Short-term response of cerebrovascular and ocular hemodynamics from micro- to hyper-gravity: A multiscale mathematical analysis

Abstract

Exposure to micro- and hyper-gravity characterizes the spaceflight environment, leading to several short- to long-term physiological alterations. However, both acute and long-term cerebrovascular and ocular responses are still being investigated and far from being understood, with experimental data being fragmented and incongruent.

In this study, we aim to shed light on cerebro-ocular hemodynamics during short-term exposure to altered gravitational acceleration (from $0g$ to 3$g$), by employing a multiscale 0D-1D model of the cardiovascular system. The modeling approach consists of a 1D representation of the arterial and coronary circulations, along with a 0D parametrization of the arteriolar, capillary, venous, cardiopulmonary, coronary, and cerebro-ocular circulations. The model is equipped with short-term regulation mechanisms (i.e., cerebral autoregulation, baroreceptors, and cardiopulmonary reflex) and includes the collapse of the neck veins.

After validating the model against experimental measurements of cerebral and ocular hemodynamic parameters, our findings indicate that micro-gravity leads to increased cerebral and ocular perfusion pressure ($CPP$ and $OPP$, respectively), whereas beat-averaged values of cerebral blood flow ($CBF$) are near-constant due to cerebral autoregulation. However, pulsatile values of pressure and flow rate are increased, especially in the distal cerebral circulation. Additionally, the equilibrium between intracranial and intraocular pressure ($ICP$ and $IOP$, respectively), which is thought to play an important role in the onset of Spaceflight Associated Neuro-Ocular Syndrome, is disrupted, resulting in reduced translaminar pressure ($TLP$). Conversely, hyper-gravity induces significant orthostatic stress that results in a reduction of $CPP$ and $OPP$. Consequently, $CBF$ abruptly drops at higher $g$ values, together with hemodynamic pulsatility. In these conditions, $ICP$ decreases more than $IOP$, leading to an increase in $TLP$.

Present results further underline the usefulness of numerical methods in the comprehension of the pathophysiological mechanisms that occur during exposure to altered gravity conditions, where clinical measurements are rare and difficult to obtain.