By The Book


November 1, 1991

The majority of our atmosphere — about four parts in five — is nitrogen. That's nice for microbes that can metabolize it, but it doesn't do much for humans, who need oxygen to survive. Or does it? Nitrogen helps provide the atmospheric pressure that makes it possible for oxygen to enter our bloodstream. And if you can't get oxygen into the blood — and thence to the brain, heart, and other tissues and organs — you will surely die. If you are cruising along at 41,000 feet in an airliner and the little yellow cup drops from the compartment above your head, put it on and prepare for a nap: Above about 35,000 feet, there simply isn't adequate atmospheric pressure to drive that oxygen into your bloodstream. (Standard procedure for the pilots, who are using 100-percent oxygen under pressure, is to rapidly descend to a lower altitude and land as soon as possible, so you shouldn't be out long.)

From sea level to 10,000 feet, a region known as the physiological zone, most people can get on fine. Even so, at 10,000 feet, atmospheric pressure has dropped from its sea-level average of 760 millimeters of mercury to 523 mmHg, and the amount of oxygen the blood can absorb (arterial saturation) has dropped 12 percent. The region from 10,000 to 50,000 feet is known as the physiological-deficient zone, and pressure drops from 523 to 87 mmHg. Here, survival depends on supplemental 100-percent oxygen or a pressurized cabin. As noted above, arterial saturation effectively reaches zero around 35,000 feet; oxygen can't get into the bloodstream. Above 50,000 feet, in the space-equivalent zone, pressure drops from 87 mmHg to 0, and survival is impossible without a pressure suit or a sealed cabin like the space shuttle's.

You needn't face an emergency in a jet to experience the effects of hypoxia, that is, oxygen deficiency in the body tissues sufficient to cause impairment of function. Its effects can be noticed as low as 5,000 feet, especially at night: Hypoxia particularly affects night vision. Heavy smokers, in fact, suffer from hypoxia sitting on the beach. But with single-piston-engine airplanes venturing as high as 25,000 feet (where arterial saturation is only 9 percent), understanding hypoxia and recognizing its onset are of literally vital importance.

Hypoxia can spring from various sources. Hypoxic hypoxia is the type most people are familiar with; it comes from the reduction in the oxygen pressure in the lungs caused by altitude. It can also be caused by fluid in the lungs, either from drowning or from ailments such as pneumonia. Hypemic hypoxia is an inability of the blood to transport oxygen and is caused by excessive smoking, carbon monoxide poisoning, anemia, or the effects of sulfa drugs. Stagnant hypoxia stems from lack of proper blood flow caused by sustained G forces, shock, heart failure, or even very cold temperatures. Histotoxic hypoxia finally, is the inability of cells to accept the oxygen the blood is bringing them or their inability to use it properly; it is caused by consumption of alcohol, narcotics (like codeine), or cyanide. You can see that pilots may fall prey to any type of hypoxia (remember the admonitions regarding cold remedies? — that's not just because they'll make you sleepy).

Know this, above all: Hypoxia is insidious. Its onset can be subtle, and the onset rate can vary from day to day and from person to person. Intellectual impairment is its hallmark, and one of its most pronounced symptoms is a feeling of well-being. That false sense of security will last right up to the moment you lose consciousness. Coma and death could follow.

A note here on a commonly used phrase: "time of useful consciousness." We're accustomed to seeing charts and graphs depicting it for various altitudes, but it's a little misleading, as we tend to focus on the "consciousness" part. The key word, however, is "useful." You may be severely hypoxic. You may be conscious. But if you are unable to recognize the hypoxia and take steps to reverse it, your time of useful consciousness has expired. A better phrase, one that aviation physiologists use, is "effective performance time." They define this as the time from loss of sufficient oxygen until you are no longer able to perform a task in a safe and efficient manner. That can be considerably shorter than the time you're actually awake.

How can you recognize hypoxia in yourself and others? That's really two separate questions.

The subjective symptoms, the ones you should be looking for in yourself, can include — separately or in combination — nausea, apprehension, hot and cold flashes, tunnel vision, headache, fatigue, a tingling sensation, dizziness, blurred vision, numbness, the aforementioned euphoria, and air hunger (your body knows you're hypoxic even if your mind doesn't). Some of those symptoms are things that happen fairly frequently to pilots, but part of staying ahead of the airplane should include staying ahead of your physical condition; if you encounter one or more of those symptoms, consider: Could it be hypoxia?

The objective signs, the ones you should always be alert for in your crewmates and passengers, include mental confusion, an increase in the rate and depth of breathing, cyanosis (a blue tint, particularly of the lips or under the fingernails), belligerence, poor judgment, loss of muscle coordination, euphoria, and (this should be a sure tip-off) unconsciousness. Don't laugh; if you're becoming hypoxic yourself, you could easily miss the fact that your partner is nodding off. It's happened, and more than once.

Several factors influence the rate of onset of hypoxia and the severity of the effects. These include flight-oriented elements like the altitude, rate of ascent, length of time spent at altitude, and cabin temperature. They also include personal items like your level of physical activity, overall physical fitness, individual tolerance, and psychological factors such as stress. It should also be added that a rapid decompression, as might occur in a pressurized airplane following loss of a door seal at altitude, for example, should be expected to cut your effective performance time in half because of the rapid drop in pressure of the oxygen in your system.

If you fly high, you can prevent hypoxia in one of two ways: by flying a pressurized airplane or by using supplemental oxygen. Remember the rules? Pilots of unpressurized airplanes must use supplemental oxygen when flying higher than 12,500 feet for more than 30 minutes and at all times above 14,000 feet. Passengers must be provided with supplemental oxygen (that doesn't mean they have to use it) above 15,000 feet. For unpressurized operations under FAR Part 135, pilots must use oxygen when flying above 10,000 feet for more than 30 minutes and at all times above 12,000 feet. (Passenger requirements also differ somewhat from Part 91 regulations.) The regulations governing pressurized airplanes are more complex but can be found with the other supplemental oxygen rules in FARs 91.211, 135.89, and 135.137.

What to do if you believe you're experiencing hypoxia? Get on 100- percent oxygen, and select a positive-pressure setting on your regulator, if available. Return your rate and depth of breathing to normal (hyperventilation won't help a bit.) Check the connections on your oxygen equipment. Most important, descend to below 10,000 feet. If you're flying a pressurized airplane, remember: Your emergency supplemental oxygen is there for one reason only — to keep you operating effectively long enough to get the airplane down to an altitude where you can breath without assistance. Descend — right away — to below 10,000 feet. It goes without saying that you must also be familiar with the functioning and capability of your oxygen equipment.

Any pilot can learn a valuable lesson by visiting an altitude chamber at the FAA's Civil Aeromedical Institute in Oklahoma City or at a number of military facilities around the country. Contact your local FSDO and ask the accident prevention counselor for AC Form 3150-7. Excellent aerospace physiology training courses are offered by the University of North Dakota Aerospace Foundation. This article is based on material presented in an UNDAF course, which the author gratefully acknowledges. For more information, contact UNDAF at the Center for Aerospace Sciences, 4201 University Avenue, Box 8009, University Station, Grand Forks, North Dakota 58202-8009; telephone 800/258-1525 or 70l/777-4740; fax 70l/777-4799.