FIGURE 2.

Relationship between Hb concentration ([Hb]) and blood oxygenation Shown is the inverse relationship between [Hb] and SpO₂ in male highlanders from Cerro de Pasco, Peru at 4,340 m (r = −0.55, P < 0.0001, n = 965; Villafuerte FC, unpublished observations). The dotted lines represent the thresholds of [Hb] to determine excessive erythrocytosis (EE; ≥21 g/dl) and of SpO₂ (<83%) to determine severe hypoxemia. The different phenotypes on these variables can be observed from the 4 quadrants. Although it is expected that individuals with severe hypoxemia develop EE (relative risk of 2.66, 95% confidence interval of 2.29–3.08, P < 0.0001), about one-third of highlanders with SpO₂ values >83% have EE. On the other hand, also a third of highlanders with SpO₂ <83% have [Hb] within the usual range relative to the altitude of residence.