How Altitude Sickness Works

By: Tom Scheve
Extreme Sports Image Gallery Altitude sickness can befall even the most intrepid athletes. See more pictures of extreme sports.
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You s­et off ­with a fri­end to do some hiking in the Rocky Mountains. Taking a red-eye out of New Orleans (7 feet/2.13 m below sea level), you arrive in Denver (5,431 feet/1,655.37 m above sea level), rent a car and, by mid-afternoon, you're driving up into the mountains, parking at a trailhead at 11,000 feet (3,352.8 m), hiking and finally setting up camp at 12,000 feet (3,657.6 m) above sea level.

The next day, you feel somewhat hungover. You have a headache and no appetite for breakfast. Your friend, on the other hand, feels great. The two of you continue ascending the mountain, gaining another 2,000 vertical feet (609.6 m) by nightfall.


That evening, you feel dizzy and nauseous and notice your feet are swollen. That night's rest is less than ideal -- you keep waking up with a start, rattled by the awareness that you stopped breathing in your sleep.

The following morning, your companion wants to continue, so you pay little heed to your reservations. While your friend is certainly no slouch, you're normally the faster, stronger and better-winded half of the duo. You can't help but wonder why you're feeling so poorly while your friend seems no worse for the wear.

You push on, but you can't seem to catch your breath -- even when you stop moving. After a labored day of climbing, the two of you camp at 14,000 feet (4,267.2 m). That night, you toss and turn in agony and feel like you're being suffocated. The following morning, you have a bad cough and tightness in your chest. By now, it's clear you have some sort of altitude sickness. Why did you -- and not your friend -- become ill? Was it preventable? Could you die?

Altitude sickness can strike suddenly and unpredictably. And it's not just for the mountain climbers among us. Any rapid change in altitude can cause the onset of sickness. While most of our knowledge about altitude sickness is geared toward climbing (because the activity is based on rapid, steady rates of ascension) even flying or driving to higher altitudes can produce symptoms. For instance, flying into Peru at the airport in Cusco (elevation 11,000 feet above sea level) can cause you to feel the common effects of high altitude that day, but you'd naturally acclimate by staying at that elevation.

The sickness is largely a traveler's disease -- locals are acclimated to the elevation.

So how does altitude affect our bodies? We'll find out in the next section.


Physiological Effects of High Altitude

HowStuffWorks 2008

What's high altitude? It's considered to be between 5,000 and 11,500 feet (1,524 and 3,505.2 m) above sea level. Very high altitude is any altitude between 11,500 and 18,000 feet (5,486.4 m), and extreme altitude is anything above 18,000 feet.

Atmospheric pressure (which is measured with a barometer and is also known as barometric pressure) is a measurement of air's force against a surface. At low elevations, the pressure is greater, since the molecules of air are compressed from the weight of the air above them. However, at higher elevations, there's less pressure and the molecules are more dispersed.


The percentage of oxygen in the air at sea level is the same at high altitudes -- roughly 21 percent. But because the air molecules are more dispersed, each breath delivers less oxygen to the body. When you take a breath at 12,000 feet (3,657.6 m), you're breathing in 40 percent less oxygen than at sea level. At 18,000, feet you're taking in 50 percent less oxygen. So during and shortly after physical activities (like mountain climbing or hiking), you should expect to feel short of breath at higher altitudes [source: Curtis].

At higher altitudes, our bodies make adjustments: creating more red blood cells to carry oxygen through the bloodstream, pushing air into normally unused portions of the lungs and producing citrate synthase, a special enzyme that helps the oxygen found in hemoglobin make its way into body tissue [source: humidity and low air pressure at high altitudes causes moisture from your skin and lungs to evaporate at a faster pace -- and your body's increased exertion requires even more water to keep it hydrated.

When your body loses fluid through its blood vessels, it tries to counteract the effect by holding water and sodium in its kidneys. As a result, even more fluid builds up in your body -- and more trickles out of your blood vessels [source: women. There's no clear reason why, but researchers suspect it has something to do with their hormones and menstrual cycles. The condition isn't always an indicator of altitude sickness (it can occur from malnutrition and other illnesses at low altitudes), so it can be discounted as a precursor of altitude sickness if no other symptoms are present. While swelling may worsen with ascent, it usually resolves after descent.

