I recently sat in a room full of aviation professionals and, unbelievably, heard the comment that “the so-called oxygen bottle is really compressed air, just like scuba divers use.” While I sat there with my mouth open, more than a few in the room nodded agreement. They may not be the only folks thinking this to be true.

In the early 1980s the company took delivery of its first Gulfstream GIII. We’d occasionally fly the airplane at FL 450 and had only the space shuttle with which to share the airspace. Today, FL 450 is a given on most any long eastbound flight and we often see the other corporates heading west at FL 470. While I once got to 59,000 ft. in an F4E (ask me sometime), I wouldn’t fly any of today’s equipment above FL 470. High-altitude flight is so comfortable, and so routine, that the perils of that realm of flight are overlooked in the “hours of sheer boredom.” Recent events show that when the shell on that artificial world is broken, reaction has to be instinctive or people will die.

Oxygen being delivered to the human brain by the bloodstream is vital for normal cognitive functioning, Beyond that, it is required for sustaining life. But did you know there are altitudes at which you could be sitting in a passenger seat of a jet with the passenger oxygen mask on your face, with the system functioning normally and delivering 100% oxygen to that mask, breathing in and out normally, and pass out in a matter of seconds and be dead shortly thereafter. It’s not about breathing, it’s about gas transfer. In this case that gas is oxygen. Lack of such a transfer, oxygen deficiency, is called hypoxia.

Some Terms

Hypoxia — Lack of oxygen to the brain. There are four types:

a. Hypoxic — Not enough oxygen is available.

b. Hypemic — The inability of a person’s cells to carry sufficient oxygen due to another chemical compound (think carbon monoxide) being more readily transferred into them than oxygen.

c. Histotoxic — The inability of the blood cells to carry sufficient quantities of oxygen because they are overloaded with other chemicals such as alcohol.

d. Stagnant — Lack of oxygen to the brain due to blood being drawn from the head due to external forces (g’s). 

In this article, we will discuss only hypoxic hypoxia.

Hyperventilation — A condition in which too much oxygen in the bloodstream causes panting, which increases oxygen, causing more panting until the amount of oxygen is large enough to create syncope. Syncope, passing out, is the body’s way of saying “slow down.”

Evolved Gases Disorders — Problems in the body due to nitrogen in the blood coming out of solution at locations other than the lungs.

Henry’s Law — Henry’s law states that the amount of gas that can pass through a membrane is dependent upon the partial pressure of that gas above the membrane. In this case, let’s think of it as the transfer that occurs when oxygen passes from the atmosphere into the bloodstream through the membranes of the lungs, called alveoli.

Dalton’s Law — This law states that the partial pressure of a gas in a considered atmosphere is equal to the percentage of the total mixture of gases in that atmosphere that the given gas represents.

Time of Useful Consciousness (TUC) — How long the brain can be deprived of oxygen and still be able to make useful decisions and complete complex tasks such as piloting an airplane.

Oxygen makes up about 20% of atmospheric gases. Nitrogen makes up about 78% of the gas mixture, and other, inert gases make up about 2% of the mixture. Leaving inert gases out of the equation changes nothing and makes the math easier. So, our “assumed ” atmosphere is made up of oxygen and nitrogen at the ratio of 80% nitrogen and 20% oxygen. At sea level, the atmospheric pressure is 14.7 psi, so, according to Dalton’s law, we receive oxygen at a partial pressure of about 3 psi (0.2 x 14.7 = 2.94).

In order to avoid hypoxia we need partial pressure of about 3.0 psi of oxygen to be present in the lungs for gas transfer. While hypoxia is a decrease in the amount of oxygen getting into the blood and into the brain, the tissues and the cells, it doesn’t have to be a fatal decrease. We took a cog railway trip to the top of Pike’s Peak a few years back and I had to guide my wife through the gift shop by the shoulder. She was having a ball (more on that later), enjoying her inability to coordinate her movements normally. She was tipsy, eventually became belligerent (we’re blaming hypoxia) and wound up just wanting to “take a short break.” All of these moods are classic hypoxia symptoms. She literally improved each few hundred yards of the trip back down the mountain and was normal when we reached the base (or as normal as a sea level person could be at nearly 7,000 ft. MSL). She was suffering some of the effects of hypoxia, but at that altitude they would not be fatal for the time she was there.

We can all understand that when we put on an oxygen mask that is sealed to our face (no beards allowed), if that mask selector switch is in “100%” oxygen we are receiving nothing but oxygen through the system. This is not normal or harmful, but the body  just doesn’t need oxygen at a greater pressure than 3 psi.

