A Medical Look at Hypoxia

A Medical Look at Hypoxia

A Medical Look at Hypoxia

Is it actually safe to operate pressurized aircraft at cabin altitudes above 10,000 feet?

Over the years, there have been numerous NTSB reports of pilots flying pressurized twin and turbine aircraft in the mid to high 20 flight levels, failing to respond to radar controllers and ultimately crashing. One of particular note occurred several years ago when a pilot of a pressurized piston twin took off from New Orleans and headed on an IFR flight plan across the Gulf of Mexico toward the central Florida coast. About an hour into the flight, when well off-shore, he stopped responding to calls from the controllers, causing them to scramble two Air Force jets to intercept the aircraft and see what was going on. The military pilots then reported seeing an elderly male pilot slumped over the control wheel and apparently unconscious. They followed the airplane as it began a gradual descent of wide circles until it crashed into the ocean and disappeared in 10,000 feet of water. The aircraft and its pilot were never seen again.

The NTSB blamed the crash on an aircraft pressurization failure, which lacking any evidence to the contrary was not an unreasonable thing to do. But when all factors are considered, the risk of that pilot having an incapacitating physiologic event at altitude may have actually been a much better explanation. Which begs the question, just how safe is it for pilots (especially of a certain age) to be flying these aircraft in the flight levels at or near their maximum operating altitude with cabin altitudes of 10,000–12,000 feet?

When a pressurized piston or turboprop aircraft operates in the high 20 flight levels, the pilot’s body is only being supplied with half of the oxygen available at sea level. This triggers the “Bohr Effect” (illustrated here), further decreasing the amount of oxygen available to the brain and heart.

The truth of the matter is, sudden depressurization events in general aviation aircraft are not common. The reason for this is that the pressure differential is quite low, and the basic structure is well designed for that purpose. Plus, most airframes simply do not have that much time or “cycles” on them. Slow depressurization events following maintenance are more common as they result from work done that compromises the cabin’s sealed structure. But these are generally carefully checked for on post maintenance flights and fairly obvious to the pilot. Particularly if the aircraft is piston-powered, engine failure can also cause a partial loss of pressurization in a twin (and a complete loss in a pressurized single) because the remaining engine simply does not put out enough air through the turbocharger to keep the cabin fully inflated. But the failure of piston engines is quite rare especially at altitude.

So, there is a tendency to think that if the airplane is pressurized, then it must be safe for any pilot to personally fly it for hours at a time near its maximal approved altitude. Though that may generally be true for the airframe and engines, it is a dangerous and false assumption for many pilots. This is because there is a series of physiologic events that accelerate and compound each other, making the risk of failure on the part of the pilot quite high relative to that of the aircraft. It is imperative both piston and turbine pilots know and understand the causes for this assumption.

In the mid-to-high 20 flight levels, many pressurized pistons, and even some turboprop aircraft, have cabin altitudes at or above 10,000 feet. If the aircraft is piston-powered, the engines will have nearly sea level air pressure being supplied to them by the turbochargers and not even “know” they are up that high. The cabin altitude, however, is way up there at about 12,000 feet, which is often as high as the pilot has ever been in his entire life. And depending upon his age and underlying health status, this is already a potentially dangerous thing to do, even if sitting there. (By comparison, the pilots and passengers in a B787 at FL410 are sitting in a cabin at no more than 6,000 feet, and should that cabin altitude climb to 12,000 feet, it is considered an emergency).

The physiologic problem that needs to be understood is that of lowered blood oxygen levels – something dangerous for all pilots but more so as age and other health problems accumulate. Although the amount of oxygen in the air is a constant 21.9 percent of volume regardless of altitude, as one goes up, the air pressure itself decreases causing the available or partial pressure oxygen also to decrease. At 12,000 feet, the useful available oxygen is about half of that at sea level (13 percent vs. 21 percent), and this shortage of oxygen triggers a number of physiologic events which gradually accelerate and compound each other, particularly in pilots with a little grey hair and other medical issues.

The best solution to recognizing the gradual onset of hypoxia is to wear a pulse oximeter anytime the cabin altitude is above 5,000 feet.

Physiologically, the first thing that happens is the level of oxygen saturation in the blood goes down, a condition known as “hypoxia.” A medical term in which “hypo-” means “low” and “-oxia” means oxygen. The body attempts to compensate for hypoxia by increasing its respiratory rate, which helps improve the oxygen deficiency a bit, but also causes a lowering of carbon dioxide (CO2) in the blood, a condition known as “hypocapnia.” As hypocapnia develops, the hemoglobin molecules in each red blood cell which pick up oxygen as they pass through the alveoli of the lungs, paradoxically start to get very reluctant to release that oxygen when they arrive at areas of the body requiring it. This is called the “Bohr” effect, named after the physiologist who described it in the early 1900s. It is also known to medical students as the “oxygen/hemoglobin dissociation curve,” the graph for which they must thoroughly understand and memorize. The graph is not linear, but has an exponential downward curve, meaning that the problem accelerates or becomes worse and worse very quickly. This results in even less oxygen being supplied to needy tissues like the heart and brain at a rate faster than the altitude is changing. So now, even though the airplane’s pressurization system is operating as it was designed to with a perfectly legal cabin altitude between 10,000 and 12,000 feet, the pilot’s body has much less oxygen in the bloodstream. And because of the oxygen/hemoglobin dissociation curve, what oxygen there is in the bloodstream is not as available to the needy tissues as it was even a couple of thousand feet lower. And this is just the beginning of the problem. 

