Medicina De Altura

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ABC of oxygen: Oxygen at high altitude
Andrew J Peacock BMJ 1998;317;1063-1066

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Clinical review

ABC of oxygen Oxygen at high altitude
Andrew J Peacock
The number of people travelling to the high altitude regions, especially South America, Nepal, and India, has risen enormously in the past 10 years. Without special climbing ability these trekkers can be exposed to altitudes they will not have encountered in their home countries. Forexample, the height of Everest base camp is 5500 m whereas the top of Mount Blanc, the highest mountain in the Alps, is only 4800 m. The areas with the highest mountains are also the areas with the poorest facilities, especially for medical care. Trekkers must therefore understand the effects of altitude on their bodies (hypoxia, cold, and dehydration), the processes of acclimatisation, andprophylaxis against and treatment of altitude illness. High altitude can also be a problem for people with cardiopulmonary disease, many of whom take long haul flights on commercial aircraft. They need to know how their condition can be affected by the cabin altitude of the aeroplane (typically 1800-2500 m). If there is any doubt they should be assessed before travel to determine whether theircondition is likely to worsen significantly during flight.

Climbing Everest
Inspired PO2 (kPa) Altitude (m) 0 5 10 15 20

Oxygen availability and altitude
Although the percentage of oxygen in inspired air is constant at different altitudes, the fall in atmospheric pressure at higher altitude decreases the partial pressure of inspired oxygen and hence the driving pressure for gas exchange in thelungs. An ocean of air is present up to 9-10 000 m, where the troposphere ends and the stratosphere begins. The weight of air above us is responsible for the atmospheric pressure, which is normally about 100 kPa at sea level. This atmospheric pressure is the sum of the partial pressures of the constituent gases, oxygen and nitrogen, and also the partial pressure of water vapour (6.3 kPa at 37°C). Asoxygen is 21% of dry air, the inspired oxygen pressure is 0.21 × (100 − 6.3) = 19.6 kPa at sea level. Atmospheric pressure and inspired oxygen pressure fall roughly linearly with altitude to be 50% of the sea level value at 5500 m and only 30% of the sea level value at 8900 m (the height of the summit of Everest). A fall in inspired oxygen pressure reduces the driving pressure for gas exchange inthe lungs and in turn produces a cascade of effects right down to the level of the mitochondria, the final destination of the oxygen.

10 000 Mount Everest

5000

Highest human habitation Pike's Peak

Ben Nevis Denver 0

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25

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Barometric pressure (kPa)

Relation between altitude and inspired oxygen pressure

Partial pressure of oxygen (kPa)

20 Sea level...