On the up

Many of us will have experienced that moment of inspiration on reaching a long strived for mountain summit. The crisp thin air adds a lot to the special atmosphere as you gaze at a stunning view. The obstacles have been overcome, but at altitude it is not just climbing that provides the challenge. Your body itself must cope with lower oxygen concentrations. It can in fact do so with amazing fortitude due to complex systems of physiological adaptation.

This first recorded description of the adverse effects of high altitude on humans dates back to 30 bc, when Tsee Hsn Shoo described the journey along the Silk Road in Karakorm.

Today, despite advances in the field of high altitude medicine, there is much morbidity and death due to high altitude. The primary cause of these problems is a shortage of oxygen available, or hypoxia. An increase in altitude changes the atmospheric environment around you; the higher you go the less oxygen there is. This reduced partial pressure of oxygen is due to a fall in overall barometric pressure in a logarithmic relationship to altitude (figure 1).

At 5500 m the barometric pressure and therefore partial pressure of oxygen is half that at sea level. The partial pressure of oxygen at a constant altitude also varies slightly with temperature, time of day, season, and other environmental changes. But these factors pose problems of their own to a mountaineer, such as dehydration, cold injuries, sunburn, and snow blindness.

The main life threatening effects of hypoxia are summarised by specific syndromes--acute mountain sickness, pulmonary and cerebral oedema, and, over …