Cerebral blood flow and oxygenation at maximal exercise: The effect of clamping carbon dioxide

During exercise, as end-tidal carbon dioxide (P-ETCO2,) drops after the respiratory compensation point (RCP), so does cerebral blood flow velocity (CBFv) and cerebral oxygenation. This low-flow, low-oxygenation state may limit work capacity. We hypothesized that by preventing the fall in P-ETCO2 at peak work capacity (W-max) with a newly designed high-flow, low-resistance rebreathing circuit, we would improve CBFv, cerebral oxygenation, and W-max. Ten cyclists performed two incremental exercise tests, one as control and one with P-ETCO2 constant (clamped) after the RCP. We analyzed, middle cerebral artery CBFv, cerebral oxygenation, and cardiopulmonary measures. At W-max, when we clamped P-ETCO2 (39.7 +/- 5.2 mmHg vs. 29.6 +/- 4.7 mmHg, P < 0.001), CBFv increased (92.6 +/- 15.9 cm/s vs. 73.6 +/- 12.5 cm/s, P < 0.001). However, cerebral oxygenation was unchanged (Delta TSI -21.3 +/- 13.1% vs. -24.3 +/- 8.1%, P = 0.33), and W-max decreased (380.9 +/- 20.4W vs. 405.7 +/- 26.8W, P < 0.001). At W-max, clamping P-ETCO increases CBFv, but this does not appear to improve W-max. (C) 2010 Elsevier B.V. All rights reserved.