My first exposure to the effect of high altitude was in Nepal severalyears ago. I was on a trek six days' journey from Kathmandu. It is a bizarre and unnerving feeling to discover that your exercise tolerance is suddenly no more than a few slow paces, that your pulse races with each step you take, and that you are obliged to stop to catch your breath every few feet, waiting for the palpitations to subside while local people of all ages - some carrying heavy loads - stop, stare, then overtake. I was a fit young medical student, but my body felt as though it belonged to the victim of some dreadful disease that I had just been studying - chronic bronchitis, perhaps emphysema or asbestosis. I developed a hammering headache, and became more and more breathless, even at rest.

I was lucky, although I didn't know it at the time; these are important warning signs of acute mountain sickness (AMS). I decided to come down. In fact, members of the medical profession have a poor track record when it comes to heeding their own symptoms, and an especially poor record at high altitude. In a report on seven deaths from mountain sickness on Himalayan treks, three of the seven who died were themselves doctors.

The tragic fact about deaths from mountain sickness is that they are preventable in every case. The purpose of this article is therefore threefold: to offer some practical information about AMS, its warning signs and prevention; to discuss the merits of the drugs that are sometimes suggested for its prevention; and to consider other approaches to emergency treatment.

Mountain sickness

The driving force for the absorption of oxygen through the lungs into the bloodstream is atmospheric pressure - the 'weight' of the column of air that extends for ten miles or so above our heads. As we ascend, atmospheric pressure is reduced. Complex mechanisms exist to compensate for the resulting lack of oxygen: these include an increase in breathing rate and depth, as well as changes in the blood and tissues that increase their efficiency in carrying and using oxygen. However, the increased breathing results in reduced levels of carbon dioxide, causing the body to become more alkaline and, in turn, causing numerous other physiological changes to occur, not all of which are clearly understood. The kidneys are able to compensate for changes in alkalinity and acidity, but the process of acclimatisation to high altitude can take several days - longer under conditions of low temperature and increased exercise.

AMS tends to occur within two days of exposure. It usually begins with loss of appetite, headache, nausea, vomiting and sleeplessness. This is the early, benign form. It may simply resolve, but may also progress to a more serious, so-called 'malignant' form. It should be regarded as an important warning.

Malignant AMS can be fatal, and it may begin with little or no warning. Pulmonary oedema develops - a build-up of fluid in the lung tissues - which further interferes with absorption of oxygen, leading to breathlessness that persists even at rest. There is also a cough, with white, pink or frothy sputum, and the lips may turn blue. A build-up of tissue may also occur in the brain - cerebral oedema. This results in headache, drowsiness, impaired co-ordination, abnormal or drunken behaviour, confusion, impaired consciousness and coma. Progression to coma may occur quite rapidly.

Benign AMS can be handled initially by remaining at the same altitude until symptoms resolve. If they do not improve, the best treatment is prompt descent. Victims of malignant AMS need to be brought down immediately, and most sufferers need to be carried down. Experts on AMS advise that descent should not be delayed while aid is summoned, and should start even at night, if necessary.

Mountain sickness is most often a problem at altitudes over 2,600 to 3,000 metres, though in some people it may occur as low as 2,350 metres. This means that a hazard exists at many popular travel destinations. Crucial factors in determining susceptibility to AMS are speed of ascent, and the altitude at which you sleep. If possible, begin by avoiding sleeping above 3,000 metres for the first few nights. 'Climb high, Sleep low', is the rule to follow. Then increase your sleeping altitude by no more than 300 or so metres per day - even this may be too fast for some people to adapt to.

High on drugs

While the most important approach to treatment is descent, there is an increasing trend towards advising trekkers and climbers to consider carrying medication. There are two drugs for the treatment and prevention of AMS, acetazolamide (Diamox) and dexamethasone. A third drug, nifedipine, has shown promising results.

Acetazolamide is a diuretic drug that increases excretion of bicarbonate by the kidney, tending to counteract the increase in alkalinity referred to above. Some experts consider that it speeds acclimatisation, while others believe that it may mask early symptoms that are not a great nuisance in themselves, but that provide useful warning signs that severe AMS may be developing. There is no consensus. I have spoken to doctors who swear by acetazolamide, and to others who are greatly troubled by such side-effects as nausea, tiredness, poor sleep, and 'pins and needles' in the arms and legs. There are many cases on record of malignant AMS occurring despite the use of acetazolamide. It may, nonetheless, provide some worthwhile benefit.

Dexamethasone is a powerful 'steroid' drug that has many actions; the most beneficial of these, as far as high altitude is concerned, is a tendency to reduce oedema. It does not affect acclimatisation, but merely alleviates some of the symptoms. It is safe for most people when taken for only short periods, but serious side-effects do occur, especially in people with diabetes. It may be useful to carry this drug for emergency use in descent.

Portable recompression chambers

The best treatment for a victim of the effects of reduced oxygen pressure is, obviously, to increase the pressure. Bringing the victim down to a lower altitude is usually the fastest and simplest way of doing this. However, a new approach also has its appeal. This is the use of a simple, portable compression chamber. These look like oversized sleeping bags that can simply be inflated with a foot-pump. It has to be pumped continuously, to eliminate waste gases, and this can be tiring at altitude. Alternatively, a carbon dioxide extractor is available for it. A larger model capable of accommodating two people is also available.

Achievable compression is roughly equivalent to a 1,500-metre descent, depending on your altitude. This would certainly buy time in an emergency, though there is no substitute for real descent for people who are seriously ill. It has already been used with great success by expeditions to remote places where rescue is difficult. Its cost makes it suitable for groups and expeditions rather than routine treks.

In one case, however, one victim of AMS died while left unattended overnight in such a chamber. It is very important for people with AMS to be carefully monitored throughout the course of their illness and to be brought down if there is no rapid response to the increased pressure.

Conclusion

The best approach to AMS is prevention, and the most important measure is gradual ascent. Problems are particularly common with people on a tight time schedule, who fly in to high-altitude destinations and try to cram in the maximum amount of sights and activity into the shortest time possible. One simply cannot expect to be able to fly in to such places as La Paz, Cusco or Leh and carry on sightseeing without allowing ample time - perhaps several days - for rest and acclimatisation. Yet there are cases on record where unfit, elderly people have been booked on tours to Peru without any warning about the dangers of high altitude, and have died as a result. Mountain sickness is a preventable illness, and all travellers to high-altitude regions should make sure they are fully informed about it.