Airframe and Powerplant

Hypoxia Lowdown

March 1, 2002

Now I use GUMPB

We all learned a little about hypoxia during flight training, but only a few of us have actually determined how, or at what altitude, hypoxia begins to affect our flying. No one disputes that oxygen usage and hypoxia are important for those high-altitude turbocharged fliers, but those of us who rarely ascend above 8,000 feet in normally aspirated airplanes may not give hypoxia a second thought. Pilots don't become hypoxic at 8,000 feet, right? I thought that way too, until I tested myself.

There are tremendous differences among pilots when it comes to how efficiently their bodies use oxygen. The variable between a pilot who can remain unaffected after two or three hours at 8,000 feet and one who gets sloppy, sleepy, and dimwitted is each body's efficiency in extracting oxygen from the atmosphere. Some pilots are affected by hypoxia much lower than the FAA's mandated oxygen usage altitudes. I'm one of them.

Pulse oximeter

I verified this after I bought a pulse oximeter. This small (two and one-quarter inches by one and one-quarter inches by one and one-quarter inches) battery-powered device quickly measures the percentage of arterial oxygen saturation of hemoglobin in the red blood cells. Within seconds after inserting my index fingertip between the two halves of the oximeter, my percentage of oxygen saturation and my pulse were displayed. As I write this (at 300 feet above sea level) my oxygen saturation is 97 percent and my pulse is 62. I've always been pretty active and have taken my pulse often over the years. I believed that my low resting pulse rate meant that I was healthy. A few sessions with the oximeter proved that heart rate was one thing and percent of oxygen saturation was another.

I took readings of these two important values during flights last year, and the numbers amazed me. Based on my readings, I realized that I needed to learn more about how hypoxia was affecting me, and that I might have to buy a portable oxygen system.

One thing I learned while monitoring my oxygen saturation levels was that changing my breathing pattern — by consciously taking a series of five or six deep, full breaths — is all it takes to again become an alert, eagle-eyed, fully conscious pilot. So one of the first changes I made was to revise my informal before-landing GUMP (gas, undercarriage, mixture, prop) check to include a letter reminding me to breathe. Now I use GUMPB (B for breathe). I'm not kidding — Gumby has replaced GUMP. It works, and it didn't cost a thing.

Oxygen requirements

Oxygen-use requirements are spelled out in FAR 91.211. Crewmembers must use supplemental oxygen when the cabin pressure altitude is above 12,500 feet for longer than 30 minutes, and must use it continuously when flying above 14,000 feet pressure altitude.

Since most normally aspirated general aviation airplanes rarely linger at these altitudes, many pilots tend to dismiss the need for supplemental oxygen. One reason for this is that pilots who are hypoxic can feel very good. For some pilots this feeling is akin to sitting at home in front of a warm fire sipping a cup of hot chocolate. This is the Catch-22 of hypoxia — our brains use approximately 30 percent of the oxygen circulated by our red blood cells and any deprivation, however slight, insidiously chips away at our brain's ability to function competently. The less competent we get, the better we feel.

Oxygen science

At sea level, the weight of the column of air piled up above us is approximately 15 pounds per square inch. At 18,000 feet the weight of the same square inch of air is about 7.5 pounds, or half of the sea level pressure. The pressure of the air (and the oxygen molecules we need to maintain visual, physical, and psychological acuity) that we inspire (breathe in) lessens as we ascend above sea level. The amount of oxygen in the air is constant at 21 percent, but the pressure differential that pushes the oxygen molecules from the alveoli in the lungs across the cell walls into the hemoglobin (oxygen-carrying molecules within our red blood cells) in our blood supply decreases as we gain altitude. We compensate for this decrease in pressure by taking in more oxygen — by taking deeper breaths, pressurizing the airplane cabin, or increasing the percentage of oxygen in the air we breathe in by using supplemental oxygen.

The decrease in blood oxygen saturation with an increase in pressure altitude is predictable. What isn't predictable is a pilot's ability to take in and distribute the available oxygen. Some of us, because of a number of factors, don't do as well as the charts say we should and become hypoxic at lower-than-predicted altitudes. Because hemoglobin likes to bond to carbon monoxide (a product of tobacco smoke) 200 to 300 times more than it does to oxygen, smokers don't have access to their full capacity for circulating oxygen. A smoker's diminished oxygen-circulating capacity results in a greater tendency to become hypoxic. There are other factors that can cause lower than normal resistance to hypoxia.

