Oxygen in the ambient air must ultimately reach the cell and does so because of gas laws, the lungs, circulatory system, blood, and hemoglobin. An inter- ruption of that process from a single problem or a combination will result in hypoxia, each situation with a different name. Ambient atmospheric par- tial pressure of oxygen decreases during ascent, which translates into a lower pressure gradient in the lungs for diffusion to take place across the alveo- lar membrane (Dalton’s and Graham’s gas laws). Hemoglobin (Hgb), the pri- mary transporter of oxygen within the blood, does not have access to adequate amounts of oxygen to attach. Or there isn’t enough Hgb available. Oxygen molecules are biochemically attached to the hemoglobin molecule, which is affected by the surrounding partial pressures of oxygen (and carbon dioxide), which, in turn, transfers these gases to and from the tissue cells. The oxygen hemoglobin dissociation curve indicates a rapid decrease of saturation—and transfer—when oxygen partial pressure drops below 60 mm Hg. Ideally, arte- rial pressure should be near 80–90 mm Hg or a saturation of 87–97 percent.
The tissue cell might be unable to take on the oxygen because the cell is impaired or diseased. Also, the amount of blood flow to the tissue cells is compromised if the circulatory system is compromised.
The entire process is reversed when removing carbon dioxide from the tissue cell and bringing back oxygen-poor hemoglobin. This is critically important,
HIGH ALTITUDE AIRPORTS
Several of the world’s airports are so high that most aircrew are required to be on supplemental oxygen during off/on loading. Cabin altitudes must be less than 8,000 ft. Check out runway lengths.
These are extremes, but there are plenty of airports above 8,000 ft, even in the U.S.A., which can cause hypoxia in the crew.
Peru: 5 airports above 12,000 ft, one at 14,422 ft (runway lengths 6,600 ft to 13,700 ft)
Bolivia: La Paz 13,333 ft, runway 13,123 ft Chile: Coposa 12,500 ft, runway 10,499 ft
GOTCHA!
Review of respiration physiology 51
52 Altitude physiology
since both directions of this transfer process of carrying oxygen and carbon dioxide affects the feedback “signals” to the brain, which adjusts the entire system to meet the needs of the body. The biochemical process is quite com- plex, and further information is available in the resources appendix.
Classification of hypoxia
No matter what the cause of hypoxia, the resulting symptoms and effects on flying skills are basically the same. The following classification is therefore intended only to review these causes so that your degree of suspicion of being hypoxic (or observing hypoxia in crew or passengers) is raised when you are subjected to these varied situations (Fig. 5-1). How you feel and function when hypoxic and what to do about it will be discussed later.
Figure 5-1
Figure 5-1 Causes and types of hypoxia. Source: R.L. DeHart, Fundamentals of Aerospace Medicine.
Philadelphia: Lea & Febiger, 1985. Reproduced with permission.
Review of respiration physiology 53 Hypoxic (altitude) hypoxia
This term is commonly used when talking about hypoxia associated with lack of available oxygen, as experienced when flying at altitude in an unpressur- ized cabin. It means that there aren’t enough oxygen molecules available to breathe with sufficient partial pressure, as when we ascend. The number of molecules of oxygen decrease, despite the fact that the percentage remains the same. This situation is particularly evident in the physiologically deficient zone of the atmosphere.
Strictly speaking, one is hypoxic even a few hundred feet above the ground. In actuality, the symptoms do not become a significant factor until about 5,000 feet, especially at night. The significance of the symptoms is related to many fac- tors, which will be explained later. Suffice to say, hypoxia must be considered as being present at all flight levels, including highly pressurized cabin altitudes.
From a gas law perspective, hypoxic hypoxia exists when the partial pres- sure of oxygen in the atmosphere or the inhaled ambient air is reduced. This reduced partial pressure is also present in the inspired air as it travels into the bronchial tree and into the lungs. In other words, the partial pressure of oxygen as it is presented to the blood within the lungs is too low to effectively carry and transfer enough oxygen to the cells of the tissues.
Hypemic (anemic) hypoxia
This occurs when the blood’s ability to carry oxygen molecules is the prob- lem, even though there is adequate oxygen available in the air to breathe and exchange. This happens for a variety of reasons.
Anemia, or a reduced number of healthy, functioning red blood cells for any reason (disease, blood loss, deformed blood cells, etc.), means less capacity for blood to carry oxygen. Hemoglobin (Hgb) physically carries 75 times more oxygen molecules than are dissolved in solution. Anything that would interfere or displace oxygen that is attached to the hemoglo- bin would also reduce the oxygen available to the cell. This occurs most commonly when carbon monoxide is inhaled unknowingly along with the cabin air. Hemoglobin accepts carbon monoxide 250 times more than oxy- gen and therefore competes with adequate oxygen transfer. Other chemi- cals such as sulfa drugs and nitrites (as in food preservatives) have the same combining activity to hemoglobin, thus competing with oxygen for attachment to hemoglobin molecules.
