Dynamic Exercise Elicits Dissociated Changes Between Tissue Oxygenation and Cerebral Blood Flow in the Prefrontal...

Neuronal activity causes changes in both cerebral metabolic rate of oxygen (CMRO₂) and cerebral blood flow (CBF). Since the relationship between tissue oxygenation and regional CBF (rCBF) during exercise has not been elucidated, we compared the data obtained using near-infrared spectroscopy (NIRS) and rCBF examined using positron emission tomography (PET). Participants in this study comprised 26 healthy young men. Changes in concentration of oxygenated hemoglobin (ΔO₂Hb) and deoxygenated hemoglobin (ΔHHb) in the prefrontal cortex (PFC) were measured using NIRS continuously during a 15-min bout of the constant-load low-intensity cycling exercise (n = 14). Under the same protocol as the NIRS study, rCBF was measured using H₂¹⁵O and PET by the autoradiographic method at baseline (Rest) and at 3 min (Ex1) and 13 min (Ex2) after starting exercise (n = 12). As systematic factors influenced by exercise, heart rate, end-tidal pressure of carbon dioxide (P_ETCO₂) and blood pressure (BP) were monitored. For each region investigated by NIRS, rCBF was analyzed quantitatively using PET-MRI co-registered standardized images. Despite inter-individual differences, changing patterns of ΔO₂Hb and ΔHHb in the PFC were similar between channels. Significant main effects for time point were identified in ΔO₂Hb, ΔHHb and changes in rCBF. While rCBF increased from rest, ΔO₂Hb was not changed at Ex1. Conversely, rCBF was unchanged from rest but ΔO₂Hb was significantly increased at Ex2. Fluctuations of P_ETCO₂ and BP evoked by exercise were not in accordance with changes in ΔO₂Hb, ΔHHb and rCBF, while BP may affect the forehead skin blood flow. Given that NIRS data are a mixture of skin and brain effects, our results suggest that CMRO₂ may differ between the phases in a bout of dynamic exercise. The present study indicates the utility of NIRS to examine the relationship between CMRO₂ and rCBF during exercise.