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Altitude sickness

NHS Direct Online Health Encyclopaedia

Introduction Altitude sickness (also known as acute mountain sickness) can occur in its mildest form at around 2500m (8000ft) above sea level (common ski resort height), but is more likely to occur, and to be more severe, at higher altitudes of 2500m and above. As you go higher the air still contains the normal amount of oxygen (21%), but atmospheric pressure decreases which results in each breath containing fewer molecules of oxygen. For example, at 5500m (18000ft), each breath will contain roughly half the normal amount of oxygen. The onset and severity of symptoms, and the altitude at which they are experienced, vary according to the individual, the rate of ascent, and the amount of time spent at high altitude. In most cases symptoms develop 24-36 hours after arrival at altitude, and begin to ease within 48 hours as the body acclimatises (gets used to it).

Symptoms Symptoms of mild altitude sickness are: headache, tiredness, feeling unsteady, feeling sick and vomiting, racing heartbeat, laboured breathing, loss of appetite, shortness of breath during exertion, breathing faster or deeper, difficulty sleeping and irregular breathing during sleep, and urinating frequently. The following symptoms indicate a medical emergency and anyone experiencing them should descend to a lower altitude as soon as possible: worsening of the mild symptoms given above, persistent irritable cough, breathlessness, bubbling sound in the chest, coughing up pink frothy liquid (sputum) - indicating fluid in the lungs, difficulty in walking and clumsy movements, irrational behaviour, double vision, convulsions, drowsiness, or confusion indicating swelling of the brain, or fluid in the brain. Other symptoms may include swelling of the face, arms or legs (known as peripheral oedema) which usually lasts a few days and disappears. Tiny blood blisters at the back of the eyes (retinal haemorrhages) are also common above 5000m (16500ft) but are rarely noticed and only occasionally interfere with vision.

Causes The main cause is going too high, too quickly. The condition is caused by the bodys response to a reduction in oxygen supply to the muscles and brain, which in turn requires the heart and lungs to work harder causing breathing and pulse rates to increase. Red blood cells, which carry oxygen around the body, are made in greater numbers. However, the bodys efforts to cope with low oxygen also results in a number of changes to blood acidity, lung pressure, electrolytes, fluid and salt balance. In a small number of cases, typically at above 3500m (11000ft), two potentially life-threatening complications occur: fluid accumulates in the lungs (high altitude pulmonary oedema) or in the brain (high altitude cerebral oedema).

Diagnosis Diagnosis is based on headache, combined with one or more other symptoms (presuming there is no other likely cause and an ascent to at least 2500m).


Blood Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.

Heart The heart is a muscular organ that pumps blood around the body.

Brain The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.

Lung Lungs are a pair of organs in the chest that control breathing. They remove carbon dioxide from the blood and replace it with oxygen.

Oxygen Oxygen is an odourless, colourless gas that makes up about 20% of the air we breathe.

Red cell Red blood cells transport oxygen around the body and remove carbon dioxide.

Acute Acute means occuring suddenly or over a short period of time.

Haemorrhage To haemorrhage means to bleed or lose blood.

Drowsiness Drowsiness is when someone feels extremely tired and uncontrollably near to sleep.

Double vision Double vision is to see two images of a single object instead of one.

Loss of appetite Loss of appetite is when you do not feel hungry or want to eat.

Inflammation Inflammation is the body's response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.

Vomit Vomiting is when you bring up the contents of your stomach through your mouth.

Onset The onset is the beginning or early stages of a condition or disease.

© Queen's Printer and Controller of HMSO, 2005

Crown copyright material is reproduced with the permission of the controller of HMSO and the Queens Printer for Scotland.



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