There is another phenomenon that also decreases potential oxygen to essential organs like the heart and brain, and that is a gradual narrowing of the inside of the blood vessels or so-called “arteriosclerosis.” This happens to everyone as they age but occurs earlier and faster in people with a history of high cholesterol, high blood pressure, poor exercise habits, diabetes, and nicotine use. So, whether the result of a perfectly normal aging process or common health issues, the size of the internal diameter of blood vessels is smaller, which further limits the amount of already poorly oxygenated blood available to key organs. And again, the issue continues to compound itself.   

The heart’s timing mechanism known as the sinoatrial node (or SA node) is a key receiver of oxygenated blood and is very sensitive to any decrease in oxygen levels. As the amount of available oxygen drops, the SA node becomes very unhappy about this and can frequently start to misfire, which is similar to upsetting the timing mechanism on a piston engine. All of a sudden, the efficiency of the heart as a pump is drastically reduced, and with it even a further decrease in oxygenated blood to both the heart itself (which in turns makes the misfiring even more pronounced) and also to the brain. The latter usually produces a loss of consciousness from which spontaneous recovery is improbable, and death usually follows.  

In summary, when a pressurized piston or turboprop aircraft is in the high 20 flight levels and operating just as it was designed (cabin altitude of 10,000–12,000 feet), the pilot’s body is only being supplied with half the oxygen available at sea level. This, in turn, triggers the Bohr effect, further decreasing the amount of oxygen available to the brain and heart, which if the pilot is of mature age, are already compromised due to the narrowing of blood vessels. Finally, the hearts timing mechanism becomes irritated causing it to misfire, which can rapidly lead to loss of consciousness and death. Depending upon who else is on board, a fatal crash of confusing cause usually follows.  

Given this physiologic reality, is it really safe for pilots with grey hair and some common health issues such as elevated cholesterol, high blood  pressure, and possible arterial narrowing, to operate pressurized aircraft at their highest legal altitudes with cabin altitudes? The answer is probably not. But, if the pilot is willing, some steps can be taken to lower the physiologic risk to a more acceptable level, and it involves the use of supplemental oxygen.

Supplemental oxygen is something that needs to be used before hypoxia is present because its effect on the brain is very insidious and makes such recognition of what is occurring, and the logical solutions that would follow, nearly impossible. The best solution to recognizing the gradual onset of hypoxia is to wear a pulse oximeter anytime the cabin altitude is above 5,000 feet and watch the numbers on the dial. These are simple to use and available from various aviation supply stores for a very nominal amount. Their application should ideally be on the “FL180 checklist” along with switching altimeters to 29.92. It is also a good idea to brief any passenger sitting in the right seat to keep an eye on the oximeter readings. If the blood oxygen level as shown on the oximetry starts to drop into the low 90s, then the pilot should automatically go on supplemental oxygen even if the aircraft is at an altitude below FL180. In the relatively low flight level altitudes piston aircraft fly, pilots do not require a huge oxygen flow to fix the problem of hypoxia; usually, just a 2-liter flow of oxygen via nasal prongs will restore the partial pressure of that gas to that of an airline cabin. 

Some pilots, however, are hesitant to use supplemental oxygen even if the pulse oximeter shows it is indicated because they feel it should be saved for an “emergency.” Part of this is due to poor training wherein the oxygen masks are never used unless there is an emergency caused by an abrupt depressurization event. The other explanation is that the typical pressurized piston aircraft has a limited inbuilt oxygen supply which is frequently difficult and expensive for the pilot to have re-filled, and therefore needs to be “saved” or at least used very conservatively. The resupply problem can be a real one, but a very convenient way around this is a portable oxygen bottle that can be filled by any local medical oxygen supplier at minimal cost. One rumored concern about using medical oxygen is that it is not “dry” enough and as a result, moisture could form in the supply tubes which can clog them shut. This concern originated in WWII when bomber crews were flying in non-pressurized and non-heated cabins in freezing temperatures in the high flight levels, but is not applicable to the heated and pressurized general aviation aircraft we have today. A portable medical oxygen tank and its supply lines will almost always work just fine in a heated aircraft cabin.

Pressurization makes a significant contribution to the potential safety of flight because it enables the aircraft to overfly otherwise bad weather, and as long as the aircraft’s altitude does not exceed far above FL180, the cabin altitudes are as physiologically safe that of an airliner or newer executive jet. Above that altitude, however, the tradeoff starts to shift the other way depending to a large extent upon the pilot’s age, medical history, and physical condition. The older the pilot, the higher the risk, particularly if he or she has some adverse health history such as known coronary artery disease, high cholesterol, diabetes, or high blood pressure. But for nearly everyone, using supplemental oxygen whenever the pressurized piston aircraft goes above about FL180 will convert the physiologic risk to that of driving across most mountain passes in the western United States. 

Now, all this discussion about physiologic risks, oximeters, and the use of supplemental oxygen may sound like a needless hassle. But if you feel that way, have your spouse or close relative read this Twin & Turbine article. It would be wise to follow the advice that I am sure will follow. Be safe out there. 

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