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Sunday, 30 June 2013 15:18

Hypoxia: The Silent Killer

Written by  Brent Crow
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July 2013 -

"I don't go that high," came the reply to my question asking why a pilot had never had hypoxia training. After asking how high the pilot typically flew, he said he never would go over 10,000 to 12,000 feet. Then I asked him if there was a possibility of carbon monoxide poisoning aboard his aircraft.

"What does that have to do with anything?" came the pilot's reply.

I've been working at the Arizona State University's Altitude Chamber for the past 10 years, given training to hundreds of personnel, as well as receiving training myself. Once upon a time, I too did not understand the dangers associated with hypoxia.

I woke up around 4 a.m. to complete flight planning and skipped breakfast as a result of running behind schedule. I arrived at the airport, completed preflight, and was en route to Winslow, Ariz. (KINW) from Phoenix-Mesa Gateway Airport (KIWA).

My cruising altitude to Winslow was a cozy 9,500 MSL. The ground below me was patchy snow dotting the mountainous landscape and I detected a hint of a headache and wished I hadn't skipped breakfast.

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After departing Winslow on my return journey to Phoenix, I leveled at my cruise altitude of 10,500 feet. Quite high for a Piper Warrior, but not uncommon in this mountainous region of Arizona. The ground bobbled only a few thousand feet below me. I noticed it was definitely colder at this altitude, and remember the OAT gauge reading around five degrees Celsius.

I clicked on a notch of cabin heat and filled out another line on my flight plan. Then I was hot, and clicked off the cabin heat knob. I debated whether I wanted to try and shed a layer of clothing, but then went back to admiring the picturesque landscape. Then I was cold again. I chuckled silently to myself as I thought about my grandma who always fought hot and cold flashes.

Then it dawned on me.

I thought back to two weeks earlier when I took my first training flight in the altitude chamber. My symptoms: light-headedness, mild headache, hot and cold flashes, tingling in my extremities.

I made a rapid assessment of my fingers. I could not feel them touching the yoke and they were beginning to tingle. I remembered my training—treat as soon as you know there is a problem because you never know how close you are to unconsciousness.

"Albuquerque Center, Warrior eight-five Quebec; request immediate descent. I'm experiencing symptoms of hypoxia."

I received immediate clearance to descend "at pilot's discretion," and proceeded to 1,000 feet AGL, where I focused on breathing deeper and all my symptoms faded away.

After landing, I called Nat Daggs, the manager of the chamber back in the day, and told him he just saved my life.

"See, I told you I would!" he chuckled.

Over the years, I've heard many stories of hypoxia survival from our past trainees, but unfortunately I've also heard the stories of pilots who didn't have hypoxia training. Some stories we've all heard about on the nightly news, like Payne Stewart's plunge into a South Dakota field, or Helios Airlines' 737 that crashed in Greece.

These stories continue to occur, and yet hypoxia training is not mandated by the FAA or the International Civil Aviation Organization (ICAO). Is simply reading a few paragraphs in a textbook sufficient for combating hypoxia? I certainly don't think so.

Hypoxia is a "silent killer," meaning you will begin to be affected by a lack of oxygen long before you realize it. Couple that with the fact that there are over one thousand documented symptoms of hypoxia—nearly all are painless, and feel good to most people—and you could be in danger without knowing it.

Most importantly, your symptoms are unique to you, meaning your flight instructor's symptoms may be completely different than yours. Therefore, you cannot read symptoms out of a book and expect to know which you will experience. The only way to recognize your unique symptoms is to experience hypoxia in a safe environment like an altitude chamber.

Another issue arises with the use of the Time of Useful Consciousness (TUC) chart found in pilot training handbooks. Most pilots see this chart and think that they have five minutes to switch oxygen canisters while flying at 25,000 feet. This is not necessarily the case, since there are numerous factors which cause these times to change such as physical activity, stress, sleep, food, altitude, etc.

We typically take people to 25,000 feet in the altitude chamber and I've seen times of useful consciousness range from 30 seconds to 20 minutes. Pilots should understand that altitude decreases their TUC, but you cannot rely on a specific period of time at a given altitude to function as a useful pilot.

One example of this was when I accompanied skydivers on a jump from 25,000 feet. I was using a continuous flow system with an oral-nasal mask that was certified up to 25,000 feet. At one point I had to move to a different seat a few feet away. The combination of the mask bouncing on my face, the lack of oxygen pressure at this altitude and the physical activity caused my head to spin with hypoxia symptoms, rendering me unable to walk. My time of useful consciousness was only a few seconds rather than the five minutes one may mistakenly expect.

This silent killer also has many forms, and each form is additive, meaning you can have all four forms at one time, further decreasing your TUC.

Hypoxic Hypoxia is the most common form of hypoxia known to pilots. It is caused by a lack of oxygen pressure available in the lung. Most pilots know there are less oxygen molecules available per cubic foot as you ascend, but don't understand the effect that pressure plays in the alveoli in the lung. You must have pressure of oxygen available in sufficient quantity to "push" oxygen across the membrane into the bloodstream where the red blood cell covered in hemoglobin will grab that oxygen for transport.

Hypemic Hypoxia is a form of hypoxia caused by the red blood cell being unable to carry oxygen. Perhaps this is because your cockpit is being flooded with carbon monoxide, which your red blood cell would rather carry when given the choice. Perhaps you just gave blood, or are bleeding, in which case you are also at risk of not having enough red blood cells to meet the demand of oxygen.

Stagnant Hypoxia is another form of hypoxia that you probably see on a more regular basis. Remember when you got up too quickly from the couch and you had a little dizzy spell? Ever had a limb fall asleep because you were in an awkward position, or pulled too many gs which caused your vision to shrink? You were actually hypoxic—or stagnant hypoxic, to be exact. This is when the blood is not circulating, causing areas of the body to become oxygen deficient.

Histotoxic Hypoxia occurs when the cells of the body are not able to utilize oxygen effectively. This is typically due to a chemical—poisons, such as alcohol or carbon monoxide—which your cells are trying to get rid of. If you've seen somebody drunk, then you know what histotoxic hypoxia looks like.

Hypoxia doesn't need to render you unconscious to kill you. It simply needs to lower your cognitive abilities to the point where you cannot deal with the problem if you finally recognize it.

At every chamber flight, we prove this fact by telling hypoxic stricken trainees that they are about to die from hypoxia, but they don't take the required actions to save their life. Instead they sit and smile at us.

After we put their oxygen mask back on and ask them why they just sat there, they always reply, "I heard you, but couldn't do anything about it."

So do something about it. You can recognize it early with hypoxia training. It may save your life like it did mine.

Brent Crow is the operations manager at Arizona State University's altitude chamber. Send questions or comments to This email address is being protected from spambots. You need JavaScript enabled to view it. .

Read 3703 times Last modified on Thursday, 30 January 2014 21:02
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