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Altitude sickness is a reaction to the lower amounts of oxygen available at high altitudes (due to the lower air pressure). Your body will respond in various ways to this: some are normal, some are illnesses. The illnesses are a serious health hazard and can result in death if ignored or left untreated.

[edit] Understand

The further you move away from sea level up into higher altitudes, the lower the air pressure is. The body has two main problems with high altitude and the corresponding lower air pressure:

• Air at lower pressure has less oxygen per lungful. Your body adjusts to this by making more red blood cells to carry oxygen more efficiently. Most of the cell-building happens while you sleep; however, the process can take days and in the meanwhile, you may be ill.
• At lower air pressure, water evaporates faster. This can lead to dehydration.

The changes to your body at altitude are complex and can be quite dramatic. Some changes are normal, some are signs of illness. The difficulty your body has maintaining a good oxygen supply and keeping related problems under control is directly related to how high up you are, and also to recent changes in your altitude. These are the two major factors that cause altitude sickness.

Hence this article talks a lot about ascent and descent. Ascending further away from sea level is the risky activity and the time you must be alert. Conversely, descending towards sea level is the single major factor that helps with moderate to severe altitude sickness.

Altitude sickness is very dangerous for four reasons: its rapid progression; its deadliness; the fact sufferers are usually some distance from medical help and are difficult to evacuate swiftly; and the fact that in many cases sufferers are reliant on their health because they're doing a lot of physical activity in dangerous environments.

[edit] How high is high?

A minority of people, about 20%, have some symptoms of altitude sickness if they ascend to about 2500 meters (8000 feet) above sea level and sleep there. However, most people will acclimatize to 3000 meters (10000 feet) with relative ease, perhaps having symptoms after the first night.

Acclimatizing to heights of 3000 – 5000 meters (10000 – 16000 feet) is much more difficult, and it is here that it is absolutely necessary to ascend slowly and return to a lower altitude to sleep if you have been travelling around at a higher altitude during the day. Over 50% of people will develop AMS (below) if they ascend rapidly from sea level to 3500 meters (11500 feet) without acclimatization, and everyone will if they ascend rapidly to 5000 meters.

It is thought to be impossible to permanently acclimatize to heights above 5500 meters (18000 feet). It's possible to spend several weeks sleeping as high as 6000 meters (19500 feet) once acclimatized, but gradual deterioration of physical well-being will still occur.

Regions above 7500 meters (25000 feet) are referred to as the death zone: you will deteriorate noticeably while you remain at such high altitudes, some of your body's major systems will shut down and climbers will only remain there for two or three days. Death rates from altitude sickness above 7000 meters are estimated at 4% of all people who venture that high.

If your home is significantly above sea level, you gain a definite leg up on ascending to higher elevations, but that doesn't make you immune to altitude problems; it just pushes the threshold for their onset higher. Most otherwise healthy people who live at elevations of 1500 to 2500 meters (5000 to 8000 feet), an elevation range containing quite a few major cities, experience little trouble going to 3000 meters (10000 feet) or a bit higher, but even they will be at risk of altitude problems at 5000 meters (16000 feet).

[edit] Risk factors

The only reasonably reliable test of whether or not you will struggle with altitude or cope with it comparatively easily is previous experience of high altitudes. If you've ascended without sickness before, you're reasonably likely to do it again; if you've had altitude sickness before, you're likely to have it again, although slowing your ascent rate may help.

Physical fitness is not necessarily a good indicator, and neither are strength or good health. You may react badly to altitude despite being fit, young and healthy. In fact, the fit, young and healthy have a hidden risk: their general physical capacity leads them to believe that they should handle altitude just fine, which is not always true. Unless you have previous experience at high altitudes, you should regard yourself as as likely to become ill from it as anyone else.

Bad health, on the other hand, is a risk factor: particularly cardiac or respiratory problems. Healthy hearts and lungs have a hard enough time getting oxygen to your tissues at high altitudes. Naturally, if you have physical problems that make exertion difficult for you, you have reason to think carefully about exertion at high altitude, where it is much harder!

Scuba diving increases risk of decompression sickness. If you have recently been diving and have not fully gotten rid of the nitrogen in your blood, you should not ascend to a higher altitude (or travel in a plane). See scuba diving for recommendations on how long to wait.

[edit] Effects of altitude

Altitude has some physiological effects on all people who are at high altitudes. These effects are not in and of themselves symptoms of illness, although they are signs of the increased difficulty that the body has getting at altitude.

[edit] Hyperventilation

You will naturally breathe faster at higher altitudes to compensate for the lower air pressure. It's possible you won't notice this: a similar effect happens during air travel.

[edit] Altitude diuresis

Your body will try and concentrate your blood to compensate for the lack of oxygen. This will make you urinate a lot at altitude. If you aren't urinating much more than you usually would, then you might actually be dehydrated.