It is more instructive to discuss what the regulator does for the pilot in the “normal” position. How much oxygen are we receiving sitting on the ground? How about at altitudes like 10,000 ft., 18,000 ft., 30,000 ft. and 43,000 ft.?

Let’s assume the oxygen regulator has been designed to produce oxygen at a partial pressure of 3.0 psi to the lungs of the pilot. At sea level that’s the pressure that already exists, so the regulator, in “normal,” delivers ambient air to the pilot at a cabin altitude of sea level. This is why I used to smile when I heard certain European inspectors would check the oxygen remaining to insure crews had oxygen masks on when above FL 370. At the cabin altitude that a properly functioning pressurization system would provide at FL 370, the regulator would be delivering almost completely ambient cabin altitude to the pilot and the amount of oxygen used wouldn’t decrease noticeably over several hours.

At 10,000 ft., atmospheric pressure is equal to 10.5 psi and the partial pressure of oxygen is about 2.1 psi (0.2 x 10.5). Let’s assume that 2.1 is about the least partial pressure of oxygen we can receive and still function normally. This is why cockpit warnings come on around the 9,000-10,000-ft. cabin altitude range. They are saying “Do something or you’re going to feel like Detwiler’s wife on top of Pike’s Peak, and shortly after that even worse things may happen.”

For normal functioning, we need 3.0 psi of oxygen. How can that be achieved if the total atmospheric pressure of the atmosphere at that altitude is only 10.5 psi? It’s simple. The design of the regulator changes the mixture of oxygen with ambient air so that at that altitude the amount of the oxygen being delivered is raised to roughly 29% of the mixture. This means there is still enough oxygen in the mixture for the partial pressure of that oxygen to allow normal transfer.

An altitude of 18,000 ft. is a useful benchmark because at that height half of the earth’s atmosphere is below us. Atmospheric pressure at that altitude is equal to 6.75 psi and the ratio of oxygen to nitrogen in order to allow 3.0 psi of oxygen must be 44% (0.44 x 6.75 = 2.97 psi). If I’m losing you here, go back to the discussion of Dalton’s law, on the previous page.

At 35,000 ft., the total pressure of the atmosphere is about 3.0 psi. In order for normal oxygen transfer to occur, we know oxygen must be present at about 3 psi.  Since the total atmospheric pressure at 35,000 feet is only equal to 3.0 psi, that means that 100% of the mixture being presented to the pilot must be oxygen. 

In the above senerio an interesting phenomenon occurs. The total pressure of the atmosphere goes below 3.0 psi. So, even with 100% of the mixture presented to the pilot being oxygen, there will not be enough partial pressure of oxygen at the alveoli to make a normal transfer. The regulator must start delivering pressurized oxygen to the pilot. Pressure breathing has to have been practiced. When used to the procedure, it’s actually fun to force the air out of your lungs, like you are blowing up a balloon, and then relax and let the regulator pressure fill your lungs back up. Almost everyone will hyperventilate the first time they try it, especially if they are under stress at the time.

Pressure breathing will allow the pilot to function into the low to mid 40s in altitude, but above that the pressures are so low that gases begin to “bubble” out of the blood regardless of what’s going on in the lungs. When these gases, mostly nitrogen, bubble out and collect around the joints, it’s called the bends. The military requires that full pressure suits be worn for flight above 50,000 ft.

Factors Affecting Hypoxia

The most-common factor affecting one’s ability to become hypoxic is the physical condition of the pilot involved. Exercise creates a need in the body for oxygen, which affects the amount of red blood cells that carry oxygen. A well-conditioned pilot has more red cells in his bloodstream and can delay the onset of hypoxia longer than a couch potato. Also, if the pilot normally functions at an altitude of over 5,000 ft., as a Denver-based pilot does during the off hours, their blood contains more red blood cells. Age and its related decrease in good circulation is also a factor. Other chemicals in the bloodstream, from medications to unknown gases, also affect this ability.

So what can go wrong?

The absolute best teacher for the potential physiologic problems associated with high-altitude flying is the hypobaric chamber or altitude chamber. Over the course of 35 years in the military, I have had the good fortune to attend over a dozen altitude chamber “rides.” In my humble opinion, a pilot who has not done this type of training is not qualified to be flying at high altitude.

The chamber lets us experience hypoxia in a carefully monitored situation with, in military chambers, at least two attendants outside watching the progress, one inside to aid with any problems that develop, and a separate mini chamber in which to place people who have experienced any problems and bring them up or down in pressure depending on the problem. Hypoxia instruction is usually a very routine operation, but if something goes wrong, it can go terribly wrong, terribly fast, so highly trained assistants and close-by medical facilities are a must.

