Influence of carbon dioxide on dynamic cerebral autoregulation during simulated microgravity

INTERVENTION: exposure to CO2 (‐), head‐down tilt (‐) exposure to CO2 (+), head‐down tilt (‐) exposure to CO2 (‐), head‐down tilt (+) exposure to CO2 (+),head‐down tilt (+) CONDITION: Healthy adult volunteers. e.g. astronauts, or exposure to carbon dioxide during head down tilt, etc. PRIMARY OUTCOME: Gain, coherence and phase between arterial blood pressure and cerebral blood flow velocity during 10‐min exposure to CO2 with head‐down tilt SECONDARY OUTCOME: cerebral blood flow velocity, pulsatility index, blood pressure variability, cerebral blood flow velocity variability, respiratory indexes, intracranial pressure, NIRS, baroreflex function, HRV INCLUSION CRITERIA: Healthy adult consented to participate in this study.