The microgravity environment during spaceflight can induce cephalic fluid shifts and alterations in cerebral blood flow, which are closely associated with visual impairment, spatial disorientation, and cognitive decline in astronauts. Although previous studies have utilized ground-based simulated microgravity paradigms such as head-down bed rest to reveal certain patterns of cerebral blood flow changes, most have focused on the steady-state phase during the middle and late stages of exposure, with insufficient understanding of the hemodynamic fluctuation characteristics during the early phase of postural transition. More importantly, cerebral blood flow, as a terminal perfusion indicator of the cerebral microcirculation, has not been systematically investigated in terms of its coupling relationship with upstream macrovascular hemodynamics in the internal carotid and vertebral arteries. Furthermore, magnetic resonance imaging cannot be employed during spaceflight due to its bulky equipment, whereas ultrasound offers the advantages of portability, real-time monitoring, and non-ionizing radiation. Therefore, elucidating the association between macrovascular hemodynamic changes and regional cerebral blood flow alterations during early simulated microgravity, and evaluating the feasibility of using ultrasound parameters as portable predictors of cerebral blood flow, holds significant scientific importance and application value.
Recently, a study published in Space: Science & Technology by researchers from Capital Medical University in collaboration with the National Space Science Center systematically investigated the cerebrovascular coupling response mechanisms during early simulated microgravity by integrating Doppler ultrasound with three-dimensional pseudo-continuous arterial spin labeling magnetic resonance perfusion imaging. The study enrolled 33 healthy male volunteers who underwent a -6° head-down bed rest protocol, with hemodynamic parameters of the bilateral internal carotid and vertebral arteries and regional cerebral blood flow values measured at baseline and after 12 hours of bed rest. The results demonstrated that after 12 hours of simulated microgravity, heart rate significantly decreased, while bilateral internal carotid artery parameters showed no significant changes; however, left vertebral artery flow velocity significantly decreased and right vertebral artery resistance index declined. Cerebral blood flow was significantly reduced across all brain regions, with more pronounced perfusion decreases in the posterior circulation compared to the anterior circulation. Regression analysis further revealed that posterior circulation cerebral blood flow changes were independently associated with the vertebral artery systolic-to-diastolic ratio, end-diastolic flow velocity changes, and blood flow volume, whereas anterior circulation changes were primarily predicted by baseline cerebral blood flow levels. This study is the first to reveal that cerebral blood flow reduction during short-term simulated microgravity exhibits regional specificity and is closely associated with upstream vertebral artery hemodynamic alterations, confirming the potential of ultrasound as a portable tool for early assessment of cerebral blood flow during spaceflight, and providing a theoretical basis 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 changes under simulated microgravity and their coupling relationship with upstream macrovascular hemodynamics, and a systematic experimental protocol was designed. The cephalic fluid shift induced by space microgravity can lead to alterations in cerebral blood flow, which are closely associated with visual impairment, spatial disorientation, and cognitive decline in astronauts. However, most existing studies have concentrated on the steady‑state phase during the middle and late stages of microgravity exposure, leaving insufficient understanding of the cerebral blood flow fluctuation patterns during the initial phase of postural transition. Moreover, the coupling relationship between cerebral blood flow and upstream hemodynamics in 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 bed rest protocol to simulate the microgravity effect. Data were collected at two time points: the baseline state before bed rest and 12 hours after the onset of bed rest. As shown in Fig. 1, the experimental procedure included physiological monitoring, carotid Doppler ultrasound examinations, and three-dimensional pseudo‑continuous arterial spin labeling magnetic resonance perfusion imaging. Ultrasound measurements were performed on the bilateral internal carotid and vertebral arteries to obtain hemodynamic parameters, including peak systolic velocity, end-diastolic velocity, resistance index, pulsatility index, and blood flow volume (Fig. 2 illustrates representative Doppler ultrasound waveforms). Magnetic resonance perfusion imaging quantified regional cerebral blood flow values in the bilateral anterior and posterior circulations. Through paired comparisons and linear regression analyses, the study systematically explored the associations between macrovascular hemodynamic changes and regional cerebral blood flow alterations.
Second, the study revealed region‑specific patterns of carotid hemodynamics and cerebral blood flow changes during early simulated microgravity. Regarding physiological parameters, heart rate significantly decreased from 74.61 bpm to 66.58 bpm after 12 hours of head‑down bed rest, while systolic blood pressure, diastolic blood pressure, and oxygen saturation remained stable. For ultrasound parameters, as shown in Table 1, no significant changes were observed in any parameters of the bilateral internal carotid arteries; however, the vertebral arteries exhibited marked alterations: left vertebral artery peak systolic velocity and mean velocity significantly decreased, while right vertebral artery resistance index, pulsatility index, and systolic/diastolic ratio all significantly declined. This indicates that the vertebral arteries are more sensitive to early microgravity, whereas the internal carotid arteries tend to maintain hemodynamic stability. In terms of cerebral blood flow, as shown in Fig. 3, arterial spin labeling magnetic resonance imaging revealed significant reductions in cerebral blood flow in both the bilateral anterior and posterior circulations. Further comparisons, as presented in Fig. 4, demonstrated that both the absolute and relative decreases in cerebral blood flow were significantly greater in the posterior circulation than in the anterior circulation, suggesting that posterior perfusion is more vulnerable during early microgravity. Anatomically, the posterior circulation supplies the brainstem, cerebellum, and occipital lobe, which are critically involved in balance regulation, autonomic control, and spatial orientation; thus, its reduced perfusion may increase the risk of orthostatic intolerance and space motion sickness in astronauts.
Finally, the study identified key predictors of cerebral blood flow 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. 5, posterior circulation cerebral blood flow changes were independently associated with multiple vertebral artery ultrasound parameters: the left vertebral artery systolic/diastolic ratio, the change in end-diastolic velocity, and the 12-hour right vertebral artery blood flow volume were all significant predictors. In addition, individuals with higher baseline heart rates exhibited smaller cerebral blood flow reductions, while those with higher body mass index showed more pronounced posterior circulation decreases. In contrast, anterior circulation cerebral blood flow changes were primarily predicted by baseline cerebral blood flow 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 large vessels. Given that magnetic resonance imaging cannot be utilized during spaceflight, the findings support the use of vertebral artery ultrasound parameters to effectively predict posterior circulation cerebral blood flow changes, highlighting the potential of portable ultrasound as an early assessment tool for cerebral blood flow in space stations. The study also has several limitations: only young male volunteers were enrolled, so generalization to female and older astronauts warrants caution; in-flight data are lacking, and subjective symptoms such as dizziness were not collected. Future studies should expand the sample size, include controls with different mission durations, and conduct on-orbit ultrasound validation to provide more comprehensive technical solutions for brain health monitoring during long‑duration crewed deep‑space exploration.
Space: Science & Technology
24-Jun-2026