Cerebral oxygenation declines at exercise intensities above the respiratory compensation threshold

During incremental exercise PaCO2 and PETCO2 begin to decline at the respiratory compensation threshold (RCT-GEX). Since Paco, alters cerebral blood flow it was hypothesized that there would be a systematic decline in cerebral oxygenation (Cox) measured by near infrared spectroscopy above the RCT (RCT-NIRS). Cardiorespiratory and NIRS responses were simultaneously monitored from the left frontal lobe during incremental exercise in 17 men. All subjects showed a decline in Cox above the RCTLGEX with a 20-40 s delay. Significant differences (P < 0.01) were observed between the RCT-GEX and RCT-NIRS for time (9.83 versus 10.39 min), power (198 versus 212 W) and oxygen uptake (2.31 versus 2.43 L min(-1)). Intra-class correlations for power and absolute VO2 were 0.97 and 0.98, respectively. Bland-Altman analysis revealed no outliers for any of the variables. The results suggested that the &-crease in Cox observed above the RCT was most likely due to a reduction in cerebral blood flow mediated by a decline in Paco, This decline in Cox could reduce neuronal activation thereby limiting maximal exercise capacity in healthy subjects. (c) 2006 Elsevier B.V. All rights reserved.