Increased exercise Sa(O₂) independent of ventilatory acclimatization at 4,300 m

Arterial O₂ saturation (Sa(O₂)) decreases in hypoxia in the transition from rest to moderate exercise, but it is unknown whether over several weeks at high altitude Sa(O₂) in submaximal exercise follows the same time course and pattern as that of ventilatory acclimatization in resting subjects. Ventilatory acclimatization is essentially complete after ~1 wk at 4,300 m, such that improvement in submaximal exercise Sa(O₂) would then require other mechanisms. On days 2, 8, and 22 on Pikes Peak (4,300 m), 6 male subjects performed prolonged steady-state cycle exercise at 79% maximal O₂ uptake [V̇(O(2 max))]. Resting Sa(O₂) rose from day 1 (78.4 ± 1.6%) to day 8 (87.5 ± 1.4%) and then did not increase further by day 20 (86.4 ± 0.6%). During exercise, Sa(O₂) values (mean of 5-, 15-, and 30-min measurements) were 72.7% (day 2), 78.6% (day 8), and 82.3% (day 22), meaning that all of the increase in resting Sa(O₂) occurred from day 1 to day 8, but exercise Sa(O₂) increased from day 2 to day 8 (5.9%) and then increased further from day 8 to day 22 (3.7%). On day 22, the exercise Sa(O₂) was higher than on day 8 despite an unchanged ventilation and O₂ consumption. The increased exercise Sa(O₂) was accompanied by decreased CO₂ production. The mechanisms responsible for the increased exercise Sa(O₂) require further investigation.