on the heart and lungs and should be avoided.
What is typical with ILD?
Interstitial lung diseases can reduce the ability of your lungs to get oxygen into your bloodstream. When less oxygen is available in the air you breathe you can have an excessively low oxygen saturation level in your bloodstream. As long as supplemental oxygen can keep your oxygen saturation levels at or above 90% then it should be safe to fly on an airplane.
Obtaining supplemental oxygen for airplane flights always requires advance planning. A supplemental oxygen flow rate above 2 LPM on a long plane flight may require an extra level of preparation and planning.
What affects test quality?
Oxygen saturation will likely be overestimated and the amount of supplemental oxygen needed during airplane travel will be underestimated if the soft rubber mask used to deliver the HAST gas mixture does not fit tightly.