Population distribution residing at different altitudes: implications for hypoxemia

BACKGROUND: Hypoxemia can adversely affect health. The present study is aimed at estimating the prevalence of altitude-related low partial pressure of oxygen in arterial blood in Mexican population by means of census and geographic data. METHODS: Population, altitude, and characteristics of communities were obtained from the Mexican Institute of Statistics, Geography, and Informatics (INEGI). The population of each municipality (municipio) was assumed to have the same age distribution as that reported for the entire country. Partial pressure of oxygen in arterial blood (PaO(2)) was estimated from altitude and from a hypothetical alveolar-arterial oxygen gradient that increases with age. RESULTS: In Mexico, 3.95% of the population lives at altitudes 2,500 m above sea level. Census data for the year 1990 recount 3.6 million people distributed among 6,150 communities. Exposure to intermediate altitudes is considerable: one half of the Mexican population resides above 1,550 m, 32% above 2,000 m, 25% above 2,230 m, and 5% above 2,440 m. It was estimated that between 0.9 and 3.4% of the healthy population (between 800,000 and 3 million persons) have a resting PaO(2) <55 torr, a criterion frequently used for prescribing chronic oxygen therapy in patients with lung diseases. CONCLUSIONS; Although the exact prevalence of hypoxemia in Mexico awaits empirical data, a large number of people live in places where altitude may expose them to low partial pressure of oxygen in inspired air. Ensuing hypoxemia may adversely affect their health. Hypoxia may be particularly harmful to elderly persons and to patients suffering from respiratory diseases.