Severe hypoxia uncovers divergent regional oxygenation patterns during exercise: A dual-site NIRS phase-plane...

Acute severe hypoxia reduces systemic oxygen availability and may impose different constraints on cerebral and muscular oxygenation. We tested whether the prefrontal cortex (PFC) and vastus lateralis (VL) exhibit distinct oxygenation patterns during exercise by combining dual-site NIRS with phase-plane vector analysis of oxyhemoglobin (HbO₂) and deoxyhemoglobin (HHb). Twelve physically active adults (11 men, 1 woman; VO_2max: 52.03 ± 9.84 mL·kg^-1·min^-1) performed maximal incremental exercise in normobaric normoxia and severe normobaric hypoxia (FiO₂ = 11%, ~5000 m equivalent). Responses were compared at lactate-matched submaximal intensities (2 and 4 mmol·L-1) and peak exercise. At rest, hypoxia reduced tissue saturation index (TSI) more in PFC than VL (10.82 ± 2.48 vs. 3.37 ± 4.75 percentage points). During, hemoglobin-derived variables mainly reflected exercise intensity and regional differences. Phase-plane angle K differed between hypoxia and normoxia in the PFC already at 2 mmol·L^-1 (118.11 ± 35.73° vs. -15.41 ± 110.08°; p < 0.001), whereas mean VL K remained within the 90-180° sector across intensities and conditions (116-133°), without significant between-condition differences. No between-condition difference in PFC K was detected at peak exercise. These findings describe regional oxygenation responses to severe hypoxia, with condition-dependent differences in the submaximal PFC response. These regional oxygenation responses may partly reflect hypocapnia-related constraints on prefrontal perfusion and compensatory vasodilation in exercising muscle, although K does not directly quantify blood flow or oxygen extraction and its interpretation warrants caution when PFC vector magnitudes are small.