Relationship Between Decrease of Oxygenation During Incremental Exercise and Partial Pressure End-Tidal Carbon...

A previous study considered that a decrease in cerebral oxyhemoglobin (O₂Hb) immediately before maximal exercise during incremental exercise is related to cerebral blood flow (CBF) and partial pressure end-tidal carbon dioxide (P_ETCO₂). This study aimed to investigate the relationship between O₂Hb, P_ETCO₂, and the estimated value of cerebral blood volume (CBV) with cerebral oxygen exchange (COE) by using vector analysis. Twenty-four healthy young men participated in this study. They performed the incremental exercise (20 W/min) after a 4-min rest and warm-up. The O₂Hb and deoxyhemoglobin (HHb) in the prefrontal cortex (PFC) were measured using near-infrared spectroscopy (NIRS). The P_ETCO₂ was measured using a gas analyzer. The O₂Hb, HHb, and P_ETCO₂ were calculated as the amount of change (ΔO₂Hb, ΔHHb, and ΔP_ETCO₂) from an average 4-min rest. Changes in the CBV (ΔCBV) and COE (ΔCOE) were estimated using NIRS vector analysis. Moreover, the respiratory compensation point (RCP), which relates to the O₂Hb decline, was detected. The Pearson correlation coefficient was used to establish the relationships among ΔO₂Hb, ΔP_ETCO₂, ΔCBV, and ΔCOE from the RCP to maximal exercise. The ΔP_ETCO₂ did not significantly correlate with the ΔO₂Hb (r = 0.03, p = 0.88), ΔCOE (r = -0.19, p = 0.36), and ΔCBV (r = -0.21, p = 0.31). These results showed that changes in the ΔP_ETCO₂ from the RCP to maximal exercise were not related to changes in the ΔO₂Hb, ΔCOE, and ΔCBV. Therefore, we suggested that the decrease of O₂Hb immediately before maximal exercise during incremental exercise may be related to cerebral oxygen metabolism by neural activity increase, not decrease of CBF by the P_ETCO₂.