Modeling steady-state intracranial pressures in supine, head-down tilt and microgravity conditions.

Introduction: The effects of microgravity are often simulated by head-down tilt (HDT). While data exist for intracranial pressure (ICP) during short-term HDT, no corresponding data exist for long-term exposure to HDT or microgravity. A mathematical model was developed to predict these unknown long-term responses. Predicted pressures include those in the cerebral vasculature, ventricular and extra-ventricular cerebrospinal fluid (CSF), and the brain tissue extracellular fluid.

Methods: A mathematical model was used to predict steady-state responses to various stimuli. Simulated CSF infusion tests were used to estimate model parameters such as the filtration coefficient of the cerebral capillary bed. Short-term HDT simulations validated the model. Further simulations predicted ICP responses to long-term HDT and microgravity.

Results: Constant-rate infusion simulations predict that the filtration coefficient of the cerebral capillary bed is between 4.46 and 5.15 x 10(-3) {[(ml x min(-1)) x mmHg(-1)] x 100 g(-1)}. Short-term HDT simulations reproduced clinical observations for venous sinus pressure and ICP. Further simulations produced the following relationships: first, ICP is unaffected by the expected changes in central artery pressure. Second, ICP changes in parallel with central venous pressure. Third, ICP increases 0.37 mmHg per 1 mmHg decrease in blood colloid osmotic pressure.

Discussion: Results suggest that despite the presence of tight capillary junctions in the brain, the cerebral capillary filtration coefficient is of the same order of magnitude as measured in the calf and forearm. Simulations also suggest that ICP in microgravity is significantly less than that in long-term HDT and may be less than that in the supine position on Earth.