Hypoxic Altitude Simulation Test (HAST)

What is it?

This 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 should you 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. 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.  If you need supplemental O2 to keep your oxygen saturation level above 90% during the HAST test then this amount of oxygen will likely be prescribed for you if you plan on flying any distance an airplane.  

For a more detailed physiological explanation of blood oxygenation and altitude see Alveolar O2 and Altitude.

Note:  If you use supplemental O2 and it is delivered by a compressed gas cylinder or a liquid oxygen system you will not be allowed to bring it with you on an airplane.  Currently airlines will only allow you to bring a personal battery-operated oxygen concentrators and even then it is best to check with the airline in advance.  Some airlines will provide supplemental oxygen for your flight but again it is best to check with the airline in advance. For information about traveling with supplemental O2 please see the Airline Oxygen Council of AmericaNational Home Oxygen Patients Association or Portable Oxygen: A User’s Perspective websites.

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.

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PFT Patient by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

2 thoughts on “Hypoxic Altitude Simulation Test (HAST)”

    1. Barbara –

      The two tests have completely different purposes. A Hypoxic Challenge Test (HCT) is used to assess an individual’s ventilatory response (usually an increase in tidal volume and respiratory rate) to breathing a mixture of gas containing a low concentration of oxygen. A Hypoxic Altitude Simulation Test (HAST) is used to find out what changes in arterial oxygenation (either SaO2 from an oximeter or PaO2 from an ABG) an individual will have at the atmospheric pressure that will be inside an airplane.

      To be honest I’m not sure what clinical value an HCT has. The primary sensors in the human body that affect ventilation are for CO2 and pH. O2 is kind of a backup system and there’s some question about where the O2 sensors are located anatomically (yes, if you look in a physiological textbook they’ll say it’s the carotid bodies but these play more roles than just in sensing O2, and what part of the carotid bodies senses O2 and how they do it – if they do it at all – remains unclear). Individuals with chronic lung disease (usually COPD) may no longer respond appropriately to changes in CO2 and pH so it’s at least theoretically possible that knowing their response to hypoxia may be helpful but it probably doesn’t have any clinical utility. Several decades ago it was realized that giving too much supplemental O2 to individuals with COPD caused them to hypoventilate (breathe too little and retain a lot of extra carbon dioxide). This was originally thought to be due to a blunting of the hypoxic response to ventilation but since that time it has come to be realized it has much more to do with how the lung distributes blood flow due to hypoxia. This is a reason that only the smallest amount of supplemental O2, only what’s needed to keep the blood oxygen level within a safe range, should ever be given. More O2 is not better.

      A HAST test however, is very clinically relevant because it will tell you immediately if you need supplemental O2 (and how much) if you are going to travel by airplane.

      – Richard

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