He was telling me that he had just come back from ferrying an airplane across the country, cruising at 10,500 MSL, and suffered a severe case of altitude sickness. I hadn’t given it much thought before; I also fly a non-pressurized airplane and have been up to the service ceiling—FL 210—a bunch of times (on O2 of course) and didn’t have any problems. It’s just not something I had commonly associated with flying, and most of the information in the medical literature on the topic is about hiking and mountain climbing, not aviation. But it can be an important topic for us in GA, and there are some things we can learn from our land-based friends.
The disease is known by the catchall phrase “High Altitude Illness” (HAI). It’s also called “Acute Mountain Sickness” (AMS), since it’s been known for a long time, back when men were climbing mountains, not flying airplanes. As far back as two thousand years ago, a Chinese official warned of the dangers of crossing over the Himalayas from China into what probably is now Nepal. Travelers, he said, would have to cross “The Great Headache Mountain” where “men’s bodies become feverish, they lose color and are attacked with headache and vomiting.” One of my favorite adventure authors is Jon Krakauer, who wrote, among a couple of great books, “Into Thin Air,” about his climb to the summit of Mt. Everest. His description of the symptoms of altitude sickness is right out of a medical textbook and even more graphic than that of our ancient Chinese observer. “Straddling the top of the world, one foot in China and the other in Nepal, I cleared the ice from my oxygen mask, hunched a shoulder against the wind, and stared absently down at the vastness of Tibet. I understood on some dim, detached level that the sweep of the earth beneath my feet was a spectacular sight. I hadn’t slept in 57 hours. I was so nauseous that the only food I’d been able to force down over the preceding three days was a bowl of ramen soup and a handful of peanut M&M’s. Weeks of violent coughing had left me with two separated ribs that made ordinary breathing an excruciating trial. At 29,028 feet up in the troposphere, so little oxygen was reaching my brain that my mental capacity was that of a slow child. Under the circumstances, I was unable to feel much of anything except cold and tired.” None of this sounds good, especially if it affects a pilot in the air, and it’s also the reason I’m not headed up to Mt. Everest any time soon.
The possibility of getting altitude sickness starts at about 8,000 ft MSL, but if you’re inclined to get symptoms, it can occur as low as 6,500 MSL. You’re especially susceptible if you’ve ever had it before. Commercial airline travelers aren’t at too much risk since the FAA mandates that carriers maintain cabin altitude pressure less than 8,000 ft. If you spend enough time over 8,000 feet the chance of altitude sickness approaches 50%, particularly if you’re not acclimated to being at higher altitudes. It can be downright dangerous, and if not appropriately treated it can be life-threatening to pilots, passengers, and mountaineers. The Journal of Travel Medicine had an article about flights to a small airport near Mt. Everest called Shyangboche—“SYH” is the identifier to program into your GPS for those of you headed that way. The airport elevation is 12,270 ft and in recent years has opened up to regular helicopter and fixed-wing flights for trekkers and tourists to service a hotel up there with flights directly from Kathmandu (4,264 ft). That means there’s a rapid ascent of 8,000 ft to the airport. That quick climb to altitude is an important factor in developing HAI. The authors report that 84% of those who flew directly to the airport developed at least some of the symptoms of HAI. The rapid ascent is one of the key factors in HAI but not the only thing, since they report that 61% of those who hiked up to the hotel over a few days also developed symptoms.
There are several other triggers in developing HAI besides the quick climb into “thin air.” Two other central elements are the decreasing pressure of the air as altitude increases (barometric pressure) and the corresponding decreases in the amount of oxygen and water vapor in the atmosphere (density altitude). We’ll unpack that a little more, but to start, those three factors lead to a response in your body that pushes fluid out of the bloodstream into body tissues where it shouldn’t be. The fancy word for this is “edema,” another one of those Latin words that means “swelling.” The fluid collecting in the organ tissues causes the “swelling,” and that’s never a good thing since soggy organs don’t function properly. The two places fluid builds up causing the dangerous symptoms of HAI are in the brain and the lungs. The medical vocabulary for this is high altitude cerebral (brain) edema (HACE) and high altitude pulmonary (lung) edema (HAPE). The fluid accumulation leads to disabling effects on breathing and oxygenation, cognition, situational awareness, problem solving, reflexes, reaction time, vision, GI function, and, in the end, loss of consciousness altogether. Headache is one of the most common symptoms of HAI and can be similar to a migraine headache that’s very incapacitating. Any or all of these symptoms can come on without any warning and if not quickly treated can lead to serious consequences.
Although the percent of oxygen in each gulp of air doesn’t change, the amount of O2 in each breath does due to the lower atmospheric pressure. This means there’s a lower O2 concentration in the blood called “hypoxia“ and that’s the trigger for a chain of compensatory mechanisms your body uses to try and get more oxygen to the tissues. It’s a complex process and the side effects end up causing that fluid build-up. The first reaction is to increase breathing rate and depth (called “minute ventilation”). Also, the arteries in your lungs (pulmonary arteries) constrict to redirect blood flow to areas in the lungs containing the highest oxygen content, something with a lot of syllables called “hypoxic pulmonary vasoconstriction.” There are two downsides to pulmonary vasoconstriction since it leads to higher blood pressure in the lungs (pulmonary hypertension). The higher pressure in the lung’s blood vessels pushes fluid from the bloodstream into the fragile little air sacs (alveoli) where air is supposed to be. Fluid in these tiny sacs is very irritating and can lead to the persistent cough Krakauer describes. The combination of fluid plugging up the air sacs so they can’t get the little bit of oxygen in the “thin air” into the bloodstream and high blood pressure in the lungs is that “high altitude pulmonary edema.” Oxygen levels in the blood plummet and that triggers more body reactions and the symptoms our ancient Chinese traveler and Krakauer describe.
Fluid collecting in the brain, high altitude cerebral edema (HACE), is also a severe and potentially fatal condition associated with HAI. It accounts for the headache, cognitive dysfunction, insomnia, fatigue, nausea, and altered mental status described with HAI. Full-blown HACE is the least common form of altitude illness, but the most critical, and must be managed quickly as it can progress to coma and death within 24 hours. The underlying mechanism of HACE is the same as in the lungs and is due to fluid build-up in the brain as it tries to compensate for low O2 levels. The special issue with fluid build-up in the brain is that the brain lives in a tight, non-stretchable box—the skull—so there’s no extra room for swelling. It’s known as the “tight fit” hypothesis, and as a result of the swelling, pressure inside the skull (“intracranial pressure” or ICP) rises above the pressure of the blood (BP) that’s trying to get blood into your head. It’s the balance of these competing pressures that results in diminishing blood flow to the brain and eventually no blood flow at all. With no blood flow there’s no oxygen delivery and the brain quickly stops functioning altogether.
There are aspects of GA operations that can help us avoid HAI but also some things working against us. Although we climb high and fast, something we brag about at lunchtime with fellow aviators, we don’t stay at altitude very long. It takes, on average, about 24 hours for the swelling and all its side effects to set in, but some symptoms of HAI can start as soon as 10 hours after getting to a high altitude. My friend had spent most of two days over the course of his flights at high altitude. We also use supplemental O2, but as Jon Krakauer described, that doesn’t always help. There is a protective benefit from being acclimated to altitude so the more time you travel up high the less the risk. There are other ways to avoid HAI altogether and some medications to pretreat aviators and trekkers who have had high altitude symptoms in the past or want to take preventative measures. Stay tuned, stay away from Mt. Everest for now, and next month we’ll go into more detail.