As your sleeping body attempts to strike some balance between oxygen and carbon dioxide levels, it falls into periodic breathing. Periodic breathing is a cycle of decreased breathing, followed by a complete absence of breathing (from three to 15 seconds). Breathing resumes once carbon dioxide has sufficiently built up in your bloodstream to prompt your brain. Periodic breathing is fairly common at higher altitudes and is not itself a sign of altitude sickness, but it often leaves afflicted individuals feeling worn out upon waking.

If these are the basic effects high altitude has on your body, what are symptoms of altitude sickness? Next, we'll learn about Acute Mountain Sickness and why a hotdog-eating champion is no more susceptible to it than a marathon runner.


Acute Mountain Sickness

A Tibetan villager plays pool in the outskirts of Basu County of Tibet Autonomous Region, China. The Tibetans are accustomed to the high altitude of their region, though many visitors are afflicted with high altitude sickness.
China Photos/Getty Images

Acute Mountain Sickness (AMS) is the most common manifestation of altitude sickness. AMS doesn't normally occur below 8,000 feet (2,438.4 m)

-- but you don't have to climb a mountain to feel its effects. In the United States, you might get symptoms of AMS in Vail, which sits at a base of 8,120 feet (2,474.98 m). In the Nepalese valleys near Mount Everest, elevation ranges from lower altitudes around 6,500 to 10,000 feet (1,981.2 to 3,048 m) to higher altitudes of 11,000 and 13,000 feet (3,357.8 and 3,962.4 m). La Paz, Bolivia, sits at 11,975 feet (3,649.8 m). Visitors to these sites -- even those who arrive by ways other than foot -- can feel the effects of altitude sickness.


AMS is an altitude-related neurological disorder, so it's nothing to take lightly. Though the causes of AMS are still not entirely understood, genetics seem to play a part. An out-of-shape person may not be vulnerable to AMS, while an Olympic athlete may find himself or herself in a life-or-death medical emergency. People with AMS sometimes attribute headache or nausea to some other cause and continue ascending -- and getting sicker. This misdiagnosis can be fatal.

There are three forms of AMS: mild, moderate and severe.

Most people who ascend to altitudes of 10,000 feet (3,048 m) or more will have at least some symptoms of mild AMS. These include headache, nausea, weakness and decreased appetite. The onset of mild AMS generally occurs within 12 to 24 hours of reaching a higher altitude and will normally disappear within three days. While there are medications that can treat the symptoms of AMS, they can't cure it. The only way to cure AMS is resting and allowing your body to acclimatize -- adjust to the change in altitude -- or descending to a lower altitude. Most people afflicted with AMS are ambulatory; only in severe cases will they need to be carried or otherwise transported down.

If your symptoms become worse, you may have progressed to moderate AMS. One alarming symptom of moderate AMS is ataxia, the loss of coordination. The hallmark of ataxia is the "drunk man's walk," or the inability to walk in a straight line.

If you suspect that you have moderate AMS, you should immediately descend. Acclimatization and medications won't help you at this stage. Even a few hundred feet of descent should quell symptoms, but the lower, the better. In one to three days, the symptoms may disappear entirely. At this point, you can return to a higher elevation -- but you must properly acclimate along your ascent.

If you or a companion has a severe loss of coordination, trouble thinking clearly and fluid buildup in the lungs, these are all signs of severe AMS. The afflicted party should immediately be taken to low altitude, preferably under 4,000 feet (1,219.2 m). Severe AMS can be deadly. But even severe AMS isn't as dangerous as the next condition we'll learn about.


High Altitude Pulmonary Edema (HAPE)

Pakistani Army soldiers walk above their military outpost at the Siachen Glacier -- 18,655 feet (5,686.04 meters) above sea level. Soldiers generally spend 90 days at a high altitude base before descending.
Robert Nickelsberg/Getty Images

In the last section, we discussed Acute Mountain Sickness, the first form of altitude sickness. Now we'll learn about the second, more serious form of altitude sickness: High Altitude Pulmonary Edema (HAPE).

HAPE is caused by excessive fluid in the lungs. For some people, the lack of oxygen at high altitudes can cause blood vessels to constrict. The body responds by forcing blood through unrestricted vessels, which results in high blood pressure and blood vessel leakage. This leaked fluid then builds up in the lungs, interfering with the normal exchange of oxygen. This results in further oxygen loss in the already-depleted bloodstream.


Because of some similar symptoms (such as fluid in the lungs and difficulty breathing), HAPE is sometimes misdiagnosed as pneumonia. This can be a fatal mistake. A person afflicted with HAPE may take a sudden turn for the worse, fall into a coma and die within a few hours. HAPE usually manifests at night, and many climbers have been found comatose in the morning by their companions.