Although I quit smoking more than four years ago, diagnostic tests revealed that my smoking had killed off some of my lung cells. This loss of lung capacity isn't noticeable during normal sports activities but may be a contributing factor to my low oxygen-saturation numbers.

Headaches and mistakes

I like to fly long cross-country flights and love to look down on the planet from aloft. Before I started considering hypoxia as the cause, I would dismiss the headaches I experienced — and my crummy airmanship at the end of these flights — as a product of fatigue, dehydration, or poor nutrition. All of those factors could contribute. But after using my oximeter for a couple of flights, I became convinced that the real cause was mild hypoxia. I knew I could get more rest, eat better, and drink more water, but I knew very little about how to control hypoxia.

According to The Pilot: An Air Breathing Mammal written by Stanley R. Mohler, M.D., and published in a copy of the Human Factors Bulletin, an AOPA Air Safety Foundation publication, normal blood oxygen saturation levels are 96 to 100 percent at sea level 93 to 95 percent at 5,000 feet pressure altitude, 90 to 93 percent at 7,500, 88 to 92 percent at 10,000, 83 to 87 percent at 12,500, and 77 to 83 percent at 14,000 feet.

Last August I took a day VFR flight — flying level at 6,500 feet msl — and took five readings over the course of a 20-minute period. My oxygen saturation averaged 88 percent and my heart rate averaged 75. On the return leg of the same flight (a 2.8-hour round trip) the oxygen saturation numbers were the same, but my heart rate had increased to an average 94 beats per minute. What's a danger point or point where you should use supplemental oxygen? Dr. Brent Blue of Aeromedix has written that any time a pilot's oxygen saturation drops 10 points below the percentage that is normal at his home airport, supplemental oxygen should be used. During these flights, I found that I could consistently increase my blood oxygen saturation by at least five percentage points by taking five or six slow, deep breaths.

What does it mean?

My blood oxygen saturation levels were low enough — at 6,500 feet — to degrade my mental functions. Since the effects of hypoxia are cumulative (the longer I fly at altitudes that result in oxygen saturation levels below 90 percent the more I'm affected) is it any wonder that, after three hours at 10,000 to 11,000 feet flying from Alexandria, Louisiana, to Mineral Wells, Texas, that I was slow to make a decision to land in the face of worsening weather, or that I made a poor landing? Imagine the outcome if a complicated instrument approach procedure with low ceilings was required at the end of that long day of flying.

In "Flying and Hypoxia," a segment of a video produced by the FAA's Civil Aeromedical Institute titled Aviation Physiology, hypoxia is cited as a known cause in aviation accidents and incidents. More information on this video is available online ( www.cami.jccbi.gov/aam-400/videoseries.html) or by calling 580/234-2845.

A search of the NTSB's Web site ( www.ntsb.gov) turned up a series of fatal accidents in which hypoxia was determined to be a causal factor. At least a couple of these fatal accidents involved pilots who seemed to think supplemental oxygen was for wimps or who claimed to be unaffected by hypoxia and regularly flew without supplemental oxygen at altitudes as high as 17,000 feet. The lesson they taught us is that hypoxia is not static — its effects vary and our abilities change. Especially as we age.

When we become hypoxic we are more prone to make mistakes, misinterpret instruments, and fail to remember basic skills. One of the fatal accidents mentioned involved a pilot who was so thickheaded after flying at 16,000 feet for 40 minutes that he didn't switch to any of three full fuel tanks when the selected tank ran dry and his engine quit. Since the ability to take in and circulate oxygen varies with each pilot, and often changes as pilots age, every pilot should know how to recognize and deal with hypoxia.

Learning our limits

What can general aviation pilots do to learn more about their blood saturation percentages and any propensity they may have toward hypoxia? They can schedule a safe and supervised session in one of the hypobaric pressure chambers that are located around the country. The cost is $50. Visit the Web site ( www.cami.jccbi.gov/aam-400/asemphys.html) for more information.

