Hypoxia equally reduces the respiratory compensation point and the NIRS-derived [HHb] breakpoint during a ramp-incremental test in young active males

This study investigated the effect of reduced inspired fraction of O2 (FiO2 ) in the correspondence between the respiratory compensation point (RCP) and the breakpoint in the near-infrared spectroscopy-derived deoxygenated hemoglobin signal ([HHb]bp ) during a ramp-incremental (RI) test to exhaustion. Eleven young males performed, on two separated occasions, a RI test either in normoxia (NORM, FiO2 = 20.9%) or hypoxia (HYPO, FiO2 = 16%). Oxygen uptake ( ˙ V O2 ), and [HHb] signal from the vastus lateralis muscle were continuously measured. Peak ˙ V O2 (2.98 ± 0.36 vs. 3.39 ± 0.26 L min-1 ) and PO (282 ± 29 vs. 310 ± 19 W) were lower in HYPO compared to NORM condition, respectively. The ˙ V O2 and PO associated with RCP and [HHb]bp were lower in HYPO (2.35 ± 0.24 and 2.34 ± 0.26 L min-1 ; 198 ± 37 and 197 ± 30 W, respectively) when compared to NORM (2.75 ± 0.26 and 2.75 ± 0.28 L min-1 ; 244 ± 29 and 241 ± 28 W, respectively) (p < .05). Within the same condition, the ˙ V O2 and PO associated with RCP and [HHb]bp were not different (p > .05). Bland-Altman plots mean average errors between RCP and [HHb]bp were not different from zero in HYPO (0.01 L min-1 and 1.1 W) and NORM (0.00 L min-1 and 3.6 W) conditions. The intra-individual changes between thresholds associated with ˙ V O2 and PO in HYPO from NORM were strongly correlated (r = .626 and 0.752, p < .05). Therefore, breathing a lower FiO2 during a RI test resulted in proportional reduction in the RCP and the [HHb]bp in terms of ˙ V O2 and PO, which further supports the notion that these physiological responses may arise from similar metabolic changes reflecting a common phenomenon.