Stagnant hypoxia
If the blood flow (the circulation of the oxygen carrying hemoglobin) is compro- mised for any reason, then sufficient oxygen cannot get to the cells and tis- sues. Stagnant implies a diminished flow, not necessarily a complete stoppage.
Such decrease in blood flow results from the heart failing to pump effectively, an artery constricting and cutting off or reducing the flow, or venous pooling of blood because of gravity, such as in varicose veins of the legs.
Another cause, unique to flying, arises during positive G maneuvers—pulling Gs and long periods of pressure breathing at extreme cabin altitudes where
54 Altitude physiology
oxygen masks are required. Another situation is in cold temperatures where blood supply to the extremities is decreased by shunting blood away to more crucial organs. All these situations can lead to stagnant hypoxia.
Histotoxic hypoxia
Histotoxic means the target cell expecting and needing oxygen is abnormal and unable to take up the oxygen that is present. This abnormality has been created as a result of a toxin or toxins present/absorbed by the cell. In other words, the oxygen might be inhaled and reach the tissue or cell in adequate amounts, but the cell is unable to accept and use the available oxygen. This can occur when alcohol is present in the blood or in the cell and prevents the essential use of oxygen by the cell. Alcohol becomes a toxin to the cell.
The same is true for some narcotics and certain poisons, such as cyanide.
Stages of hypoxia
No matter what the reason for oxygen not getting to the cell or being used in its metabolism, the lack of oxygen (hypoxia) results in a variety of subtle and not-so-subtle symptoms: any single or concurrent combination of sev- eral situations causing some degree of incapacitation. The danger of hypoxia is that the pilot is probably unsuspecting that he/she is hypoxic.
The key to flying safe at altitude is to recognize:
• The conditions under which you could be hypoxic.
• The physical and mental symptoms that indicate you are hypoxic.
• When a crewmate is susceptible to hypoxia in those conditions.
Because the nervous system tissues have a heavy requirement for oxygen, especially the brain (and eyes), most hypoxic symptoms are directly or indi- rectly related to the nervous system. If hypoxia is prolonged, serious prob- lems develop with ultimate death (Fig. 5-2). In extreme cases (prior to death), some brain cells are actually killed, and they cannot be regenerated. It bears repeating. The single most dangerous characteristic of hypoxia is that if the crewmember is hypoxic and engrossed in flight duties, the pilot might not
DEEP VEIN THROMBOSIS (DVT)
Deep vein thrombosis is also known as “traveler’s thrombosis” or erroneously “economy class syndrome” (research shows occurrence in any class), although it’s not clear if there is a relationship only with travelers. Any situation where one’s leg activity is limited for long periods increases the risk. Occurrence is about 1 per 1,000 people in the general population from all causes. The cause of DVT is similar to the cause of stagnant hypoxia. The ultimate concern is that a throm- bus (or blood clot) can break off and flow to the lungs and become a pulmonary embolus, a medical emergency.
Review of respiration physiology 55 even notice the impairment. Only through education, continued awareness, and actual exposure to hypoxia in controlled conditions (as in an altitude chamber) can the pilot truly respect this insidious hazard.
Because hypoxia can and often does develop gradually, the pilot must recog- nize its various stages, allowing some degree of anticipation if symptoms in the early stages are identified. The earlier that hypoxia can be recognized, the sooner that corrective action can be taken before the pilot becomes unable to act appropriately. Keep in mind that although altitude is the common denominator in being hypoxic, the health of the pilot can affect his tolerance.
Other abuses, such as smoking, alcohol, and stress, can reduce this toler- ance in the otherwise healthy pilot; therefore, hypoxia can be very unpredict- able. Saying you had no problem at 10,000 feet last week does not mean you will safely function today at the same altitude.
Indifferent stage
One of the earliest symptoms of hypoxia is its effect on the eye. Vision, espe- cially night vision, will deteriorate even at altitudes less than 5,000 feet. And you won’t know you are having problems. Other classic symptoms can be present at lower altitudes if the body is less tolerant, as will be explained later. Suffice to say, at any altitude, at night, be aware that your vision is compromised. For example, acceptable night vision is lost by 5–10 percent at 5,000 feet, 15–25 percent at 10,000 feet, and 25–30 percent at 12,000 feet.
