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Scaling the heights: an altitude problem

Carolyn Driver
RGN RM RHV FPCert MSc(TravelMed)
Independent Travel Health Specialist

In medical terms, "high altitude" is defined as an altitude that causes physiological changes and a reduction of arterial oxygen saturation to less than 90%. Generally this will occur at or above 2,500m. Extremely sensitive individuals may experience problems between 1,500m and 2,500m as some slight physiological changes will be occurring despite O(2) saturation remaining above 90%. Altitudes between 3,500m and 5,800m are described as "very high", and those above 5,800m are "extreme".(1)
The physiological effects of altitude
As altitude increases and the barometric pressure falls, hypoxia (oxygen deficiency) occurs. The body compensates for this by increasing ventilation with more rapid respirations and an increase in tidal volume. This higher respiratory rate will cause an alkalosis (an increase in blood alkalinity), which then slows the increased ventilatory effort until renal compensation occurs by excretion of bicarbonate. The circulatory system also responds by increasing the heart rate, and haemoconcentration enhances the oxygen-carrying capacity of the blood. Initially this haemoconcentration is due to reduced plasma volume, but over time it is a result of increased red cell production stimulated by erythropoetin.(2)
Acclimatisation is the process by which the body makes these adjustments, and this can take up to four days.

Acute mountain sickness
It is not possible to predict who will experience symptoms at high altitude, and there is no obvious relationship between susceptibility and age or physical fitness. It is estimated that 25% of all tourists to moderate altitude (AMS is a collection of symptoms that appear typically 6-12 hours after arrival at high altitude. They generally last between one and three days if no further ascent occurs. Occasionally the onset of symptoms may be delayed for one to two days after arrival at altitude. Rate of ascent is the single most important risk factor, and in particular the altitude at which the individual sleeps.(1)
The main symptoms of AMS are headache, nausea, vomiting, anorexia, fatigue, dizziness and sleep disturbance. Individuals may not necessarily suffer from them all; however, typically a sufferer will have a headache, be off their food and have difficulty sleeping. The headache is usually frontal and throbbing, is worse during the night and is aggravated by stooping.

Prevention and treatment of AMS
The best way to prevent AMS is by gradual ascent, especially once an altitude of 3,000m has been achieved. Above this level, sleeping altitude should be no more than 300m higher than the previous night, with a rest day every 1,000m. It can be helpful to spend at least one night at intermediate altitude (1,500-2,500m) before ascending to higher levels. Allowing adequate time for acclimatisation before taking on strenuous activity is very important - all the more so for those who have flown directly to altitudes above 2,500m. Avoiding alcohol for the first two days above 2,500m and drinking plenty of water is advisable to assist acclimatisation. Travellers need to be familiar with the symptoms of AMS and advised that should they experience these they should not ascend any further until they resolve, and should descend if there is no improvement or symptoms worsen. Painkillers can be taken for the headache. A flexible approach to their itinerary is also important, so that full acclimatisation can occur before attempts are made to travel to more than 3,000m.

Acetazolamide (Diamox; Goldshield) has been used successfully to speed up acclimatisation and is useful for those who have to ascend quickly or who are known to be especially susceptible to AMS. It helps by speeding up the excretion of bicarbonate and assisting the return to normal pH. Doses of 125mg twice daily have been shown to be effective and should be started at least one day before ascent to 2,500m, and continued until maximum altitude has been reached or acclimatisation has occurred. There can be side-effects such as paraesthesia and mild diuresis, but it is generally well tolerated. Individuals who are allergic to sulphonamides should not use this drug.(4) Prevention of AMS, although well researched, is an unlicensed use of the drug in the UK. Travellers should also be warned that it is not a substitute for proper acclimatisation.

