How Hypoxia Can Ruin A Flight

by Dr. Bill Blank, MD

In the last issue, Midwest Flyer Magazine Editor Dave Weiman wrote about a general aviation oxygen system, so I thought an article about “hypoxia” would be helpful.

Hypoxia means lack of oxygen and in our case, we are talking about the brain not getting enough oxygen. The brain uses 20-25% of the oxygen consumed by the human body. The consumption does not vary depending on our level of physical activity. The brain is always on the job. On the other hand, oxygen usage of our muscles and heart does vary with physical activity.

The problem with hypoxia in aviators is its insidious onset. That is not to say that its onset is undetectable, but the airman must know his own individual symptoms of onset. These vary between individuals. Once hypoxia sets in and the brain isn’t functioning well, the ability of the airman to deal with it is greatly reduced. The time of useful consciousness has passed.

How does hypoxia occur?

With each breath of air, we need a certain minimum number of oxygen molecules for good brain function. Air normally contains 21% oxygen and 78% nitrogen. As we climb and the outside pressure decreases, air is still 21% oxygen. The problem is we have less oxygen molecules in each breath. We finally get to an altitude where symptoms of hypoxia appear. The altitude at which this starts varies between individuals.

In order to be able to fly at higher altitudes where hypoxia can occur, oxygen systems have been developed.

If we breathe 100% oxygen, all of the molecules in the air we breathe are oxygen molecules. We can therefore go higher before we become hypoxic. Eventually, even 100% oxygen at the low pressure of high altitudes cannot do the job. In order for the oxygen to reach the brain, the pressure must be high enough to drive the oxygen into the blood. If not, the oxygen wants to leave the blood for the lungs, exactly the opposite of what we want. To fly at these altitudes, a pressurized breathing system is needed. The other solution is to have a pressurized airplane. In this case, the air pressure in the cabin provides an environment similar to standing on the top of an 8,000 ft. mountain. This altitude varies between airplane designs, but is where many airline cabins are designed to be.

Much of the research on hypoxia was done after World War I and up to World War II.  It was done on young, healthy men and was used to develop regulations to decrease the likelihood of hypoxia-related accidents. It is a bit of a stretch to say that the current group of GA pilots is characterized as being a group of young, healthy individuals.

Military pilots receive extensive altitude training. They experience simulated high-altitude flight in altitude chambers. They learn what happens if a mask malfunctions or they experience sudden decompression. A major goal of the training is to teach pilots to recognize their own symptoms of incipient hypoxia, while they have time to react.

What are the symptoms of hypoxia?

1. Increased breathing rate, headache, fatigue.
2. Lightheaded or dizzy sensations, listlessness.
3. Tingling or warm sensations, sweating.
4. Poor coordination, impairment of judgment.
5. Loss of, or reduced vision, sleepiness.
6. Blue color of skin, fingernails, and lips.
7. Behavior changes, euphoria.

In addition, night vision is affected. The rod cells in our retina used for night vision are much more sensitive to lack of oxygen than the cones used for daylight vision. That is why supplemental oxygen is recommended when flying above 5,000 ft. at night.

The rules for GA pilots regarding the use of supplemental oxygen:

1. For flight above a cabin pressure of 12,500 ft. and below 14,000 ft. for more than 30 minutes, the minimum flight crew must use oxygen.
2. For flight above 14,000 ft., the minimum flight crew must use oxygen at all times.
3. Above 15,000 ft., each occupant must be provided with oxygen.
4. Flight in pressurized aircraft requires special training and an altitude endorsement.

Those of you who fly at high altitudes may want to know your own incipient symptoms of hypoxia. A free course is available at the Civil AeroMedical Institute (CAMI) in Oklahoma City. They can be contacted at 405-954-4837 for further information. A valid medical is required.

I took the course several years ago. My first symptom was euphoria. I was quite happy. When they ask me to add 2+1, I knew that I should know the answer, but could not answer the question. I actually thought it was funny that I could not answer it. When I looked at how I had written my name afterwards, a 2-year old could have done better. In addition to altitude training, as a pilot, you may wish to consider a “supplemental oxygen system.” The regulations specify the altitude above which supplemental oxygen use is mandatory. That doesn’t mean we can’t use supplemental oxygen at lower altitudes. This is another way to compensate for age.

EDITOR’S NOTE: William A. Blank is a physician in La Crosse, Wisconsin, and has been an Aviation Medical Examiner (AME) since 1978, and a Senior AME since 1985. Blank is a retired Ophthalmologist, but still gives some of the ophthalmology lectures at AME renewal seminars. Flying-wise, Blank holds an Airline Transport Pilot Certificate and has 5300 hours. He is a Certified Instrument Flight Instructor (CFII), and has given over 1200 hours of aerobatic instruction. In addition, Blank was an airshow performer through the 2014 season, and held a Statement of Aerobatic Competency (SAC) since 1987.

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