Figure 5-2
Figure 5-2 Stages of hypoxia related to arterial blood oxygen levels.
56 Altitude physiology Compensatory stage
During this stage, the body and mind can be severely affected and in increas- ing and subtle ways; however, the circulatory system and, to a lesser degree, the respiratory system can provide some defense against hypoxia while in this stage. This happens as a result of increased heart rate, enhanced circulation, and a more productive cardiac pumping of blood. Respiration (breathing rate and depth) also increases. Although these body responses are automatic, one should not assume recovery without taking immediate conscious corrective action whenever you suspect hypoxia.
At 12,000 to 15,000 feet the effects of hypoxia on the nervous system can become increasingly incapacitating, especially for the unacclimated. As time at this unhealthy altitude continues (as little as 10–15 minutes), impaired skills are very evident. A variety of symptoms develop (a complete list follows), many causing impairment. Such symptoms as drowsiness, poor judgment, and frequent subtle errors in flying skills become apparent. More dangerous is a feeling of well being and indifference (euphoria).
Once again, the crucial characteristic about hypoxia, especially in this stage of potential recovery and compensation, is the recognition of an increasing oxygen deficit before you are aware you are in trouble. The same holds true in your crewmates. Keep checking on each other, especially in an environment conducive for hypoxia. Beyond this stage, you will not suspect hypoxia but be so impaired that you won’t be able to correct the situation by descending, putting on an oxygen mask, or declaring an emergency.
Disturbance stage
This is the stage when chances of recovery are greatly diminished. Symptoms become more severe, with headaches, hyperventilation, impaired peripheral vision, marked fatigue, sleepiness, and especially euphoria. By now you might not even recognize you are hypoxic, but you don’t care and have little incentive or energy to take corrective action such as getting to a lower alti- tude or going on oxygen.
Critical stage
This is when you have lost it—you’re unconscious. All this can happen within 3–5 minutes after you failed to recognize you were hypoxic during the com- pensatory and disturbance stages. Some will faint as a result of circulatory failure, or something more serious will occur: failure of the central nervous system, which also results in unconsciousness. Convulsions can also occur before or after unconsciousness.
Symptoms of hypoxia
The following are some of the more common symptoms experienced by pilots with hypoxia (not necessarily in any order of severity):
• Change in peripheral vision, even noting “tunnel vision.”
• Visual acuity impairment, images slightly blurred, can’t focus.
Review of respiration physiology 57
• Difficulty in visual accommodation, focusing from near to distant and back.
• Weakness in muscles, more difficult to change the airplane seat.
• Feeling very tired and fatigued, sleepy for no reason (not boredom).
• Sense of touch is diminished, the controls feel different.
• Sense of pain is diminished (the aching sprained ankle is better).
• Headache, especially if hypoxic for a long period (2+ hours).
• Lightheaded and mild dizziness, reacting poorly in tight turns.
• Tingling in fingers and toes.
• Muscular coordination decreased, sloppy at controls.
• Stammering, can’t get the right words out to ATC.
• Cyanosis, bluish lips and fingernails (notice in your crewmate).
• Impaired judgment, doing dumb things, slow thinking.
• Loss of self-criticism, no longer care if you’re a great pilot.
• Overconfidence, “No problem!”, taking risks not normally acceptable to that individual.
• Overly aggressive, too much in charge, challenging ATC.
• Depression, small irritants become perceived as major problems.
• Altered respiration, breathing faster, shallower.
• Reaction time decreased, you’ve lost your touch.
• Greatly reduced color discrimination and night vision (even at 5,000 feet).
• Euphoria, you settle for less, who cares?
Note that many of these symptoms are very subjective; that is, most symp- toms are not definitive of how hypoxic you are. For example, a change in heart rate is objective evidence, but it can also change with exertion, stress, and other situations. One might not think of being hypoxic. A common symp- tom is fatigue, which is a very subjective sign. How fatigued must you be for you to consider hypoxia? Are you fatigued from hypoxia or from the length of the flight? And if you are indeed hypoxic, you will be indifferent to any degree of fatigue. Also, these symptoms are not listed in order of importance or ease of recognition. Some pilots experience a specific kind of headache, often lasting for a very long time; others only feel fatigued before they become incapacitated and lose control.
Hypoxia is an easy trap to fall into, and it happens to all pilots, many times, in varying degrees. It is unfortunate, but the fact is that the human body does not have an effective warning system to alert one to the onset of hypoxia.