[edit] Periodic breathing

Because of the disruption to oxygen and carbon dioxide levels in your blood due to the body chemistry changes and hyperventilation that occurs at altitude, your body's "when to breathe" chemical signals become confused. While you're awake you will remember to breathe, but when you sleep it is common to have interrupted breathing: holding your breath for up to fifteen seconds and then breathing very rapidly when you start breathing again.

This can be very alarming when you wake up knowing that you weren't breathing or were short of breath; or when you notice someone else has stopped breathing. But it is a normal response to altitude, and happens to almost everyone. Acclimatisation only improves it a little.

[edit] Illnesses at altitude

As well as the less dangerous physiological effects, altitude makes you susceptible to actual illnesses, several of them very dangerous. While not all the effects of altitude can be avoided, you should take sensible steps to avoid actual illness, and take it very seriously if it does occur.

[edit] Dehydration

You need to drink a lot of water at high altitudes. The loss of appetite, a precursor to nausea, can lead you in to a dehydration headache. Unfortunately, it is easy to mistake dehydration headaches for AMS (below) headaches and vice versa. If a headache does not improve after drinking a liter of water it should be considered as an AMS effect. Dehydration headache can also be recognised by comparing pulse rates: if your pulse rate goes up more than 20% when standing up after lying down for five minutes, you need more fluids.

[edit] AMS

Acute mountain sickness (AMS) is the most common unhealthy response to altitude: it's a collection of signs that your body is becoming ill and has not adapted successfully to a higher altitude.

For your own safety, assume any illness at altitude is AMS. The most common reasons that people fail to descend as soon as they should are bad assumptions. They assume that having AMS is a sign of weakness; that their level of fitness means they can't have AMS; or mistake their symptoms for the flu or another illness. Assume AMS first: it happens to healthy strong people, and if it turns out you are indeed sick with something else, descending to a lower altitude will make it easier for your body to heal anyway.

In particular, if you've recently ascended, and you have a headache and any other symptom, you have AMS. The other signs of AMS vary for different people, but include:

• fatigue
• dizziness
• loss of appetite
• nausea or vomiting
• confusion
• difficulty walking (called gait ataxia)
• rattling breath
• feeling generally extremely ill

The last three signs in particular are signs that you are becoming quite ill, but you should not wait for the onset of these symptoms before acknowledging you have AMS: they're fairly reliable indicators of the onset of HACE or HAPE.

You and your party should keep an eye on each other for signs of AMS, and if you have AMS, for signs of it worsening. Very sick people can become confused and not realise how ill they are. Loss of appetite is a particularly good sign: anyone who has been walking or climbing at altitude for a day should be hungry for a good meal in the evening.

If you have symptoms of AMS, do not ascend further. Consider descending.

If you have signs of HACE or HAPE, descend immediately. Your life may depend on it.

[edit] HACE

High altitude cerebral edema (HACE) is the end-stage of AMS (conversely AMS can be thought of as the mild form of HACE). When you have HACE, your brain swells and stops working properly.

HACE symptoms include a number of signs of mental functions failing: confusion, fatigue and weird behaviour. But the most reliable one is gait ataxia, and you can test it by walking heel to toe along a straight line on the ground. Healthy people can pass this test easily, anyone who has difficulty balancing while they do it is showing signs of HACE.

HACE is extremely serious, and you may only have a few hours to help someone with HACE.

[edit] HAPE

High altitude pulmonary edema (HAPE) is another severe altitude illness. It sometimes occurs in conjunction with AMS or HACE, but sometimes doesn't — it's thought to have different causes. When you have HAPE, your lungs fill with fluid. Signs include extreme fatigue; breathlessness (when not due to periodic breathing — give yourself 30 seconds to recover upon waking); a cough, especially if it is wet and has blood in it; rattling or gurgling breath; chest congestion; and blue extremities. A fever is sometimes present. It most commonly sets in at night.

HAPE is another extremely serious illness, and like HACE should be treated as a critical emergency.

[edit] DCS

Decompression sickness (DCS, also known as the bends or caisson disease) is a severe illness in which bubbles of nitrogen forms in your blood, blocking blood supply to parts of your body. Symptoms include persistent tingling or joint pain, fatigue, itching, rashes, confusion and collapse. Decompression sickness is brought on by extremely sudden changes in air pressure (effectively an increase in altitude), such as loss of cabin pressure in a plane you are flying in. Even a fast ascent to most altitudes (such as by plane) would not normally cause decompression sickness. The exception is for anyone who has recently been scuba diving, who should avoid ascents above the altitude that their dive took place at for between 12 and 24 hours depending on dive activity. See the Scuba diving article for more information.

[edit] Prevention

[edit] Keep hydrated

Remember to drink a lot. At 3500 meters (11 500 feet), make sure you drink at least 3 litres of water every day. That means, two large bottles of water. Do not leave anything in the bottom, drink it all, even if it feels bad. If you ascend from there, drink even more. A head-ache is the punishment for those who do not heed this advice, and that feels worse.

