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 (PETCO2). This study aimed to investigate the relationship between O2Hb, PETCO2, 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 PETCO2 was measured using a gas analyzer. The O2Hb, HHb, and PETCO2 were calculated as the amount of change (ΔO2Hb, ΔHHb, and ΔPETCO2) 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, ΔPETCO2, ΔCBV, and ΔCOE from the RCP to maximal exercise. The ΔPETCO2 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 ΔPETCO2 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 PETCO2.
Keywords: Incremental load exercise; NIRS vector analysis; Near-infrared spectroscopy; Partial pressure end-tidal carbon dioxide; Prefrontal cortex oxygenation.