This chapter discusses the influence of the autonomic nervous system (ANS) in adaptive process to hypobaric hypoxia. This process is complex, and the integrated response depends on a number of factors, including the extent and duration of hypoxic exposure. Acute responses are modified by chronic adaptations that restore circulatory function toward normoxic levels, over periods that may range from a few days or weeks for the sea-level sojourner to years for the high-altitude native. Tolerance to hypoxia varies greatly among individuals, and subjects with a particular susceptibility may develop inappropriate responses, leading to acute mountain sickness and even life-threatening conditions, such as high-altitude cerebral and pulmonary edema (HAPE). HAPE patients have a marked increase in sympathetic activity during acute exposure to hypoxia, even before the development of HAPE. Sympathetic activation may thus play a facilitatory role in HAPE, presumably by contributing to the development of pulmonary hypertension in susceptible individuals.
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