HAPE symptoms include:

  • Trouble breathing when at rest
  • Persistent cough
  • Weakness
  • Feeling of tightness in the chest

If two or more symptoms are present, it's best to immediately descend and get medical attention [source: Shlim]. You should be aware of other signs of HAPE -- you may have a fever, make a "gurgling" sound while breathing or find that lying flat on the ground is extremely uncomfortable.

With advanced symptoms, you probably won't have the strength to descend on your own -- you'll need help. This is just as well, since HAPE can result in high blood pressure that only worsens with physical exertion. As with severe Acute Mountain Sickness, the most important thing is to descend as soon as possible. Even after medical treatment, there is still a 10 to 15 percent chance that HAPE will be fatal [source: Dietz].

Stop before you reach the summit! If you thought blood vessels leaking into your lungs were bad, wait until you read about blood vessels leaking into your brain.


High Altitude Cerebral Edema (HACE)

Ascending to great heights can give you an exhilarating feeling -- or a very ill one.
©iStockphoto/Bernhard Weber

The third and most serious form of altitude sickness is High Altitude Cerebral Edema (HACE). Acute Mountain Sickness is actually a mild case of HACE.

Much like High Altitude Pulmonary Edema (HAPE), HACE occurs when a combination of higher altitudes and the resulting lower air pressure causes fluid to leak from blood vessels [source: Curtis]. But instead of the fluid collecting in the lungs, as it does with HAPE, it collects in the brain. While the brain is naturally surrounded by cranial fluid, HACE introduces fluid inside the brain. CT scans of HACE patients have shown fluid in pockets of their brains. HACE is a very dangerous condition and needs immediate medical attention. A person with this condition could become comatose or die if not soon treated.


The most obvious symptom of HACE is ataxia, which we've already discussed. To determine if someone has this symptom, you can test his or her coordination. Stretch a rope along the ground or draw a line in the dirt or snow and ask him or her to walk in a straight line beside it. Someone with ataxia will be unable to walk a straight line. (Interestingly, HACE doesn't affect the performance of a finger-to-nose test.) As we learned earlier, loss of coordination is also a sign of severe Acute Mountain Sickness. A mild amount of trouble walking or moving could very well be severe AMS, but it's safest to assume HACE, especially if other symptoms -- like difficulty thinking -- are present.

A person with HACE should immediately descend at least to the last elevation where he or she woke up symptom-free. A person can die from HACE in as little as six to 24 hours after showing signs of ataxia [source: CDC].

Medical attention should be sought immediately. Once a person with HACE is taken to a safe, lower elevation, his or her mental confusion usually clears up quickly. It may, however, take up to several weeks for physical coordination to be fully restored. Lasting neurological damage is rare but can occur. This damage is specific to the patient and the severity of the case, but possible effects are impaired memory, motor skills and cognitive functioning.

Once fully recovered at a lower elevation, a person can safely reascend to (and beyond) the elevation that previously caused him or her so much trouble, so long as he or she properly acclimates on the trip.

If you're suffering from altitude sickness and have the choice of being stuffed into a pressurized bag or consuming ginkgo, what should you do? Find out in the next section.


Preventative Treatment

A persistent headache can be a warning sign of altitude sickness.
©iStockphoto/Diane Diederich

There are some steps you can take to prevent or diminish some of the normal effects of high altitude, as well as more serious conditions, such as AMS, HAPE and HACE.

The best rule of thumb in preventative treatment? If you begin having symptoms, stay put until they disappear. If symptoms get worse, descend. Don't make any sudden changes in altitude -- someone who races up to an elevation of 15,000 feet (4,572 m) will be worse for the wear than someone who slowly acclimates himself or herself to 20,000 feet (6,096 m). If you need to stay at a given elevation, it's better to engage in some light activity than it is to sleep. Generally, your symptoms will worsen as you sleep.


The most important factor in determining your likelihood for altitude sickness is acclimatization. To become acclimated, sleep one night below 10,000 feet (3,048 m) after you begin your ascent. Above 10,000 feet, don't sleep more than 1,000 to 1,500 feet (304.8 to 457.2 m) higher than the elevation at which you previously slept. For every 3,000 feet (914.4 m) gained, sleep two consecutive nights at that altitude. The climbing maxim "climb high, sleep low" refers to the practice of ascending during the day to new heights and then descending at night to sleep. Dehydration is common at high altitudes and can prevent acclimatization. Drink plenty of water before and during your ascent.

