The physiologic basis of high-altitude diseases.

Many physicians are surprised to learn how many people live, work, and play at high altitude. Some 140 million persons reside at altitudes over 2500 m, mainly in North, Central, and South America; Asia; and eastern Africa (1). Increasingly, people are moving to work at high altitude. For example, there are telescopes at altitudes over 5000 m (2) and mines at over 4500 m (3), and the Golmud–Lhasa railroad being constructed in Tibet will have 30 000 to 50 000 workers at high altitudes, including many who work at more than 4000 m. Skiers, mountaineers, and trekkers go to altitudes of 3000 m to more than 8000 m for recreation, and sudden ascents to high altitude without the benefits of acclimatization are common. All of these groups are prone to high-altitude diseases that sometimes have fatal consequences. In addition, the physiology of hypoxia, which is at the basis of high-altitude medicine, plays an important role in many lung and heart diseases. HYPOXIA OF HIGH ALTITUDE Relationship of Altitude to Barometric Pressure Evangelista Torricelli (1608–1647) was the first person to realize that the atmosphere above us creates a pressure that can, for example, support a column of mercury. In a memorable sentence, he stated, “We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight” (4). Figure 1 shows the relationship between altitude and barometric pressure in the regions where human exposure to high altitude is common. Table 1 lists some of the barometric pressures and the consequent inspired PO2. At an altitude of 3000 m, which is commonly encountered in ski resorts, the barometric pressure and inspired PO2 are only about 70% of the sea level value. At an altitude of 5000 m, the highest at which humans reside, the inspired PO2 is only about half of the sea level value. On the summit of