Hypoxia - The Underestimated Danger
From a newsgroup - author unknownNowadays, altitude-savvy pilots are starting to carry a tiny instrument called a pulse oximeter that clips on the finger and, by passing a light beam through the vascular bed of the fingertip, measures the oxygen saturation of the blood and displays it on a digital readout. Think of it as a "hypoxia meter" that allows you to see precisely how hypoxic you are at any given time.
Types of Hypoxia
The effects of hypoxia upon flying skills and the symptoms of its onset are the same no matter what the cause of the hypoxia. It is useful, however, to look at some varying causes of this condition so we can be alert to its possible onset when of one or more of these factors is present.
The three kinds of hypoxia
Hypoxic hypoxia - occurs because there is a smaller and smaller pressure differential between the pressure of oxygen in the inspired air in the lungs and the pressure of the oxygen in the blood and tissues. Remember that the combining power of hemoglobin and oxygen is influenced by this pressure differential. The greater the differential, the more efficient the hemoglobin becomes. As this pressure differential lessens, it becomes harder and harder for the hemoglobin to pick up and transport the oxygen.
Hypoxic hypoxia is also referred to by aviators as "altitude hypoxia." This is the hypoxia that results when there is a lack of available oxygen or partial pressure of oxygen in the breathing air. This is the type hypoxia experienced when flying in an unpressurized cabin or when flying at altitude in a jet with a cabin pressurized to a cabin altitude above 5000 feet. Although strictly speaking, we are somewhat hypoxic when operating even a few hundred feet above the altitude of acclimatization, this becomes most evident when flying unpressurized aircraft. In reality, the symptoms of hypoxic hypoxia do not, in the absence of other contributing factors, become significant until about 5000 feet.
Hypemic hypoxia (also calle anemic hypoxia) - occurs whenever the blood's ability to carry oxygen is reduced although there is sufficient oxygen at a sufficient pressure in the inspired air. There are a variety of conditions that can cause this to happen.
Any condition that would cause a reduction in the number of healthy, functioning red blood cells (anemia or reduced production of red blood cells, blood loss, deformed blood cells, disease, etc.) will impair the blood's ability to supply the tissues with oxygen. Remember the old advertisements warning about "iron poor blood?" Iron is the functional part of the hemoglobin molecule and it is the iron which renders the hemoglobin absolutely indispensable for life. In addition to a reduction in the number of red blood cells available, anything that would interfere with the ability of hemoglobin to transport oxygen or anything that would displace the oxygen that is bound to the hemoglobin will affect the oxygen available to the cells.
The most common impairment to oxygen transport by the hemoglobin is carbon monoxide. Carbon monoxide combines with hemoglobin 200-300 times more readily than does oxygen and once bound is extremely hard to eliminate. Smokers will find that the carbon monoxide bound to their hemoglobin will lower their altitude for onset of hypoxic symptoms by 2000-3000 feet. This effect is not limited to smokers, however. Anyone exposed to a smoky atmosphere will suffer somewhat. (Remember this next time you volunteer to go along as a designated driver for a group of drinkers. Just sitting in that smoky bar for several hours is going to affect your performance the next day, even without alcohol and fatigue!) Other chemicals, among them sulfa drugs and nitrites (found in food preservatives) can have an adverse effect on the ability of hemoglobin to combine with and transport oxygen.
Histotoxic hypoxia - is a disruption of cellular respiration. There may well be sufficient oxygen of sufficient pressure in the inspired air to fully saturate the blood and hemoglobin, but the cells expecting and needing the oxygen are unable to use it due to the presence or absorption of cell toxins. The most common toxin found at the cellular level that can cause this effect is alcohol. Although other toxins, notably cyanide and some narcotics, also can cause this disruption of cellular respiration, alcohol is by far the most common culprit.
Now, we are all aware of the hazards associated with alcohol and flying and I'm not suggesting that any true professional would knowingly violate these rules and guidelines. Many pilots, however, may be impaired by alcohol at the cellular level and not be aware of the problem -- or its cause. Remember the iron poor blood mentioned earlier? Be cautious of the "tonics" or "elixirs" offered as remedies. Carefully read the labels on any over-the-counter medications or nutritional supplements you propose to ingest. Although many more manufacturers are eliminating or reducing the alcohol content of the liquid medications, you may be surprised at the percentage of alcohol some still contain. One popular vitamin supplement for "iron poor blood" contains 12% alcohol!
The Effects of Hypoxia at Various Altitudes
Hypoxia is an insidious and progressive condition and is almost undetectable by the pilot. You should always be aware that without supplemental oxygen at sufficient pressure you will gradually and progressively lapse into incompetence while maintaining an absolutely euphoric faith in your own ability.
As blood saturation of oxygen drops there is a steady disruption of life functions. From a blood saturation of 93%, considered the low limit of normal functioning, where visual problems begin to occur, there is a rapid deterioration into unconsciousness with decreasing saturation.
