Modeling steady-state intracranial pressures in supine, head down tilt, and microgravity conditions.
S.A. Stevens, Penn State Erie, The Behrend College, School of Science
W.D. Lakin, The University of Vermont, Department of Mathematics and Statistics
P.L. Penar, The University of Vermont, Division of Neurosurgery
ABSTRACT
Introduction: The effects of microgravity are often simulated by head down tilt (HDT). While data exists for intracranial pressure (ICP) during short-term HDT, no corresponding data exists for long-term exposure to HDT or microgravity. A mathematical model is 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: The mathematical model is used to predict steady-state responses to various stimuli. Simulated CSF infusion tests are used to estimate model parameters such as the filtration coefficient of the cerebral capillary bed. Short-term HDT simulations validate the model. Further simulations predict 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/min)/mmHg)/100g). Short-term HDT simulations reproduce clinical observations for venous sinus pressure and ICP. Further simulations produce 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 one 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.
Key Words: lumped-parameter model, fluid shifts, cerebrospinal fluid, brain, filtration coefficient