Physiology and pathophysiology with ascent to altitude.

With increasing altitude, there is a fall in barometric pressure and a progressive fall in the partial pressure of oxygen. Acclimatization describes the physiologic changes that help maintain tissue oxygen delivery and human performance in the setting of hypobaric hypoxemia. These changes include a marked increase in alveolar ventilation, increased hemoglobin concentration and affinity, and increased tissue oxygen extraction. In some individuals, these physiologic changes may be inadequate, such that the sojourn to altitude and the attendant hypoxia are complicated by altitude-associated medical illness. The rate of ascent, the absolute change in altitude, and individual physiology are the primary determinants whether illness will develop or not. The most common clinical manifestations of altitude illness are acute mountain sickness, high altitude pulmonary edema, and high altitude cerebral edema.