altitude There is no precise definition of high altitude. However, many people feel lightheaded and have other symptoms if they ascend from near sea level to 3000 metres (about 10 000 feet). Some individuals are affected at as low as 2000 metres. Nearly 140 million people live at altitudes above 2500 metres (about 8000 feet). Substantial numbers live permanently at altitudes as high as 4500 metres in the Peruvian Andes, and caretakers of a mine in Chile have lived at nearly 6000 metres. The highest point on Earth is the summit of Mt. Everest (8848 metres), and well-acclimatized climbers can just reach that altitude without using supplementary oxygen.

The two regions of the world with the largest high-altitude populations are the South American Andes and the Tibetan plateau. It is estimated that between 10 and 17 million people live at over 2500 metres in the Andes, and that over 50 000 people in Peru reside above 4000 metres. Lhasa, in Tibet, altitude 3658 metres, has over 130 000 inhabitants. Other parts of the world with substantial high-altitude populations include Central and North America, Europe, Russia, Africa, and Indonesia.

It is useful to divide people at high altitude into two groups; those who live there permanently (‘highlanders’) and those who have moved up temporarily from sea level (‘lowlanders’). Lowlanders who go to high altitude undergo a process known as acclimatization, which greatly assists them in tolerating the high altitude. Some permanent residents who have been at high altitude for generations have probably undergone true Darwinian adaptation. Many features of acclimatization and adaptation are similar, but there are some differences.

Altitude and oxygen

High altitude provides physiological and medical challenges because the amount of oxygen in the air is reduced. As we go higher, the barometric pressure falls, for the same reason as, when we are submerged deeper in water, the pressure rises. Indeed, Torricelli, who invented the mercury barometer in the mid-seventeenth century, stated, ‘We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight.’ For example, if we go to an altitude of about 5800 metres, the pressure falls to half the normal sea level value of 760 mm Hg (1013 millibars or hectopascals). At the summit of Mt. Everest the pressure is about one-third of the sea level value. Since oxygen accounts for one-fifth of the volume of the air, and this fraction does not alter with altitude, the pressure of oxygen decreases proportionally with the total barometric pressure. A decrease in this ‘partial pressure’ of oxygen in the lungs results in a decrease in the amount of oxygen in the blood — the state of oxygen shortage, or hypoxia. This hypoxia is responsible for almost all the physiological changes and the potential medical problems that occur at high altitude.

The relationship between barometric pressure and altitude is not the same over the whole surface of the globe. Because of the warming of the atmosphere by the sun near the Equator, the column of air is higher there, and therefore the barometric pressure at any given altitude is higher than at the poles. These differences are important to the mountain climber. For example, it can be shown that if Mt. Everest were at the latitude of Mt. McKinley (Denali) in Alaska, which is 60° N, the summit would in effect be over 950 metres (3000 feet) higher because the barometric pressure at high altitude at latitudes far from the Equator is so much lower. This would make it impossible to climb the mountain without supplementary oxygen. On the other hand, the use of the International Civil Aviation Organization (ICAO) Standard Atmosphere (often used to calibrate altimeters) considerably underestimates the barometric pressure on the summit of Everest: the severity of hypoxia was therefore overestimated in some predictions based on this method in the past.

Acclimatization

The role of acclimatization in enabling lowlanders to tolerate high altitude is critical; indeed it is one of the classical examples of how the human body can adapt to hostile conditions. A normal person who is acutely exposed to the barometric pressure of the summit of Mt. Everest in a low-pressure chamber will lose consciousness within 2 or 3 minutes, but with the advantages of acclimatization, many climbers have now reached the summit without supplementary oxygen.

The most important feature of acclimatization to high altitude is an increase in the rate and depth of breathing. The product of the volume of each breath multiplied by the frequency of breathing is known as the total ventilation. The increase in ventilation is brought about by stimulation of chemoreceptors by the low oxygen pressure in the arterial blood. A chemoreceptor is a specialized tissue which responds to its environment by sending nerve impulses to the brain. In order for a climber to reach the Everest summit, he must increase his ventilation some 5-fold. Anecdotal evidence of this comes from tape recordings of climbers on the summit; they are so short of breath that they need to breathe after every two or three words! The reason why the increase in ventilation is so important is that it raises the pressure of oxygen in the alveoli in the depths of the lung, where the exchange between the air and blood takes place. At the same time, the increase in ventilation greatly reduces the pressure of carbon dioxide in the lungs and in the blood.

