Coupling between carotid hemodynamics and cerebral blood flow during early simulated microgravity

With the advancement of long-term crewed deep-space exploration missions, the effects of the microgravity environment on astronauts' brain function have become an important topic in aerospace medicine. The disappearance of the hydrostatic pressure gradient during spaceflight induces cephalad fluid shifts, which can alter cerebral blood flow (CBF) and are closely associated with visual impairment, spatial disorientation, and cognitive decline. However, existing studies have predominantly focused on the steady-state phase during the middle and late stages of microgravity exposure, with insufficient understanding of the characteristics of CBF fluctuations during the early phase of postural transitions. Moreover, as an indicator of terminal perfusion in the cerebral microcirculation, the coupling relationship between CBF and the upstream hemodynamics of the internal carotid and vertebral arteries has not been systematically investigated. In addition, magnetic resonance imaging (MRI) cannot be applied in real time during spaceflight due to its bulky equipment, whereas ultrasound offers the advantages of portability, real-time capability, and radiation-free operation. Therefore, elucidating the association between early macrovascular hemodynamic changes under simulated microgravity and regional CBF alterations, and evaluating the feasibility of using ultrasound parameters as portable predictors of CBF, holds significant scientific importance and application value.

In a recent study published in Space: Science & Technology, a joint team from Beijing Friendship Hospital, Capital Medical University, and the National Space Science Center systematically investigated the cerebrovascular coupling response mechanisms during early simulated microgravity by integrating Doppler ultrasound and three-dimensional pseudo-continuous arterial spin labeling (3D pCASL) MRI perfusion imaging. The study enrolled 33 healthy male volunteers in a -6° head-down tilt bed rest experiment. Hemodynamic parameters of the bilateral internal carotid and vertebral arteries, as well as regional CBF values, were measured at baseline and after 12 hours of bed rest. The results demonstrated that after 12 hours of simulated microgravity, heart rate decreased significantly, with no notable changes in the parameters of the bilateral internal carotid arteries. However, the flow velocity of the left vertebral artery decreased significantly, and the resistive index of the right vertebral artery declined. CBF decreased significantly across all brain regions, with a more pronounced perfusion reduction in the posterior circulation than in the anterior circulation. Regression analysis further revealed that changes in posterior circulation CBF were independently associated with the systolic-to-diastolic ratio of the vertebral artery, changes in end-diastolic velocity, and blood flow volume, whereas anterior circulation changes were primarily predicted by baseline CBF levels. This study is the first to reveal that short-duration simulated microgravity induces region-specific CBF reductions that are closely correlated with upstream vertebral artery hemodynamic alterations, confirming the potential of ultrasound as a portable tool for early CBF assessment during spaceflight, and providing theoretical support and an engineering-feasible solution for the development of brain health monitoring technologies in space stations.

First, this study focuses on the characteristics of early cerebral blood flow (CBF) changes under simulated microgravity and their coupling relationship with upstream macrovascular hemodynamics, and designs a systematic experimental protocol. Cephalad fluid shifts induced by space microgravity can alter CBF, which is closely associated with astronauts' visual impairment, spatial disorientation, and cognitive decline. However, existing studies have predominantly focused on the steady-state phase during the middle and late stages of microgravity exposure, with insufficient understanding of CBF fluctuations during the early phase of postural transitions. Moreover, the coupling relationship between CBF and the upstream hemodynamics of the internal carotid and vertebral arteries has not been systematically investigated. To address this, the research team recruited 33 healthy male volunteers and employed a -6° head-down tilt bed rest experiment to simulate microgravity effects. Data were collected at two time points: baseline before bed rest and 12 hours after the onset of bed rest. As shown in Fig. 1, the experimental protocol included physiological monitoring, carotid Doppler ultrasound examination, and three-dimensional pseudo-continuous arterial spin labeling (3D pCASL) MRI perfusion imaging. Ultrasound measurements assessed hemodynamic parameters of the bilateral internal carotid and vertebral arteries, including peak systolic velocity, end-diastolic velocity, resistive index, pulsatility index, and blood flow volume. MRI perfusion imaging quantified regional CBF values in the bilateral anterior and posterior circulations. Through paired comparisons and linear regression analysis, the study systematically investigated the associations between macrovascular hemodynamic changes and regional CBF alterations.

Second, the study reveals the region-specific patterns of carotid hemodynamics and cerebral blood flow alterations during early simulated microgravity. In terms of physiological parameters, after 12 hours of head-down tilt bed rest, heart rate decreased significantly from 74.61 bpm to 66.58 bpm, while systolic blood pressure, diastolic blood pressure, and oxygen saturation remained stable. Regarding ultrasound parameters, no significant changes were observed in either side of the internal carotid arteries. In contrast, the vertebral arteries exhibited notable alterations: the peak systolic velocity and mean velocity of the left vertebral artery decreased significantly, while the resistive index, pulsatility index, and systolic-to-diastolic ratio of the right vertebral artery all declined markedly. This suggests that the vertebral arteries are more sensitive to early microgravity, whereas the internal carotid arteries tend to maintain hemodynamic stability. With respect to cerebral blood flow (CBF), as shown in Fig. 2, arterial spin labeling MRI measurements revealed significant reductions in CBF in both the bilateral anterior and posterior circulations. Further comparison, as illustrated in Fig. 3, demonstrated that both the absolute and relative decreases in CBF were significantly greater in the posterior circulation than in the anterior circulation, indicating that posterior circulation perfusion is more vulnerable during early microgravity exposure. Anatomically, the posterior circulation supplies the brainstem, cerebellum, and occipital lobe, which are closely associated with balance regulation, autonomic control, and spatial orientation; thus, its perfusion reduction may increase the risk of orthostatic intolerance and space motion sickness in astronauts.

Finally, the study identified key predictors of cerebral blood flow (CBF) changes through linear regression analysis and explored the potential application value of ultrasound as a portable monitoring tool. As shown in the forest plot of Fig. 4, posterior circulation CBF changes were independently associated with multiple vertebral artery ultrasound parameters: the systolic-to-diastolic ratio of the left vertebral artery, the change in end-diastolic velocity, and the 12-hour blood flow volume of the right vertebral artery were all significant predictors. In addition, higher baseline heart rate was associated with smaller CBF reductions, while higher body mass index (BMI) was associated with more pronounced posterior circulation CBF decreases. In contrast, anterior circulation CBF changes were primarily predicted by baseline CBF levels rather than by acute hemodynamic adjustments. These results suggest that the anterior circulation possesses stronger autoregulatory capacity, whereas the posterior circulation is more dependent on the perfusion status of upstream macrovessels. Given that MRI cannot be applied during spaceflight, the finding that vertebral artery ultrasound parameters effectively predict posterior circulation CBF changes supports the potential utility of portable ultrasound as an early assessment tool for CBF in space stations. The study also has certain limitations: only young male volunteers were enrolled, so generalization of the results to female and older astronauts should be undertaken with caution; in-flight data are lacking, and subjective symptoms such as dizziness were not collected. Future research should expand the sample size, include control groups with different mission durations, and conduct on-orbit ultrasound validation to provide more comprehensive technical solutions for brain health monitoring in long-term crewed deep-space exploration.