There are some drugs you can take to prevent altitude sickness -- and a few that can alleviate mild to moderate symptoms that onset as you ascend.

  • Ibuprofen helps alleviate the pain of normal altitude-related headaches, but it will not affect the underlying causes of the headache. If you treat a headache but it gets worse, descend immediately.
  • Diamox can increase your rate of breathing, leading to increased oxygenation of your blood. It also diminishes symptoms -- especially when you're sleeping. If you're taking it preventively, you should take it a full day before your ascent to allow your body to begin processing it. Like all medications, it may cause side effects and allergic reactions, so you should try it first at low altitude to see if it affects you adversely. In most cases, a careful and cautious rate of ascent will not necessitate the use of Diamox.
  • Acetazolamide or low doses of oral furosemide treats edema in swollen faces, feet and legs [source: Dietz].
  • And speaking of drugs, don't ingest alcohol or any other drug that may alter or decrease your rate of breathing.

Some people are advised to avoid high altitudes altogether. If you have heart disease, lung disease or sickle cell anemia, it's recommended that you not ascend to high altitudes. Diabetics can safely go to high altitudes but must be especially vigilant in monitoring their blood glucose levels -- glucose monitors may be adversely affected by high altitude, and altitude sickness may trigger ketoacidosis (a serious condition caused by a lack of insulin). The CDC recommends that pregnant women shouldn't attempt to go higher than 12,000 feet (3,657.6 m). Researchers don't know whether the effects of altitude sickness can afflict fetuses, but there are concerns that blood vessel leakage could harm them.

If preventative measures fail, you'll need to know how to treat altitude sickness. We'll take a look at some treatments in the next section.


Advanced Treatment

Athlete Brent Sherwin is treated inside a hyperbaric during training at Belmore Sports Ground, Sydney, Australia, in 2001. Portable hyperbaric chambers or gamow bags can be used to treat altitude sickness.
Chris McGrath/ALLSPORT/Getty Images

High altitude can wreak havoc on our bodies -- and you can only do so much to prevent it. Once you've got altitude sickness, you've got to treat it. Here's how.

While medications can alleviate symptoms of mild or moderate Acute Mountain Sickness (such as headaches), they aren't cures. Descent or acclimatization is the only cure for AMS. Pure oxygen reduces the effects of altitude sickness, though many climbers report that ginkgo biloba prevents or alleviates symptoms of altitude sickness. Ginkgo has proven an ineffective treatment in recent tests, however [source: Dietz].


There are three important things to remember about altitude sickness:

  1. If you have symptoms of altitude sickness -- even if you're the only member of the party feeling afflicted -- you should assume that you are, indeed, sick.
  2. Never ascend to a new altitude to sleep if you have symptoms (remember: "climb high, sleep low").
  3. If you remain at the same altitude and your symptoms worsen, descend as soon as possible.

Anyone with High Altitude Pulmonary Edema should immediately descend. If the case is severe, he or she may need to be carried down or airlifted. If immediate descent isn't possible (due to inclement weather, for instance), there are some options to help treat the condition. You can use pressurized oxygen or a portable hyperbaric chamber to help restore breathing. Someone afflicted with HAPE should rest as much as possible. Since lying flat will cause discomfort, you should elevate one end of his or her sleeping bag or bedroll.

Some drugs that may help are nifedipine and frusemide. Nifedipine is a blood pressure medication that lessens the pressure in the pulmonary artery that causes HAPE, thus improving oxygenation. Its use will result in a lower blood pressure, so one should keep this in mind and not stand up too quickly after it has been administered. It has been used to prevent HAPE in those who have a previous history with this condition [source: Rees].

Frusemide may help remove excess fluid from the lungs, but it can have serious side effects in those who are dehydrated.

Like HAPE, a person with High Altitude Cerebral Edema needs to descend immediately and seek medical help. Alternate courses of action can be taken to alleviate the conditions and prevent HACE from worsening.

The afflicted party can take Dexamethasone, which reverses swelling, including brain swelling. Though it can be used in a preventative manner, Dexamethasone is a powerful drug that may cause serious side effects. But in extreme conditions, it can save the life of someone suffering from HACE -- especially if that person has to stay put through the night before descending. It is reputed to revive people who have fallen into a coma from HACE. You can also provide oxygen to the HACE patient and put him or her into a portable hyperbaric chamber.


Lots More Information

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