The extent to which people increase their breathing when they go to high altitude depends on their genetic make-up: some increase it much more than others. There is some evidence that climbers who have a poor ventilatory response to hypoxia tolerate very high altitudes badly. A test for this can be administered at sea level by giving people a low-oxygen mixture to breathe.

Interestingly, many people who are born at high altitude have a relatively low ventilatory response to hypoxia. This seems to be paradoxical, although it may protect them from periodic breathing during sleep (see below). It may be that these permanent residents have other adaptations at the tissue level which are not yet understood.

Another feature of acclimatization is an increase in the concentration of red cells in the blood. For example, a permanent resident at 4600 metres in the Peruvian Andes typically has a 30% increase in red cell concentration. This polycythaemia increases the oxygen carrying capacity of the blood. However, the value of polycythaemia in the acclimatization process is not as clear as it was once thought to be. Severe polycythaemia increases the viscosity of the blood and probably leads to problems with unloading oxygen from the blood to the tissues. Some permanent residents of the Andes can actually do more physical work when the concentration of red cells in their blood is reduced by bloodletting.

The mechanism responsible for polycythaemia is the release of the hormone erythropoietin from the kidney as a result of the shortage of oxygen there. The erythropoietin then stimulates the production of red blood cells by the bone marrow. The evolutionary pressure for the development of this mechanism probably occurred at sea level, promoting survival after injury, because blood loss also causes inadequacy of oxygen supply. It may be that its value as an adaptation to high altitude has been overemphasized.

The cardiac output increases with acute hypoxia, compensating for the shortage of oxygen in the blood by increasing the rate of blood supply, but in acclimatized subjects it returns to the sea level value. Other features of acclimatization include an increase in the concentration of capillaries in peripheral tissues, and increases in the amount of oxidative enzymes within cells. It is likely that some of the acclimatization processes at the cellular level have not yet been discovered.

Many features of the adaptation of permanent residents to high altitude are similar to those of acclimatization. Interestingly, there is some evidence that Tibetans have progressed further in the adaptation process than Andeans, consistent with the much longer period that they have spent at high altitude. Features that suggest better adaptation include less polycythaemia, greater ventilation, lower pressures in the pulmonary circulation, and an apparent lower incidence of ‘chronic mountain sickness’ (see below). However, this is an active area of research and there is some controversy.

Mount Everest

One of the great sagas of this century has been the ascent of Mt. Everest without supplementary oxygen. In 1920 the mountaineer–physiologist Alexander Kellas predicted that it could be done if the technical difficulties were not too great. During the 1924 British expedition to Everest, E. F. Norton climbed to within 300 metres of the summit without supplementary oxygen, but in the 1930s several physiological studies suggested that the summit could not be reached. It was not until 1978, 54 years after Norton, that the last 300 metres were conquered by Messner and Habeler.

The critical factors on the summit are the barometric pressure, the extent of the increase in the climber's ventilation, and his maximal oxygen uptake. The first measurements of these were obtained by the American Medical Research Expedition to Everest in 1981. In 1985 a simulated climb in a low-pressure chamber, Operation Everest II, greatly clarified the physiological adaptations at extreme altitudes, particularly in the pulmonary circulation and skeletal muscle. For example, the resistance of the pulmonary circulation was greatly increased at very high altitudes, because hypoxia constricted the blood vessels. Also, muscle biopsies showed an increase in the concentration of capillaries because the muscle fibres became thinner.

Altitude sickness

Various forms of altitude sickness are recognized. Newcomers to high altitude frequently complain of headache, fatigue, dizziness, palpitations, nausea, loss of appetite, and insomnia. This is known as acute mountain sickness, and usually resolves after 2 or 3 days at medium altitudes. It is probably caused by the combination of the low oxygen and the alkalosis in the blood resulting from the reduced pressure of carbon dioxide. Administration of the drug acetazolamide reduces the incidence of acute mountain sickness.

