Interindividual differences in dyspnea during acute hypoxic exercise in healthy adults: A single exposure to simulated altitude at ~9000 ft.

The growing accessibility of simulated altitude facilities (~13%-16% fraction of inspired oxygen [FIO2]) has expanded their use to broader recreational populations. Although acute normobaric hypoxia reduces arterial oxygen saturation and increases ventilatory and sympathetic responses, interindividual variability in dyspnea during hypoxic exercise remains poorly characterized. Twenty-two healthy individuals (24 ± 4 years, 50% female) completed one 90-min exposure at an altitude facility (FIO2 ≈ 15 ± 0.1%) that included an assessment of heart rate variability (autonomic activity), a vascular occlusion test (microvascular responsiveness), and submaximal cycling at 60% heart rate reserve. Vastus lateralis (VL) oxygen saturation and heart rate were measured throughout. Marked interindividual variability in dyspnea derived from the Multidimensional Dyspnea Profile was observed across the cycling protocol. While resting autonomic activity (all p > 0.326) and microvascular responsiveness (all p > 0.138) were not associated with dyspnea perception, greater VL deoxygenation was strongly correlated with dyspnea during cycling (all r > -0.558, all p < 0.007) with the interpretation unchanged when adjusted for wattage. Additionally, males exhibited higher dyspnea (p = 0.022) and greater VL deoxygenation (-32 ± 14% vs. -15 ± 18%, p = 0.022) compared to females. Combined, these findings highlight the importance of interindividual variability in dyspnea during hypoxic exercise and warrant further investigation into its functional implications.