Mid-Teens Physiology

Climbing to the flight levels puts your body through major changes that mere oxygen cant address. Heres whats going on and some tips on how to minimize the effects.


by Brent Blue, M.D.

Higher, faster and farther are some of the capabilities pilots who use personal airplanes for transportation are always seeking. Often, to get that performance, well add a turbocharger, a second engine, pressurization and maybe even bite the financial bullet and start flying behind one or between two turbine engines. Tip tanks, oxygen bottles, electronic oxygen saturation monitoring devices and some liquid refreshments help us stay at altitude once we get there. While this equipment can make the higher altitudes easier to reach and safer to stay in, our bodies may not be as happy there as our airplane.

Indeed, spending hours at time at or above altitudes requiring oxygen in an unpressurized cabin can have a debilitating physical effect on a pilot. Even when fortunate enough to be able to afford and operate a pressurized airplane, those effects can be present. And if you lose pressurization-either gradually or catastrophically-its a real emergency requiring immediate action. But, what are these effects? How can they manifest themselves? How can a pilot proactively prevent or minimize them? And what should we do when we get into an altitude-induced medical problem?

The Environment
The first thing to understand about flying a personal airplane in or near the flight levels is the different environment were in. At sea level, oxygen comprises about 21% of ambient air. As altitude increases, the oxygen concentration doesnt change but the number of oxygen molecules per cubic foot of air does. At 12,000 feet, there are roughly 40% fewer oxygen molecules per breath. Especially when performing a high-concentration activity like flying an airplane, we somehow have to supplement the ambient airs oxygen content.

Rules the FAA has handed down require supplemental oxygen for the flight crew after 30 minutes above 12,500 and continuously when above 14,000 feet. But these rules dont mean altitudes effects dont start at lower levels. They also dont mean we shouldnt be concerned about problems related to oxygen use as well as other concerns brought on by flying at high altitude.

More importantly, it is hard to know who may be affected at altitude. There are no specific factors such as age, sex, or physical condition that correlate with susceptibility to altitude sickness. Some people get it, some people dont, and some people are more susceptible than others.

In addition to there being less oxygen available at altitude, the air is also drier. As a result, the skin and membranes exposed to the dryer ambient air at altitude will tend to dry out. Also, studies show that the amount of moisture lost to respiration can be about double of that at sea level. This means we will get dehydrated much more easily, especially when breathing supplemental oxygen. More on dehydration in a moment.

Another characteristic of higher altitudes is the pressure difference. When at sea level on a standard day, the atmosphere exerts a pressure of 14.7 PSI. But climb to FL200 and the ambient pressure drops to around 6.8 PSI. The lower ambient pressure means you have to work harder to breathe well-your lungs will not absorb the low-pressure air as efficiently. For that reason, pressurization was invented-a turbocharger for your lungs. Because you have to work harder-just like your engine, turbocharged or not-there are implications for fatigue (which is something Aviation Safety will address in more detail in a month or so – Ed.).

Rapid Pressure Changes
Scuba divers know that a rapid ascent from depth can result in the bends. This decompression sickness is caused when nitrogen combines with bodily tissues under pressure and then the body is subjected to reduced pressure over a short period of time. As the pressure is reduced, the excess nitrogen escapes the tissues. If it escapes too fast-the pressure is reduced too rapidly-bubbles form. Large bubbles within tissues and the circulatory system can cause the bends.

Thankfully, few personal aircraft-at least few non-pressurized aircraft-can subject their human cargo to high rates of pressure changes because they simply cant climb that fast. But, a rapid climb to altitude can cause a variant of the bends due to nitrogen bubbling. We really havent thought much about this in non-Scuba divers. Of course, any Scuba instruction should stress that flying to high altitude within 24 hours after a dive can bring on the bends.