Potentially life-threatening complications of altitude sickness
While AMS in itself is not life-threatening, the reason why travellers should be aware of its importance is because of the possibility of it progressing to high- altitude cerebral oedema (HACE) or high-altitude pulmonary oedema (HAPE). These are life-threatening conditions and require prompt response in order to prevent a fatal outcome.
HACE is thought to occur in approximately 1-2% of individuals who ascend above 4,500m but can occur at altitudes of 3,500m, and although rare has been reported as low as 2,500m. Symptoms and signs of AMS progressing to HACE include: confusion, disorientation, ataxia (defective muscular coordination), clumsiness, and lethargy that can rapidly lead to coma. Hallucinations have also been reported, and behavioural changes such as becoming unusually quiet or especially aggressive. Ataxia is one of the most important early signs, and its presence should be taken seriously and HACE considered as the most likely cause unless there is an obvious alternative cause. Symptoms of AMS/HACE can resemble inebriation, hypoglycaemia or adverse reactions to antimalarial drugs such as mefloquine. It is important that symptoms are not merely attributed to these other situations. Heel-toe walking (placing one foot directly in front of the other) is an important diagnostic test.
Immediate descent is the most important action in cases of HACE and, if available, dexamethasone may be administered (8mg immediately followed by 4mg six hourly). If oxygen is available it should be administered. Larger expeditions may carry portable hyperbaric oxygen chambers ("Gamow bags"), but all of these interventions should be carried out only during descent and should not prevent the victim being taken down to lower altitude.
HAPE can occur at altitudes over 2,500m; in fact, 10% of those who ascend rapidly to 4,500m are likely to develop it.(1) The incidence in those who ascend at standard rates is around 1-2%. It is possible that there is increased susceptibility to this condition if respiratory infections occur during or immediately before ascent.(6)
Although HAPE may be preceded by AMS, it can also occur alone. Early symptoms are dyspnoea on exertion and reduced tolerance to exercise over and above what would be expected for that altitude. Recovery times will be prolonged, and there is progression to breathlessness at rest, especially at night. A dry cough, which is common at altitude, may progress to a bubbly productive cough with bloodstained sputum.
The condition can deteriorate rapidly and, as with HACE, rapid descent is the most important form of treatment. Oxygen and drugs such as nifedipine have been shown to improve symptoms.
Prevention of HAPE is the same as for AMS, but those who have experienced the condition before need to ascend with great caution as they seem to be especially susceptible to recurrence.

General advice for travel to high altitude
In addition to advice about the potential for AMS, travellers to altitudes much greater than those they are normally used to need to be reminded of other potential hazards:

  • Temperature can vary enormously in high-altitude environments, with extreme heat during the day followed by a rapid drop in temperature to below zero after sunset. There is also an increased risk from ultraviolet radiation as there is less atmosphere to absorb the harmful rays. There may also be snow present that will reflect the rays, thus increasing their harmful effect.
  • Appropriate clothing for coping with the extremes of temperature and high-factor sunscreens or preferably sunblock should be used appropriately. A hat and adequate sunglasses or goggles are also very important. Footwear should be broken in before the trip to reduce the likelihood of blisters.
  • Maintaining adequate hydration is important in order to prevent heat exhaustion, and the trekker should ensure they have a personal water supply at all times.
  • Travellers should only ever tackle high-altitude treks with local guides and should ensure that their insurance covers them for their activities and includes full medical repatriation.
  • Travellers should also consider their environment while on such trips and give careful thought to all waste disposal - which may require them to carry their own rubbish with them until somewhere suitable is found to dispose of it.

Travel to high-altitude destinations is achievable for most travellers, and they should have a thrilling experience on their trip. However, they do need to be thoroughly prepared and have realistic itineraries in order to prevent their trip being marred by AMS or worse.



  1. Murdoch D, Pollard A. The high altitude medicine handbook. 3rd ed. Oxford: Radcliffe Medical Press; 2003.
  2. Dietz TE, Hackett PH. Altitude. In: Keystone JS, et al, editors. Travel medicine. Philadelphia: Mosby; 2004.
  3. Honigman B, Thesis MK, Kpoziol-McLain J. Acute mountain sickness: in a general tourist population at moderate altitude. Ann Intern Med 1993;118:587-92.
  4. Barry PW, Pollard AJ. Altitude illness. BMJ 2003;326:915-19.
  5. Hauser M, Mueller A, Swai B, Moshi E, Nguyaine OS. Deaths due to high altitude illness among tourists climbing Mount Kilimanjaro. Transactions of the Africa European Conference on Travel Medicine; Cape Town; February 2004.
  6. Rashid H, Hashmi SN, Hussain T. Risk factors in high altitude pulmonary oedema. J Coll Physicians Surgeons Pakistan 2005;15(2):96-9.

British Mountaineering Council
Mountain medicine centre

International Society of Mountain Medicine
Provides good tutorial aimed at the public on the effects of altitude

Tourism Concern
Charitable organisation aiming to improve the effect of tourism on host countries