Determination of the Respiratory Compensation Point by Detecting Changes in Intercostal Muscles Oxygenation by Using Near-Infrared Spectroscopy

This study aimed to evaluate if the changes in oxygen saturation levels at intercostal muscles (SmO₂-m.intercostales) assessed by near-infrared spectroscopy (NIRS) using a wearable device could determine the respiratory compensation point (RCP) during exercise. Fifteen healthy competitive. triathletes (eight males; 29 ± 6 years; height 167.6 ± 25.6 cm; weight 69.2 ± 9.4 kg; VO₂-máx 58.4 ± 8.1 mL·kg^{−1·min−1}) were evaluated in a cycle ergometer during the maximal oxygen-uptake test (VO^.₂-máx), while lung ventilation (VE),^. power output (watts, W) and SmO₂-m.intercostales were measured. RCP was determined by visual method (RCP_visual: changes at ventilatory equivalents (VE·^. VCO^.−1. 2, VE·VO^.−1₂) and end-tidal respiratory pressure (PetO₂, PetCO₂) and NIRS method (RCP_NIRS: breakpoint of fall in SmO₂-m.intercostales). During exercise, SmO₂-m.intercostales decreased continuously showing a higher decrease when VE^. increased abruptly. A good agreement between methods used to determine RCP was found (visual vs NIRS) at %VO^... 2-máx, VO₂, VE, and W (Bland-Altman test). Correlations were found to each parameters analyzed (r = 0.854; r = 0.865; r = 0.981; and r = 0,968; respectively. p < 0.001 in all variables, Pearson test), with no differences (p < 0.001 in all variables, Student’s t-test) between methods used (RCP_visual and RCP_NIRS). We concluded that changes at SmO₂-m.intercostales measured by NIRS could adequately determine RCP in triathletes.