What is the central question of this study? An interaction exists between the regulatory systems of respiration and cerebral blood flow (CBF), because of the same mediator (carbon dioxide, CO ) for both physiological systems. The present study examined whether the traditional method for determining cerebrovascular reactivity to CO (cerebrovascular reactivity; CVR) is modified by changes in respiration. What is the main finding and its importance? CVR was modified by voluntary changes in respiration during hypercapnia. This finding suggests that an alteration in the respiratory system may under- or over-estimate CVR determined by traditional methods in healthy adults.The cerebral vasculature is sensitive to changes in the arterial partial pressure of carbon dioxide (CO ). This physiological mechanism has been well established as a cerebrovascular reactivity to CO (CVR). However, arterial CO may not be an independent variable in the traditional method to assess CVR since the cerebral blood flow (CBF) response is partly affected by the activation of respiratory drive or higher centers in the brain. We hypothesized that CVR is modified by changes in respiration. To test our hypothesis, in the present study, ten young healthy subjects performed hyper- or hypo-ventilation to change end-tidal CO (P CO ) under different concentrations of CO gas inhalation (0, 2.0, 3.5%). We measured middle cerebral artery mean blood flow velocity (MCAVm) by transcranial Doppler to identify the CBF response to change in P CO during each condition. At each F CO condition, P CO was significantly altered by changes in ventilation, and MCA Vm changed accordingly. However, the relationship between changes in MCV Vm and P CO as a response curve of CVR was reset upwards and downwards by hypo- and hyper-ventilation, respectively, compared with CVR during normal-ventilation. The findings of the present study may provide the possibility that an alteration in respiration under- or over-estimates CVR determined by the traditional methods