Buying and using a pulse oximeter helps pilots monitor themselves. I bought mine, a Nonin FlightStat, from the Aeromedix Web site ( www.aeromedix.com), but they're also available from most of the oxygen system suppliers listed at the end of this article. Oximeters aren't cheap ($360 to $400) but they can produce usable numbers in almost any situation. Keep a record of your saturation percentages for a while — if your levels are consistently low, as mine were, you should invest in an oxygen setup and use it. Your flying will be more enjoyable (no more headaches) and safer, especially if you fly at night. Vision is the first system to show the effects of lowered oxygen saturation; at 10,000 feet night vision is degraded by 15 to 25 percent. Breathing oxygen for a few minutes before an approach, or when flying after sunset at altitudes as low as 5,000 feet, generally restores normal vision and brain function.

Oxygen systems

The owners of airplanes with permanently installed oxygen systems are required by the FARs to perform inspections — in accordance with Department of Transportation (DOT) guidelines — to test and certify the soundness of the oxygen bottles at regular intervals. The intervals are every five years for normal weight bottles and every three years for lightweight bottles.

Normal weight bottles (identifiable by ICC or DOT-3AA1800 being stamped into the bottle near the neck) have no life limit — as long as they pass the prescribed pressure test, they're approved for further use. The lightweight bottles (identifiable by ICC or DOT-3HT1850 being stamped near the neck of the bottle) must be retired after 24 years or 4,380 refill cycles.

Some pressurized general aviation airplanes have chemical oxygen generators instead of storage bottles. These self-contained units can produce breathable oxygen during an emergency situation (such as cabin depressurization or smoke in the cockpit) for up to 30 minutes. They're life-limited (Scott Aviation, the manufacturer, recommends replacement every 10 years) and should be inspected during annuals. Pay especially close attention to the actuating lanyards and the oxygen delivery tubing. Any evidence of brown residue in the tubing indicates that a chemical change has taken place and that it's time to replace the generators.

For most VFR fliers and renters, a portable system is the most reasonable option. There are a number of good systems available. At altitudes below 18,000 feet, nasal cannulas can legally be used in place of a full mouth- and nose-covering mask. Conserving cannulas collect oxygen between inspirations and, through a series of automatically controlled check valves, deliver a charge of oxygen each time the pilot inhales. Cannulas have the advantage of being more comfortable than full-face masks and also permit better communications, both through the intercom system and while transmitting, because they permit use of the pilot's headset microphone instead of a separate face-mask microphone.

A typical portable oxygen system includes a storage cylinder, a regulator/on-off valve, outlets for the required number of masks or cannulas, the masks and/or cannulas, a flow meter for each mask or cannula, associated tubing, and a fabric carrying pack that is usually secured to the back of the pilot's or copilot's seat. Two-place systems start at about $450, but prices vary and it pays to shop around.

Oxygen-conserving cannulas, marketed under the names Oximyzer and Oxysaver, and the Nelson adjustable-flow meters contribute to extending cylinder endurance times. Remember, though, the point of supplemental oxygen is to ensure sufficient oxygen saturation of the blood, and the best way to determine if you're getting enough oxygen is by using an oximeter in flight.


E-mail the author at steve.ells@aopa.org.

Resources

Aerox Aviation Oxygen Systems Inc., 206 Ossipee Trail, Post Office Box 206, Limington, Maine 04049; telephone 800/237-6902 or 207/637-2331; fax 207/637-2329;
www.aerox.com.

Scott Aviation, 225 Erie Street, Lancaster, New York 14086; telephone 716/683-5100; fax 716/681-1089;
www.scottaviation.com
.

Sky Ox Limited, Post Office Drawer W, 27328 May Street, Edwardsburg, Michigan 49112; telephone 800/253-0800 or 616/663-8544; fax 616/663-2579;
www.skyox.com.

Precise Flight Inc./Nelson Oxygen Equipment, 63120 Powell Butte Road, Bend, Oregon 97701; telephone 800/557-2558 or 541/382-8684; fax 541/388-1105;
www.preciseflight.com
.

Mountain High Equipment and Supply Company, 625 Southeast Salmon Avenue, Suite 2, Redmond, Oregon 97756; telephone 800/468-8185 or 541/923-4100; fax 541/923-4141;
www.mtn-high.com.

Aeromedix, Post Office Box 3370, 974 West Broadway, Jackson, Wyoming 83001; telephone 888/362-7123; fax 307/733-0032;
www.aeromedix.com
.