A more severe illness is high-altitude pulmonary oedema, in which the capillaries in the lung are damaged and leak high-protein fluid into the alveolar spaces. The damage is due to high pressures in the pulmonary circulation, which develop in response to the hypoxia. This potentially fatal condition is best treated by taking the patient down to a lower altitude as rapidly as possible, though oxygen is given if this is available. An even more serious problem is high-altitude cerebral oedema due to leakage of fluid into the brain tissues. Again, descent is by far the best treatment. Long-term residents at high altitude sometimes develop an ill-defined syndrome characterized by fatigue, reduced ability to exercise, very low levels of oxygen in the blood, and marked polycythaemia. This is called chronic mountain sickness, and again descent is the best treatment if this is practicable.

Newcomers to high altitude often complain that the most distressing period is during the night when they try to sleep. Periodic breathing frequently occurs. This is characterized by a gradual waxing and then waning of breathing movements, and often there is a period of no breathing at all (apnea) which may last for 10 sec or more. Sometimes people wake up at the end of the apneic period feeling smothered. Treatment with acetazolamide reduces the incidence and severity of periodic breathing. Permanent residents of high altitude who have a reduced ventilatory response to hypoxia develop less periodic breathing.

Factors other than hypoxia

All of the physiological and medical problems of going to high altitude described above have their root in the low partial pressure of oxygen in the air. This is an inevitable consequence of going to high altitude unless, of course, supplementary oxygen is breathed. However there are other potentially hostile factors at high altitude. One is cold. The air temperature falls at the rate of about 1 °C for every 150 metres of altitude. The effects of cold can of course be mitigated by warm clothing and shelter, but if there is a high wind the resulting chill factor makes it impossible to climb at great altitudes.

Another potential problem results from the low absolute humidity of the air because of the low temperatures. Climbers frequently become dehydrated because they lose a great deal of water vapour as a result of their high ventilation, and it is difficult to obtain water by melting snow. Solar radiation is increased at high altitude because of reduced absorption by the thinner atmosphere, and reflection of the sun from snow. Ionizing radiation, for example by cosmic rays, is also increased because of the thinner atmosphere.

Working at high altitude

Recently there has been a large increase in commercial and scientific facilities, such as mines and telescopes, at very high altitudes. Much of this development has taken place in the Andes, particularly in north Chile. As an example, the Collahuasi copper mine at an altitude of 4500 metres was being greatly expanded in the late 1990s. The workers live at sea level and are taken by bus up to the mine in a few hours, where they spend the next seven days working. They are then taken down to their families at sea level for seven days, and the cycle continues indefinitely. This cycling raises many physiological and medical problems which are poorly understood as yet. Another example is a new radiotelescope being planned for an altitude of 5000 metres in north Chile. The workers will live at an altitude of about 2400 metres and commute up to the telescope each day.

An interesting innovation is the addition of oxygen to the air conditioning in these facilities, in dormitories, offices, conference rooms, laboratories, and even in the cabins of large trucks and mechanical shovels. Every 1% of enrichment (for example increasing the oxygen concentration from 21% to 22%) is equivalent to reducing the altitude by 300 metres. Five per cent oxygen enrichment in a mine at 4500 metres could therefore reduce the equivalent altitude to 3000 metres, which is much more easily tolerated. Oxygen enrichment has become feasible because large amounts of oxygen can easily be produced from air by oxygen concentrators, and also because liquid oxygen is relatively inexpensive. The potential value of this proactive approach to dealing with the hypoxia of high altitude is still being clarified.

John B. West

Bibliography

Ward, M. P.,, Milledge, J. S.,, and and West, J. B. (2000). High altitude medicine and physiology, (3rd edn). Arnold, London.
West, J. B. (1985). Everest — the testing place. McGraw-Hill, New York.

al·ti·tude / ˈaltiˌt(y)oōd/ (abbr.: alt.) • n. the height of an object or point in relation to sea level or ground level: flying at altitudes over 15,000 feet. ∎  great height: the mechanism can freeze at altitude. ∎  Astron. the apparent height of a celestial object above the horizon, measured as an angle. ∎  Geom. the length of the perpendicular line from a vertex to the opposite side of a figure.DERIVATIVES: al·ti·tu·di·nal / ˌaltiˈt(y)oōdn-əl/ adj.

altitude In astronomy, angular distance of a celestial body above the observer's horizon. It is measured in degrees from 0 (on the horizon) to 90 (at the zenith) along the great circle passing through the body and the zenith. If the object is below the horizon, the altitude is negative.

altitude XIV. — L. altitūdō, f. altus, high; see OLD, -TUDE.