Cerebrovascular response to CO2 during moderate-intensity exercise measured by performing transcranial Doppler Ultrasonography

Previous studies demonstrated that cerebral blood flow (CBF) changes during dynamic exercise and a physiological basis for this observation may be explained by the tight control of CBF by arterial carbon dioxide tension (PaCO?). This study examined whether the steady state of the middle cerebral artery blood flow velocity (MCAVmean) and PaCO?could be observed during constant work rate cycling exercise and to investigate the cerebrovascular reactivity (CVR) to CO?. Seven young volunteers performed a 10-min exercise session with constant workload using a cycle ergometer, with intensities corresponding to the level below the lactate threshold. Respiratory gas analysis and MCAVmean were measured simultaneously using the transcranial Doppler (TCD) method. PaCO?was estimated (ePaCO?) using the end-tidal pressure of CO?(PETCO?) and the tidal volume (VT). On-transient phase II of VO?and the corresponding responses of MCAVmean and ePaCO? were investigated simultaneously with the monoexponential model. Since the responses in ΔMCAVmean or ePaCO?had the overshoot phase within 3min in some cases, analysis for the fitted curves of the monoexponential model was performed during the first 5 min as well as during the total 10 min. CVR to CO?during the dynamic cyclic exercise was 5.33 % mmHg-1 and 4.78 % mmHg?1 in the 5-min and the 10-min analysis, respectively. In the 5-min and the 10-min analysis, CVR to CO?significantly correlated with the exercise intensity during the 10-min bout (r2 = 0.89 and 0.75, respectively). During the on-transient phase of dynamic exercise, CBF would be influenced by PaCO?and other factors such as the neuronal activation and cardiac output would also be involved in changing CBF