– Document – lessons from high altitude brain anoxia

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For over a century, scientific investigation at high altitude has provided so many clues to understanding the pathophysiology of hypoxemic states at low altitude that it is difficult to pick just a few for a manageable presentation. Somewhat arbitrarily, and for reasons of personal familiarity, I have selected a few issues out of a large universe of possibilities that illustrate the impact of high-altitude research on the development of concepts of, and investigation into, pathogenesis of respiratory failure or hypoventilation at low altitude. As an example, the functional and ventilatory adaptation to high altitude is variable and reflects, at least in part, a variation in ventilatory responsiveness to the hypoxic stimulus.Brain anoxia

this adaptation leads to a consideration of the sources of variation in hypoxic chemosensitivity at high and low altitude and how such variation in chemical control of breathing might influence or determine clinical adaptation to chronic obstructive pulmonary disease. Finally, such adaptation leads to a consideration of some important unanswered questions concerning general aspects of adaptation to hypoxia. The ambient hypoxia of high altitude triggers a number of responses with potential adaptive value. These range from adjustments that are immediately available, active, and energy costly (such as increased ventilation and cardiac output) on the one hand, or to passive, potentially energy efficient responses (such as polycythemia and changes in peripheral tissue architecture, and increased capillary and mitochondrial density) on the other.Brain anoxia there are important differences among individuals and species as to which of these are evoked. Often, these varying alternatives seem to develop in a reciprocal or complementary fashion illustrated by the common observation, both at high altitude and in patients with lung disease, that polycythemia is often minimal when ventilation is high and vice versa. In addition to variation in the pattern of these responses, there is also considerable variability in their extent, and it is a familiar observation that the scatter or variability in measured functions including cardiac output, blood gas tensions, ventilation, and hematocrit is substantially enhanced at high altitude. An important example of this can be seen in measurements of hematocrit, which in residents of the high-altitude community of leadville, colorado (3,100 m), span a range several-fold greater than is seen at sea level.[1] reasons for this greater dispersion of hematocrit values at high altitude were addressed in a study which demonstrated that variation in hematocrit and red cell mass was correlated with, and partially explained by, variation in arterial oxygenation such that higher values for hematocrit and red cell mass were generally found in patients with more severe hypoxemia.[2] thus, the variable erythrocytosis at high altitude seemed to reflect, in part, variation in oxygenation; and this, in turn, raised the question of reasons for variable oxygenation.Brain anoxia several studies had previously provided clues to potential reasons for differences in oxygenation at a fixed altitude. Barcroft et al, in 1922,[3] described greater arterial hypoxemia in natives at high altitude than in newcomers to that same altitude; and chiodi[4] had shown that hypoxemia was commonly associated with…