Acute Cardiovascular Response to Gravity Changes: A Multiscale Mathematical Model for Microgravity and Hypergravity Applications

In recent years, several studies revealed that long-term human spaceflights induce many cardiovascular alterations (from blood volume decrease to cardiac atrophy), which seriously threaten the health of the astronauts. Among all the hazards of the space environment, continuous exposure to gravity changes (from micro- to hyper-gravity) plays a crucial role in the cardiovascular response to such a hostile environment. However, hemodynamic characterization is still incomplete, and no definitive data are available to date. In this context, numerical modeling provides a useful tool for evaluating cardiovascular and cerebral adaptation to spaceflight environments. To investigate the acute cardiovascular response in standing posture to gravity changes (from 0g to 2.5g), we propose a multiscale 0D-1D model combining a closed-loop central-systemic cardiovascular 0D-1D model with two 0D cerebrovascular and ocular models. The overall model is equipped with short-term regulation mechanisms (baroreflex, cardiopulmonary, cerebral and CO2 reactivity), and explicitly accounts for the action of venous collapse, and gravity and posture changes. Our analysis reveals that: (i) as gravity increases from 0g to 2.5g a worsening of several cardiac and mechano-energetic parameters is observed: heart rate (HR) increases from approximately 65 bpm to 110 bpm, while stroke volume (SV), ejection fraction (EF) and cardiac output (CO) decrease respectively by about -47%, -7% and -9%. Furthermore, these parameters show a non-linear behaviour with g; (ii) gravitational acceleration greatly affects both pressures and flow rates, in terms of mean and pulsatile values, along the arterial tree; (iii) in microgravity (0g-1g), ICP and IOP rise due to a head-ward fluid shift caused by the absence of gravitational force, while in hypergravity (1g-2.5g) they exhibit a significant reduction. Conversely, cerebral blood flow (CBF) remains relatively stable within the range of 0g-1g, but decreases rapidly with higher g values, suggesting an inability of cerebral autoregulation to maintain an adequate CBF. In conclusion, in the short-term, the 0g condition is less demanding, yet it exposes the cerebral-ocular circulation to heightened intracranial and intraocular pressure. Conversely, hypergravity conditions induce strong orthostatic stress, decreasing both SV and cerebral perfusion.