What goes up, must come down. While descending back to or near sea level isnt usually associated with a health risk, coming down can bring its own challenges. This can be especially true after being at a relatively high cruise altitude for an extended period in an unpressurized cabin. How many times have you landed after a flight at even a moderate altitude and been unable to equalize the pressure in your ears? In extreme cases-usually brought on by some sort of sinus infection-failure to equalize the ears can cause extreme pain.

Deep vein thrombosis, also called deep venous thrombosis (DVT), mainly affects veins in the lower leg and the thigh and results from sitting, dehydration and inactivity. In DVT, a clot (thrombus) forms in the larger veins of the area. This condition-which can more quickly affect seniors and relatively sedentary individuals before younger, more active ones-saw a lot of attention in the popular media a few years ago as commercial flights grew longer and longer.

There is some theoretical and practical increase in the risk of DVT among pilots flying unpressurized aircraft at high altitudes. Although extensive studies on DVT among this population have not been made, mountain climbers, especially those who have had long stints in tents due to bad weather, have been affected. Dehydration and immobility increase the risk of DVT. Taking one aspirin theoretically would decrease the risk of DVTs as would performing some isometric exercises (see sidebar).

Dehydration is a significant issue, not only for pilots but for anyone who spends time at high altitudes, since humidity can be essentially zero. Plus breathing dry oxygen accelerates the drying of the mouth, lips and sinus passages. Other effects of dehydration range from dry lips and mouth membranes in mild instances to rapid breathing, a rapid, weak pulse and confusion in extreme cases. Symptoms can include fatigue and headaches.

Dry eyes can be another problem brought on by prolonged exposure to high altitudes. This can especially affect those who have had LASIK surgery since LASIK affects the tearing mechanism of the eye.

Combating the effects of dehydration is obvious: Carry liquids-and drink them-when operating at high altitudes for hours at a time. Of course, drinking too much water, especially on a long-endurance flight, can create its own problems, but would you rather have to urinate into a cup or suffer the possibly debilitating effects of dehydration?

Any kind of in-flight emergency at high altitude presents a new challenge-its a long way to the ground and help. But operating at high altitude can create new problems and exacerbate those that would be minor at lower levels. Most significant of these would be any kind of oxygen delivery failure, which could range from exhausting an O2 tank to someone sliding back a seat back and slicing through an oxygen supply hose lying across the track. This would also include removing a mask or cannula to drink and eat.

Perhaps even more likely-especially with the proliferation of wires connected to portable electronics and headsets in the modern cockpit-is a pinched hose. An example could include removing a headset and kinking the supply hose. If it happens at FL190 to a 65-year-old person, they could get pretty wacky pretty quick and stress their coronaries in the bargain.

Abdominal gas also can be a problem, and not just for the others aboard the aircraft. Expanding gas can cause severe pain in the abdomen due to the very sensitive stretch pain receptors. These pain receptors are what can make appendicitis so painful as the appendix stretches.

Anyone with heart or lung problems-whether the FAA knows or not-are exposed to a greater risk if the oxygen system does not perform as it should. At the flight levels, even healthy folks can have problems.

Finally, alcohol and drugs can have increased potency at altitude. In recent years, so-called air rage aboard an airliner has made news. Often these situations merely involved passengers who had too much alcohol at a relatively (for them) high altitude. Even in mountainous areas-especially resorts-alcohol and drugs can have greater potency for tourists-the same thing happens in aircraft. Something as simple as an antihistamine could have serious side effects at altitude.

Using the mid-teens and higher can be the most efficient way to travel. There is less traffic, often less weather, the routings tend to be more direct and having some altitude means more options in the case of a mechanical failure or other kind of non-medical emergency. To help prevent any medical problems, drink liquids, eat, perform isometric exercises and take an aspirin.

As always, it pays to understand the environment youre in, take along the right tools and monitor your health.

Also With This Article
“Top Ten Tips For The Mid-Teens”
“Boutique Oxygen Use?”
“High-Altitude Tools”


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