The chamber also lets us experience hypoxia’s onset symptoms. These symptoms include, but are not limited to, a feeling of happiness or well being (à la Mrs. D), a feeling of over confidence, a feeling of tenseness, perspiring, changes in breathing rate and heart rate, belligerence and, eventually, unconsciousness and death.

These symptoms are vital to know because they can clue a pilot into something being wrong before any of the warning systems of the airplane tell him. Hypoxia can occur at normal pressures when other gases are present that are more easily absorbed by the bloodstream.

The training also teaches us that differences in age greatly determine how much hypoxia tolerance a person can have. In the book, The Great Muckrock and Rosie, I describe a scene in which I took an altitude chamber ride with a bunch of my friends. This was done at the end of a month and a half of grueling basic cadet exercise and physical exertion at the U.S. Air Force Academy — campus elevation 7,200 ft. (“Far, far above that of West Point or Annapolis, sir.”) One of my good friends was asked to do a hypoxia demonstration at 35,000 ft. Old Hugh sat there for about a minute when the outside monitor asked him if he was beginning to feel any of the symptoms of hypoxia. “Nope, fine,” he said.

About 30 sec. later the instructor again asked him if he was feeling OK, and Hugh just signaled a big thumbs up. Thirty seconds after that he was instructed to put on his mask as he had been without oxygen far beyond the normal time.

He refused at first and finally the instructor just put the mask on. This was in 1962. We were all between the ages of 18 and 20.

The last altitude chamber ride that I took was in the spring of 1994, some 32 years later. The chamber was full of middle-aged men and the hypoxia demonstration was done at 18,000 ft.

“When you begin to experience some of your symptoms, put the mask back on.”

We all had our masks back on our faces in less than 30 sec.

Hypoxic Encounters

Let’s view a couple of potential scenarios.

First, a high-performance turboprop airplane, cruising at an altitude in the high twenties, receives a warning telling the pilot that the cabin pressurization system, for whatever reason, is failing to keep the cabin below the desired 5,000-ft. elevation. He glances at the cabin altimeter and notes that it is climbing through 8,000 ft. With no panic he decides to make a slow descent and asks ATC for a lower altitude. After a few minutes they grant the descent. At 25,000 ft., the TUC is 3 to 5 min. Although the cabin doesn’t get that high, 5 to 10 minutes have gone by until he descends. When he reaches that lower altitude, say 16,000 ft., he notices that the cabin is also at that altitude. Feeling very competent and resourceful and capable of handling the situation (hypoxia symptoms) he continues. After about 15 min. he again calmly requests a lower altitude and eventually thinks he hears ATC issue that, but as he keys the mic to confirm, an overwhelming feeling of happiness engulfs him and he sits back and smiles, marveling at how close that fighter is on his wing. Life is good. A few minutes later, life is over.

This pilot had become hypoxic and in order to recover had to put on a mask or get to an altitude well below 10,000 ft. The feeling of well-being, if he’d been well-trained, should have told him to declare an emergency and descend now and not stop until he was below 5,000 ft., terrain permitting.

A second scenario finds a well-trained crew with a group of older passengers cruising comfortably at 43,000 ft. when the cabin pressurization system signals that it is developing problems. The cabin altitude page is displayed and it’s noted that the cabin is maintaining its normal pressure of about 4,000 feet, but  is going up and down rather than being stable. The flight attendant comes forward and asks the pilot if there is a problem as the passengers are feeling the effects of the climbs and descents. Still hoping to continue his trip, he descends to 40,000 ft. as a systems precaution. A few minutes later, the cabin begins to climb and cannot be manually controlled. Both pilots are aware that, at that altitude TUC is measured in seconds and is cut in half in a rapid decompression. As the cabin goes through 8,000 ft. the crew simultaneously declares an emergency, dons oxygen masks and begins the classic “high dive.” The cabin altitude continues up, but due to the increasing outside pressure and other system anomalies, it stabilizes around 9,800 ft. as the airplane descends through 30,000 ft. Not wanting to continue alarming the passengers, the crew continues with a normal descent, while the cabin continues to descend. By the time the airplane reaches 15,000 ft., the malfunctioning system has put the cabin below sea level. Knowing that too much pressure, when released, can be dangerous for reasons of evolved gas disorders, the crew selects bleeds off and continues with a normal approach and landing at an en route divert base. Good hypoxia training and awareness of the systems leads to an “inconvenient” divert from a much higher altitude than the turboprop.

Lastly, I hope you will take my opinion to heart. If you haven’t been through an altitude chamber, you are incompetent to be operating above 10,000 ft. Get the training. B&CA