BACKGROUND: Mild hypercapnia combined with a cephalad fluid shift [e.g., that occurring during spaceflight or laparoscopic surgery with head-down tilt (HDT)] might affect cerebral autoregulation. However, no reports have described the effects of the combination on dynamic cerebral autoregulation. Therefore, we tested the hypothesis that the combination of mild hypercapnia and a cephalad fluid shift would attenuate dynamic cerebral autoregulation.METHODS: There were 15 healthy male volunteers who were exposed to 4 10-min protocols in which they received air in the supine position (Placebo/Supine), 3% carbon dioxide (CO₂) in the supine position (CO₂/Supine), air with −10° HDT (Placebo/HDT) and 3% CO₂ with −10° HDT (CO₂/HDT). Dynamic cerebral autoregulation was evaluated using a transfer function analysis of the beat-to-beat variability in mean arterial blood pressure (ABP) and mean cerebral blood flow (CBF) velocity.RESULTS: The phase in the low-frequency range was significantly lower during CO₂/HDT than all other protocols, where CO₂/HDT was −25% lower than Placebo/Supine (CO₂/HDT, 0.49 ± 0.21; Placebo/Supine, 0.65 ± 0.16 radians). The transfer function gain in the low-frequency range was significantly higher during CO₂/HDT than all other protocols, where CO₂/HDT was 26% higher than Placebo/Supine (CO₂/HDT, 1.08 ± 0.34; Placebo/Supine, 0.86 ± 0.28 cm · s⁻¹ · mmHg⁻¹). However, neither the CO₂/Supine nor Placebo/HDT showed significant differences compared with the Placebo/Supine.DISCUSSION: Even short-term exposure to 3% CO₂ plus HDT increased synchrony and the magnitude of transmission between ABP and CBF in the low-frequency range. Thus, the combination of mild hypercapnia and a cephalad fluid shift attenuated dynamic cerebral autoregulation.Kurazumi T, Ogawa Y, Yanagida R, Morisaki H, Iwasaki K. Dynamic cerebral autoregulation during the combination of mild hypercapnia and cephalad fluid shift. Aerosp Med Hum Perform. 2017; 88(9):819–826.