Keep in mind, as mentioned earlier, that although the partial pressure of oxygen in the atmosphere decreases somewhat less than linearly with increasing altitude, the hemoglobin's ability to combine with this oxygen follows a much different and more deadly curve. This property of hemoglobin bears heavily on oxygen requirements at altitude. Let's look at some common hypoxic effects an average, healthy individual can expect with increasing altitudes.
5000 feet - This is considered by most to be a "low" altitude. The retina of the eye is more demanding of oxygen than any other organ of the body -- even the brain itself which demands 30% of the supply. At this "low" altitude this extension of the brain will begin to suffer degradations in function which will be most noticeable at this point in night vision.
Instruments and maps are more easily misread during night flight at this altitude and ground features and lights are more easily misinterpreted. It is always an eye-opening shock to my students when I bring them in over the Mojave Desert after a long flight at a cabin altitude of 8000 feet. After having them note the discernible features on the dark desert floor, I have them breath 100% oxygen for a few minutes. Without exception, all are shocked and amazed to note the features that "jump" out of the blackness after a little O2. Most have heard of this little demonstration before it actually takes place, but none are totally prepared for its dramatic effect. This level of hypoxia is extremely insidious because most pilots feel they are functioning at peak efficiency at this point. Extra vigilance is necessary to prevent missing critical fixes on charts or misreading instruments.
10,000 feet - Night vision is now degraded by 15-25 percent. The blood saturation has dropped to 90 percent and your brain is receiving the absolute minimum supply of oxygen. This is the absolute highest altitude at which you should have any trust at all in your own performance although your judgment is already severely compromised. Euphoria will prevent true self-assessment of your abilities. Physical hypoxic symptoms such as tingling and headache may not become apparent for four hours or more at this altitude, although judgment has long gone by the wayside. Above 10,000 feet blood oxygen saturation and performance degrade steeply.
14,000 feet - Blood oxygen saturation is down to a dangerous 85%. You will be increasingly disabled at this altitude. Vision will dim. You will experience serious degradation of judgment, memory and thought. The impairment of judgment will leave you feeling just fine and confident in your performance, however. If hypoxia is not recognized and corrected at this stage of impairment, it is unlikely that it will be recognized. You are in serious danger.
16,000 feet - Only 2,000 feet higher than the last assessment, but you will behave as though you had ingested a full load of gin and tonics. Your blood oxygen saturation will have dropped to 79 percent and you will be seriously disabled. You will be euphoric, belligerent, disoriented or perhaps all three. You will be irrational, unreliable and dangerous. If you are alone, your chances of survival are decreasing rapidly.
18,000 feet - At this altitude you are incapable of any useful function although you may still feel great! Blood saturation has fallen to 71 percent and your brain is suffering. You will pass out in about 30 minutes.
20,000 feet - If you have not already collapsed, it will not be long now. Five to 15 minutes is about the time of useful consciousness at this altitude and prolonged exposure can result in death. Blood saturation has dropped to 71%.
25,000 feet - Forget it! Blood saturation has now dropped to lethal levels. Time of useful consciousness is three to six minutes with death following not long after that. Above this altitude, suffering a rapid decompression may also result in a condition divers know as the bends and various other pressure related maladies. Remember, this is only HALF as high as some modern civilian aircraft are certified to fly!
What Determines Your Response to Hypoxia?
It is impossible to tell exactly when hypoxic reactions will begin to affect you. Individual reactions to hypoxia vary greatly not only among people, but in the same person on a day to day basis due to differences in body chemistry, general health and diet. Some of the determining factors are somewhat under the pilot's control and some are dictated by the flying environment itself.
Absolute Altitude
This is the easy one. The severity of hypoxia will depend directly upon the absolute altitude of the environment you're in. This may be the aircraft altitude in an unpressurized cabin or the cabin altitude in pressurized craft. As the altitude of the environment climbs and the partial pressure of the oxygen in that atmosphere drops, the risk of hypoxia rises. Acclimatization (to normal living altitudes) can help only to a limited extent. The Denver folks may have an advantage up to about 15,000 feet, but after that everyone is more or less equal -- equally impaired. This factor is usually somewhat controlled by the pilot; however, mountains or weather can cause a climb to a previously unplanned altitude. This is of little consequence in a pressurized aircraft.
Acclimatization
One question I am often asked concerns the permanent residents in the Andes above 17,000 feet. How do these people remain conscious, much less do work? The answer is acclimatization. The Peruvians who live in these villages have extremely high red corpuscle counts and so have much more hemoglobin with which to transport oxygen. These residents also have much larger pulmonary ventilation volumes and increased cardiac output. Some acclimatization shown by this population is reversible -- they will be lost with acclimatization to a lower altitude and consequently a higher atmospheric pressure. Other of their adaptations, however, seem to be permanent evolutionary responses to life at lowered atmospheric pressures, are not lost with residence at lower altitudes and are passed on genetically to subsequent generations.
