Limits of respiration at high altitude.

Under most conditions, the lungs compensate for the stresses of illness to ensure adequate acquisition of oxygen. Even with exposure to high altitude, the lungs' adaptations ensure that this process takes place. This process is challenged by global hypoxia, especially if there is impairment in the three processes needed for adequate tissue oxygenation: (1) intact ventilatory drive to breathe; (2) sufficient increase in alveolar ventilation, which is stimulated by that drive; and (3) intact gas exchange at the alveolar-capillary interface. This article reviews the mechanisms that make the study of high altitude relevant to patients who have heart or lung disease at low altitude.