Hypoxia is painless. Each pilot will react differently to the same degree of hypoxia, and he or she, in turn, will experience different symptoms even under similar conditions. These individual symptoms can also change as one gets older. It is for these reasons that the military continues to expose its flight crews to the condition of hypoxia in an altitude-chamber ride periodi- cally. The main reason is for the individual pilot to experience what his or her symptoms are for hypoxia before he or she reaches the disturbance stage and then cannot (or will not) recover or take appropriate action. No amount of lecturing or reading will prepare you for how to detect your own level of
58 Altitude physiology
incapacitating hypoxia; therefore, it is strongly encouraged that at least once in your flying career you take a chamber ride, preferably early in your flying education. This will be discussed later in this chapter.
In summary, the signs and symptoms of hypoxia are many and varied, and they differ from individual to individual and are unpredictable at any given time and altitude. Be aware, be suspicious, and watch your crewmates.
Hypoxia is not a black or white condition. It is insidious, with various levels of impairment.
Factors influencing tolerance to hypoxia
Recall that it is impossible to predict exactly when, where, or how hypoxic reactions will occur in an individual. One of the reasons is that individuals vary widely in their susceptibility to oxygen deficiency. This susceptibility is related to many factors now to be defined that are controllable by the pilot, in most cases. Avoidance of these factors becomes part of the pilot’s respon- sibility in being safe. In other words, given a cabin altitude that the pilot will be at, his tolerance to oxygen deficiency will be diminished by any of the following:
Self-imposed factors
Although the cabin might be at less than 10,000 feet (pressure altitude), there is a “physiological altitude,” which is an altitude that the body “feels” it is at. The presence of these self-imposed factors effectively raises this physi- ological altitude. Consequently, the mind and body react accordingly with some incapacitation. So instead of the body responding to the 10,000 foot altitude, it responds as if it were at 13,000 feet.
Alcohol in the body can result in histotoxic hypoxia because the alcohol is a toxin to the cells. It has been observed that one ounce of alcohol can equate to about an additional 2,000 feet of physiological altitude. It interferes with oxygen uptake and metabolism at the cell level, and, of course, is dependent on the amount of this toxin circulating in the body. Furthermore, the usual depressant effects of alcohol on behavior can cloud the pilot’s recognition of his own hypoxia, adding to his decreasing tolerance. It’s possible to have alcohol in your system and be legal, but still be impaired.
An individual who is mentally or physically fatigued tolerates hypoxia poorly because that person is already bordering on performance decrement. And since fatigue is a symptom of hypoxia, it becomes difficult to discriminate how hypoxic one is. The pilot will often erroneously reason that his fatigue is not a symptom of hypoxia and not consider any preventive actions.
The carbon monoxide in cigarette smoke, whether in a smoker or from second hand smoke before a flight, is a great threat to the smoking pilot (Fig. 5-3).
Carbon monoxide has an affinity for hemoglobin 210–250 times more than oxygen, and this results in hypemic hypoxia. As with alcohol, it has been observed that smoking three cigarettes in rapid succession or smoking 20–30 cigarettes in a 24-hour period prior to flight can saturate from 8 to 10 percent
Review of respiration physiology 59 of the hemoglobin in the body. In addition, approximately 20 percent of a smoker’s night vision is lost even at sea level. This can translate into a physi- ological altitude of an additional 3,000–5,000 feet.
The pilot who is in good shape physically (not overweight, has been exercising, and has a fairly nutritious diet) will be much more tolerant of the effects of low oxygen. The symptoms and potential incapacitation from hypoxia are still present, but the pilot will find them less severe, and more important, she will be able to take appropriate action to recognize and avoid increasing hypoxia before she is truly at risk. Another way of looking at this is that the physi- ological ceiling is reduced. Also, keep in mind that it might be more than just improved tolerance. The healthy body itself might be more efficient in the use of oxygen and therefore requires less oxygen in its metabolism. In any case, being in good medical condition is a true investment in safe flight. Being in less than optimal shape becomes a self-induced deterrent to tolerance.
Other factors affecting response to hypoxia
The following are conditions that determine the degree of hypoxia one can expect. One has some control over these factors but to a lesser degree, since they are a part of our flying environment and working conditions. When tol- erance is described, it refers to how susceptible the body is to low levels of oxygen. In other words, as a result of the following, one could become hypoxic at lower altitudes sooner than expected.
Acclimatization
People who live at high altitudes develop an increased tolerance to the condi- tions that would lead to hypoxia in people living at lower altitudes; therefore, pilots who live in Denver (the “Mile-High City”), are more adapted when fly- ing at, say, 18,000 feet than someone who lives in Los Angeles. By the same Figure 5-3
Figure 5-3 Smoking increases a pilot’s physiological altitude. (*Altitudes are in mul- tiples of 1,000 feet.)