[edit] Acclimatize to altitude gradually

Acclimatization is the process of getting your body to adapt to the lower oxygen levels by ascending slowly into higher altitudes, spending some time at each one to adapt. The most important factor is to increase your sleeping elevation (the altitude where you're spending the night) slowly. If you're on a hiking or climbing holiday, a typical strategy is to spend a day (or initially part of a day) at a higher altitude and return to a lower altitude to sleep. This also works for people doing winter sports at high altitudes: ski at the top of the resort and sleep at the bottom.

Here are the recommended maximum increases in your sleeping elevation will stop most people from proceeding to AMS:

• No higher than 2400 meters (8000 feet) the first night
• Increase your sleeping elevation by 300 meters (1000 feet) per night after 3000 meters.
• Every 1000 meters (3000 feet), you should spend a second night at the same altitude. This will be every fourth night if you've been ascending at the maximum pace recommended above.

You can, of course, ascend more gradually than these rates. Many people ascending from sea level choose to spend several nights at 2500–3000 meters (8000–10 000 feet) before beginning acclimatization to higher altitude.

During acclimatisation, drink a lot of non-alcoholic drinks. Some people find vegetarian food slightly accelerates the acclimatisation. Local recipes like mate de coca in Peru (tea from coca leafs) are fine, but its effect is doubtful.

[edit] Avoid rapid ascents

Rapid ascents are the opposite of acclimatization; you make a rapid ascent when you're gaining altitude faster than recommended. This may mean climbing and camping higher than recommended, but you can also make an even more rapid ascent by driving to a high altitude location, and flying from low altitude to high altitude is an even more rapid ascent. For example, flying from sea level to Lhasa Tibet (3650 meters/12 000 feet) is distinctly unwise. Consider spending a week or so at an intermediate altitude; see Overland to Tibet for some possibilities. If you are going to travel around Tibet — where some inhabited areas are over 5000 meters and some mountains over 8000 — do not set out until you are thoroughly acclimatised in Lhasa.

Where possible, avoid ascents more rapid than recommended above, particularly any sudden ascent to 3000 meters (10 000 feet) or higher. Even if you are taking Acetazolamide (below) a rapid ascent makes it more likely you'll get AMS and makes AMS progress to serious illness faster, so you will have less time to respond and descend.

Be particularly wary with oxygen equipment: some tourists have died at altitude when their equipment failed and they were utterly unacclimatized.

Consider road or rail travel rather than flying directly to somewhere with a very high altitude — but remember that the surface option often involves a lot higher altitudes : both the Manali-Leh road and the Beijing-Lhasa train will take you from 2000 meters to 5000 in less than a day. Or fly in stages, stopping somewhere at moderate altitude in between. If you must fly to any destination about 3000 meters (10 000 feet) at least spend a few days at some intermediate destination en route. If flying to a more moderate altitude above 2500 meters (8000 feet), you will still want to spend several nights at that altitude before setting off into higher country.

Refrain from smoking and alcohol when you arrive by plane in a high altitude area from lower altitudes.

[edit] Treatment

As soon as the symptoms of AMS appear, your first priority is recovering. You must not ascend any further until the symptoms have disappeared. This may take up to 48 hours, if it takes longer, descend. You could also descend on the onset of symptoms, this will make them disappear much faster, probably within hours.

If you are getting sicker or showing signs of HACE or HAPE, you must descend to a lower altitude as quickly as possible. If it is nighttime, do not wait for morning if you have a choice at all. You should descend at least as far as you were the last night you had no AMS symptoms. You may need to seek hospital care.

People with HACE and HAPE are frequently confused or exhausted, and are likely to need help with the descent. Help them down!

There is some equipment available to treat people with HACE or HAPE at high altitudes, including hyperbaric bags in which the sufferer can lie in a higher pressure atmosphere. Likewise, because the main cause of these illnesses is a lack of oxygen, breathing oxygen from a tank will slow their onset and may provide some temporary relief of symptoms. Either treatment buys some time if it is too dangerous to descend, but they are not a substitute for descent.

Sufferers of DCS needed to be hospitalised and treated in a recompression chamber: descent to sea level is not sufficient to alleviate DCS symptoms. As with HACE and HAPE, breathing oxygen may provide some temporary relief of symptoms allowing for rescue. Scuba diving organisations can advise further.

[edit] Acetazolamide

This drug (sold as Diamox) stimulates your breathing. It has the effect of increasing acclimatisation rates; improving periodic breathing; and helping people recover from AMS more quickly. There are some side effects but it's useful for people who have had AMS in the past; people on a forced ascent (for example, flying into Tibet); and anyone who has AMS, particularly if they are choosing not to descend. Many climbers also take it preventively.

Acetazolamide is not an absolute preventative measure, particularly in the case of forced ascents.

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