Diurnal changes of arterial oxygen saturation and erythropoietin concentration in male and female highlanders

In Caucasians and Native Americans living at altitude, hemoglobin mass is increased in spite of erythropoietin concentrations ([Epo]) not markedly differing from sea level values. We hypothesized that a nocturnal decrease of arterial oxygen saturation (SaO(2)) causes a temporary rise of [Epo] not detected by morning measurements. SaO(2) (continuous, finger oximeter) and [Epo] (ELISA, every 4 h) were determined in young highlanders (altitude 2600 m) during 24 h of usual daily activity. In Series I (six male, nine female students), SaO(2) fell during the night with the nadir occurring between 01:00 and 03:00; daily means (range 92.4–95.2%) were higher in females (+1.7%, P < 0.01). [Epo] showed opposite changes with zenith occurring at 04:00 without a sex difference. Mean daily values (22.9 ± 10.7SD U/L) were higher than values obtained at 08:00 (17.2 ± 9.5 U/L, P < 0.05). In Series II (seven females), only SaO(2) was measured. During follicular and luteal phases, SaO(2) variation was similar to Series I, but the rhythm was disturbed during menstruation. While daily [Epo] variations at sea level are not homogeneous, there is a diurnal variation at altitude following changes in SaO(2). Larger hypoventilation‐dependent decreases of alveolar PO(2) decreases during the night probably cause a stronger reduction of SaO(2) in highlanders compared to lowlanders. This variation might be enlarged by a diurnal fluctuation of Hb concentration. In spite of a lower [Hb], the higher SaO(2) in women compared to men led to a similar arterial oxygen content, likely explaining the absence of differences in [Epo] between sexes.