Effects of the angle of head-down tilt on dynamic cerebral autoregulation during combined exposure to cephalad fluid shift and mild hypercapnia.

Astronauts experience combined exposure to a cephalad fluid shift and mild hypercapnia during space missions, potentially contributing to health problems. Such combined exposure may weaken dynamic cerebral autoregulation. The magnitude of cephalad fluid shift varies between individuals, and dynamic cerebral autoregulation may be affected more by greater cephalad fluid shift during combined exposure. We evaluated the dose-dependent effects of head-down tilt (HDT) on dynamic cerebral autoregulation during acute combined exposure to HDT and 3% CO₂ inhalation. Twenty healthy participants were randomly exposed to three angles of HDT (-5°HDT+CO₂, -15°HDT+CO₂ and -30°HDT+CO₂). After 15 min of rest, participants inhaled room air for 10 min in a horizontal body position, then inhaled 3% CO₂ for 10 min under HDT. The last 6 min of data were used for analysis in each stage. Arterial pressure waveforms were obtained using finger blood pressure, and blood velocity waveforms in the middle cerebral artery were obtained using transcranial Doppler ultrasonography. Dynamic cerebral autoregulation was evaluated by transfer function analysis between waveforms. Statistical analysis was performed by two-way repeated-measures analysis of variance. The index of transfer function gain in the low-frequency range increased significantly with -15°HDT+CO₂ and -30°HDT+CO₂, but no changes were seen with -5°HDT+CO₂. Phase in the low-frequency range decreased significantly with all three protocols. These results of significant changes in indexes of both gain and phase during combined exposure to steep HDT (-15° to -30°) and 3% CO₂ inhalation suggest weakened dynamic cerebral autoregulation with the combination of moderate cephalad fluid shift and mild hypercapnia.