AltitudeOmics: effects of 16 days acclimatization to hypobaric hypoxia on muscle oxygen extraction during incremental exercise

Acute altitude exposure lowers arterial oxygen content (Ca_O2 ) and cardiac output (Qc) ^_ at peak exercise, whereas O₂ extraction from blood to working muscles remains similar. Acclimatization normalizes Ca_O2 but not peak Qc ^_ nor peak oxygen consumption (V^_ O_2peak). To what extent acclimatization impacts muscle O₂ extraction remains unresolved. Twenty-one sea-level residents performed an incremental cycling exercise to exhaustion near sea level (SL), in acute (ALT1) and chronic (ALT16) hypoxia (5,260 m). Arterial blood gases, gas exchange at the mouth and oxy- (O₂Hb) and deoxyhemoglobin (HHb) of the vastus lateralis were recorded to assess arterial O₂ content (Ca_O2 ), Qc, ^_ and V^_ O₂. The HHb-V^_ O₂ slope was taken as a surrogate for muscle O₂ extraction. During moderate-intensity exercise, HHb-V^_ O₂ slope increased to a comparable extent at ALT1 (2.13 ± 0.94) and ALT16 (2.03 ± 0.88) compared with SL (1.27 ± 0.12), indicating increased O₂ extraction. However, the HHb/Ca_O2 ratio increased from SL to ALT1 and then tended to go back to SL values at ALT16. During high-intensity exercise, HHb-V^_ O₂ slope reached a break point beyond which it decreased at SL and ALT1, but not at ALT16. Increased muscle O₂ extraction during submaximal exercise was associated with decreased Ca_O2 in acute hypoxia. The significantly greater muscle O₂ extraction during maximal exercise in chronic hypoxia is suggestive of an O₂ reserve.