Cerebral blood flow reactivity to CO 2 during exercise

The arterial CO 2 partial pressure (PaCO 2 ) is associated with cerebral blood flow (CBF). However, the relationship between PaCO 2 and CBF during exercise remains unclear. To investigate the mechanisms relevant to a control of the CBF during exercise, we measured the blood velocity in the middle cerebral artery (MCAV) in six males under various PaCO 2 conditions by controlling inhaled CO 2 fraction (F I CO 2 = 0, 3.5, and 5.0 %) and controlling ventilation (V E = 9 ‐ 38 L/min at rest and 12 ‐ 68 L/min during exercise) for 12 min at rest and during 40 W ergometer cycling. The reactivity of MCAV to PaCO 2 change at rest was 1.08 ± 0.48 and 0.96 ± 0.17 cm/s/mmHg obtained from F I CO 2 and V E control, respectively, while that during exercise was 1.41 ± 0.43 and 1.00 ± 0.30 cm/s/mmHg. The reactivity during exercise was significantly greater when obtained from F I CO 2 control trial than that obtained from V E control. The range of PaCO 2 was lower in V E control than in F I CO 2 control. These results suggest that the MCAV reactivity to PaCO 2 is greater in high PaCO 2 range during exercise, implying a possibility that increased MCAV reactivity has a role in washing‐out CO 2 against great amount of CO 2 product during exercise.