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. 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. 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. buy ortho tri-cyclen online
Several studies had previously provided clues to potential reasons for differences in oxygenation at a fixed altitude. Barcroft et al, in 1922, described greater arterial hypoxemia in natives at high altitude than in newcomers to that same altitude; and Chiodi had shown that hypoxemia was commonly associated with an increased Pco2 (by standards of high altitude), suggesting that relative hypoventilation was a contributor.