The native Denverites we spoke of earlier will have more tolerance to altitude and become hypoxic later and at a higher altitude than coastal pilots because they are already partially acclimated to altitude by virtue of their residence in the Mile High City. They are only 5000 feet above their physiologically adapted altitude at 10,000 feet. The LA pilots are 10,000 above theirs. (Then again, there are some who claim that those of us in LA are permanently in the ozone, but that is probably the topic of a different discussion.)
Rate of Ascent
The quicker you (or your environment) climb, the more rapid the onset of hypoxic symptoms. The climbers who ascend Mt. Everest are well aware of this phenomenon. They spend several weeks in their climb, stopping at several intermediate altitudes to acclimate. An explosive decompression in an aircraft with the resultant rapid climb of the cabin altitude can reduce the time of useful consciousness to one-third to one-half of that normally expected. A rapid ascent can cause the symptoms of hypoxia to quickly accumulate and incapacitate a pilot before awareness of the encroaching disability dawns on the dimming consciousness.
Duration of Exposure
Staying at 8,000 feet for several hours (not uncommon in a jet at flight level cruising altitudes) can cause the same symptoms and incapacitation as staying at a higher altitude for a shorter duration. The symptoms of hypoxia are cumulative and time related, but there is no reliable means to predict the exact relationship or effect. The only certainty is that the higher the altitude the shorter the time of exposure before symptoms begin to occur. This, too, will vary on an individual basis.
Physical Activity
Any physical activity will obviously cause the body to demand more oxygen for normal functioning. The muscles will rob the brain of the marginal amounts of oxygen available in the blood and the time of onset of hypoxic symptoms will be shortened. Although not much physical activity is expended by pilots, the extra amount required to fly in turbulence or with a failed autopilot can dramatically reduce the already minimal oxygen supply to the brain and the retinas of the eyes. This factor is usually not under the pilot's control.
Temperature
The temperature in the cabin has a great affect on an individual's tolerance for and response to hypoxia. Either extreme -- the cold of a cockpit at altitude at night with a failed heating system or the greenhouse environment of a poorly air conditioned pressurized aircraft at high noon -- will cause the body to expend energy in an attempt to maintain its core temperature within acceptable limits. This expended energy is just another form of increased physical activity and will decrease a pilot's tolerance to hypoxic conditions.
Self-Imposed Factors
Pilots vary widely in their susceptibility to oxygen deficiency and the same person will show variances from day to day. This is primarily due to many factors influencing susceptibility to hypoxia which are under direct control of the pilot. It is the pilot's responsibility to avoid these factors as much as possible. Your tolerance of oxygen deficiency will be reduced by any one or more of the following factors. The affect of combining these factors cannot be accurately assessed.
Fatigue
Fatigue is both a exacerbating factor of and a symptom of hypoxia. A mentally or physically fatigued pilot will have less tolerance for hypoxia and its associated decrements in performance and perception because the fatigue will already have degraded performance, perhaps to unacceptable levels. Hypoxia will deepen the fatigue and the cycle continues on a downward spiral of increasing fatigue and degradation of performance. Also, there is a chance that the pilot will not attribute increasing fatigue to the effects of hypoxia and will not take prompt corrective actions.
Alcohol
This cellular toxin is a risk factor even after the blood alcohol level has returned to zero. Of course, as we noted above, any alcohol in the blood or cells will hamper their ability to take up and utilize oxygen. One ounce of alcohol in the blood will raise the body's perceived altitude by 2,000 feet. However, the aftereffects of the alcohol can be just as debilitating. The fatigue caused by the disturbance of normal sleep cycles by alcohol will diminish tolerance to hypoxia as discussed above. Additionally, the depressant effect of this drug will remain after the toxin is cleared from the blood. This will cloud the pilot's judgment and delay the recognition of the problem.
Carbon Monoxide
Again, it is important to remember that carbon monoxide (CO) will combine with hemoglobin in the red blood cells 200-300 times more readily than will oxygen. Once bound it is almost impossible to rid the red blood cells of their cargo of this toxin -- in fact, most carbon monoxide remains bound to the red cells until the cells die and are scavenged by the liver. Of course, the major source of carbon monoxide in the blood stream is cigarette smoke -- either your own or second hand smoke from other smokers in the vicinity. Probably no other self-imposed risk factor is as deadly -- or as controllable -- as is the CO level in the blood. Smoking a pack of cigarettes in the 24 hours preceding a flight can saturate as much as 8-10 percent of the available hemoglobin. This will raise the body's perceived altitude by as much as 5,000 feet! You can be effectively in Denver on the ground at LAX. You will suffer the effects of hypoxia at sea level. An 8,000 foot cabin of a jet cruising in the flight levels is going to be just the same as cruising unpressurized at 13,000 feet. Is it any wonder that most accidents happen in the landing phase? One wonders how much hypoxia contributes to the accident rate yet is rarely listed as a cause -- especially in modern jet transports.
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