Role of gravity stress on pressure and flow wave patterns in the arterial tree: from micro- to hyper-gravity conditions

The evolution of the human cardiovascular system has taken place under Earth’s gravitational conditions, leading to an optimal configuration which results highly sensitive to deviations from this environment. As a matter of fact, alterations in gravitational acceleration (ranging from microgravity to hypergravity) induce a cas cade of physiological responses affecting both cardiac and vascular functions. The mechanism of fluid shift toward the cranial or caudal body compartments, triggered by microgravity and hypergravity respectively, has been identified as the driver of critical cardiovascular alterations, such as cardiac atrophy, hypovolemia, orthostatic intolerance, and diminished venous return. It is widely known that, under increasing gravitational loads, the cardiovascular system experiences pronounced orthostatic stress, resulting in marked changes in several central hemodynamic parameters (such as stroke volume reduction and heart rate elevation), as well as influencing the dynamics of pressure and flow wave propagation, which are inherently delayed due to vessel wall viscoelasticity. While the general framework of cardiovascular adap tation to the gravitational environment is widely recognized, detailed descriptions and insights into hemodynamic changes still remain limited due to the scarcity of in-vivo data from spaceflight as well as ground-based analogues. The present work fits within this scenario, resorting to the use of a validated multiscale (0D-1D) cardiovascular model to explore acute hemodynamic responses in upright posture to varying gravitational levels, ranging from 0g to 3g environment. The analysis will be conducted with a specific focus on wave propagation phenomena, at specific positions along the arterial network. The model combines a 1D representation of major systemic and coronary arteries with a lumped-parameter (0D) depiction of distal compartments, venous return, cardiopulmonary and ocular-cerebrovascular circulations. Key short-term regu latory mechanisms, including baroreflex, car diopulmonary reflexes, and cerebral autoregulation, are incorporated within the model, along with posture- and gravity-related effects. By means of waveform analysis and decomposition of blood pressure and flow signals along the 1D arterial domain, the characterization of the gravity-dependent variations in forward and backward wave components is carried out, along with the assessment of altered spatio-temporal patterns implications at the cardiac level. Furthermore, specific attention is given to wave trapping mechanisms (known to preserve the heart region from adverse wave reflections under normal gravity) which interestingly may undergo alterations or impairment in hypergravity conditions. Present results suggest that the computational approach is a promising tool increasingly adopted, which can affordably and reliably contribute to filling the current in vivo gaps associated to the cardiovascular response to altered gravity induced by human spaceflight.