Technicalities: Hypoxia at Your Fingertips

A deeper look at the physiological effects of hypoxia.

Hypoxia Big

Hypoxia Big

In last month's Aftermath, which concerned fatal accidents that the NTSB had linked to hypoxia, I was puzzled by a few that involved experienced pilots who had been flying, in one case for a rather short time, in the 12,000- to 18,000-foot altitude range without supplemental oxygen. If scrambled fighters find a maskless pilot slumped in his seat, clearly unconscious, as his airplane cruises past its destination at 25,000 feet, it's reasonable to suspect an oxygen problem. On the other hand, if a pilot with minimal instrument experience gets tangled up in clouds at night at 15,000 feet, is hypoxia to blame? You can't really tell, since plenty of pilots get tangled up in clouds at night at 5,000 feet with the same outcome.

Part 91 allows you, the pilot, to fly between 12,500 and 14,000 feet without supplemental oxygen for no more than 30 minutes. Pilots are required to use oxygen above 14,000 feet at all times. Passengers are permitted to gasp for breath indefinitely below 15,000 feet; above 15,000, everybody in the airplane must be provided with supplemental oxygen.

It is important to note that when hypoxia is discussed in terms of altitude, it is pressure altitude, not density altitude, that matters. There is always plenty of oxygen around — a whole atmosphere of it — but what controls the amount that reaches the brain is pressure differences across various membranes in the body. The process by which oxygen reaches the brain is affected by a great many factors that vary widely among individuals. By some definitions, normal people acclimatized to sea level begin to be hypoxic at 7,000 feet or so. Many people experience symptoms of altitude sickness at 10,000 feet; others are comfortable and seem to function normally at 15,000. The time of useful consciousness — a standard metric — at 29,000 feet is said to be around two minutes, and yet people — thoroughly acclimatized — have climbed Mount Everest without supplemental oxygen.

Some authors find Part 91 far too liberal. One doctor-pilot, Fred Furgang, declares in an essay on hypoxia that “supplemental oxygen is needed at altitudes lower than those required by the Federal Aviation Regulations. It is a good idea to consider oxygen for flights above 5,000 feet at night and about 8,000 feet during the day.” Well and good, but safe flying is about the careful management of risk, not about its complete elimination. How many pilots do you know who would turn back and land rather than cross a 10,000-foot ridge without oxygen?

In attempting to measure the hazards of hypoxia, we progress through a series of rough approximations. Pressure altitude is a very approximate surrogate for the amount of oxygen in the blood; the relationship varies greatly among different individuals. Oxygen in the blood — which can be measured in flight with a cheap gadget called a fingertip oximeter — is in turn a surrogate for performance, but even at the same level of oxygen saturation, different people experience different degrees of impairment. And impairment itself is difficult to evaluate; a highly skilled pilot, impaired, may outperform a mediocre one at the top of his form.

Yet another step is required to relate impairment to safety. It’s easy to say that none should be acceptable, but in fact people fly all the time with all kinds of impairments due to fatigue, minor illnesses, life stresses, circadian rhythm, medications permitted or proscribed — all factors that the NTSB has at one time or another associated with accidents. Since most of these are unavoidable, it is assumed that pilots’ skills are adequate to allow them to perform their tasks even when they are not in peak condition. Differences in ability and performance are taken for granted; one does not have to be Charles Lindbergh to earn a pilot’s license.

A typical altitude-chamber ride takes you up to 25,000 feet and has you do sums and sign your name while you’re there. When you return to ground pressure, you discover that your work product resembles a fourth-grader’s. But the purpose of the ride is to scare you into following the rules. It happens that 25,000 feet is close to the level at which the effects of hypoxia are quite marked and cease to be extremely variable among individuals; in fact, most people will eventually pass out at 25,000 feet. A chamber ride to 15,000 feet might be less dramatic, but it might be more representative of the everyday experience of pilots.

