Relationship Between Decrease of Oxygenation During Incremental Exercise and Partial Pressure End-Tidal Carbon Dioxide: Near-Infrared Spectroscopy Vector Analysis.

A previous study considered that a decrease in cerebral oxyhemoglobin (OHb) immediately before maximal exercise during incremental exercise is related to cerebral blood flow (CBF) and partial pressure end-tidal carbon dioxide (PCO). This study aimed to investigate the relationship between OHb, PCO, 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 OHb and deoxyhemoglobin (HHb) in the prefrontal cortex (PFC) were measured using near-infrared spectroscopy (NIRS). The PCO was measured using a gas analyzer. The OHb, HHb, and PCO were calculated as the amount of change (ΔOHb, ΔHHb, and ΔPCO) 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 OHb decline, was detected. The Pearson correlation coefficient was used to establish the relationships among ΔOHb, ΔPCO, ΔCBV, and ΔCOE from the RCP to maximal exercise. The ΔPCO did not significantly correlate with the ΔOHb (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 ΔPCO from the RCP to maximal exercise were not related to changes in the ΔOHb, ΔCOE, and ΔCBV. Therefore, we suggested that the decrease of OHb immediately before maximal exercise during incremental exercise may be related to cerebral oxygen metabolism by neural activity increase, not decrease of CBF by the PCO.