I suspect that the original impulse for studying the physiology of hypoxia may have come from World War II bomber crews who flew in unpressurized (and unheated) airplanes at 30,000 feet. Dozens of crew deaths were attributed to inadvertently disconnected oxygen supplies, and the conversation about hypoxia coalesced about its most dire, and potentially fatal, aspects. The fact that the most common standard of hypoxia is “time of useful consciousness” reveals how irrelevant much hypoxia discussion is to normal flight experience. Hardly anyone today climbs to 25,000 feet without oxygen and expects to get away with it; but many pilots, faced with a line of clouds, a high inversion or the Rocky Mountains, will climb to 15,000 and rationalize staying there as long as necessary. It is not a question of their losing consciousness; more likely, it is a matter of their misreading a heading on a chart or taking longer than usual to copy a clearance.

To make an already murky subject more murky still, the relationship between altitude and oxygen saturation is not at all linear. It has a distinct knee, remaining fairly flat up to 8,000 feet or so and then trending downward with increasing rapidity. The knee is located at different altitudes for different people. Thus, you not only don’t know how much oxygen your brain is getting at a given altitude, but you also don’t know how rapidly the amount is changing as you climb higher.

A while ago, in an effort to get some sort of subjective grasp of the relationship between altitude, blood oxygen saturation and performance, I bought a fingertip oximeter on the Internet for $38. When I first got it, I climbed to 13,500 feet and performed some impromptu self-tests, which consisted of holding a heading and altitude and squaring two-digit numbers in my head. (I have done this for mental exercise ever since some grammar-school teacher told my classmates and me that if we did not constantly use our brains we would lose them. You would think that by now I would know the squares of all two-digit numbers by heart, but I don’t. Maybe she was wrong: We lose them anyway.)

I was interested to see, when I landed, that I had two incorrect answers out of three. On the other hand, my measured oxygen saturation — the percent of the oxygen-holding capacity of the blood that is actually being used — had been higher at all altitudes than the standard chart said it should be. (Of course, the oximeter could be wrong.) I was interested too to see that the saturation lagged by a minute or so as I climbed or descended; the often-heard statement that “recovery is immediate” with administration of supplemental oxygen may apply to waist gunners whose masks came unplugged, but it does not strictly apply to saturation percentages.

Fingertip oximetry does not tell how much oxygen is actually getting to the brain, but it’s better than nothing. The real problem is correlating saturation with impairment. Furgang recommends going on oxygen when saturation, normally 98 to 99 percent at ground level, drops below 92 to 93 percent in daytime or 94 to 95 percent at night. A study of workers building an observatory at the 16,000-foot level in Chile identifies “minimal mental impairment” with a range of oxygen saturations between 85 percent and 75 percent, which correspond, on average, to altitudes between 10,000 feet and 13,000 feet (but your results may vary). One study (McFarland, 1972), with specious exactitude, gives a table of percentages of sea level performance in various categories. For example, at 13,650 feet (4,200 meters) visual sensitivity — this would correspond to night vision — retains only 56 percent of its sea level value; short-term memory drops to 83 percent and arithmetic ability to 92 percent. Interestingly, the last thing to go was found to be “decision-making ability,” which was at 95 percent at 13,650 and 90 percent at 16,250 feet (5,000 meters). This should be a comfort to scofflaws, since decision-making ability — assuming that this means making the right decision, not just any decision — would seem to be the factor with the greatest relevance to flight safety.

One well-known aspect of hypoxia is that a hypoxic person feels euphoric and is unaware of any impairment. It is therefore pointless to say to yourself, “I will descend (or put on the mask or cannula) as soon as I start to feel odd.” You won’t feel odd. You have to accept as a given that once you are at 10,000 feet or higher, you are already hypoxic. That doesn’t mean that you are incapable of flying an airplane, but you may be incapable of reliable self-assessment. An oximeter is valuable because it provides a measure of objectivity — and it doesn’t hurt to have some oxygen aboard, just in case.