The primary objective of this application was to make the first direct, invasive measurements of ICP and cerebral hemodynamics during changes in hydrostatic gradients induced by simulated (bed rest) and real (parabolic flight) microgravity. In order to accomplish these objectives, the following hypotheses was investigated:
Hypothesis 1: The transition from upright to supine posture increases intracranial and venous pressures that result in minimal changes in cerebral blood flow, oxygen delivery, and cerebral autoregulation. Additional gravitational loading and unloading by maneuvers result in small additional changes compared to the difference from standing to supine.
Hypothesis 2: True microgravity induced by parabolic flight will produce cerebrovascular changes that are qualitatively, and quantitatively similar to those observed during bed rest.
Specific Aim 1: The subject pool was from cured of a cancer or brain infection and whom have had direct access to the brain obtained via an Ommaya reservoir placed to allow prophylactic brain chemotherapy. Such patients have no intracranial or cardiovascular pathology, and have easy access to direct measurement of ICP with little risk. Simultaneous measurement of jugular venous pressure (PICC line) and intra-arterial pressure (arterial line or Finapres) was combined with echo-Doppler ultrasound of the middle cerebral artery (transcranial Doppler) carotid and jugular size, velocity, and flow, plus near infrared spectroscopy to assess oxygen delivery/utilization to allow a comprehensive assessment of cerebrovascular hemodynamics during routine gravitational transients. Measurements were repeated with modest increases in arterial carbon dioxide tension (PaCO2), and during aerobic and strength exercise to determine the independent and additive effects of these baffling stimuli.
Specific Aim 2: The same patients as in Aim 1 were invited to participate in a second study involving parabolic flight to achieve true microgravity. The same instrumentation allowed contrast between real microgravity, and usual terrestrial changes in hydrostatic gradients during daily life. Similar to aim 1, measurements were obtained at rest, during exercise, and during small increases in PaCO2 (10 parabolas each).
RESULTS: The investigators have clarified the key component of the physiology of the intracranial space in microgravity, which will go a long way to eliminating the Vision Impairment and Intracranial Pressure (VIIP) syndrome. They have developed a collaboration with Under Armour, Inc., to develop a wearable garment to deliver lower body negative pressure (LBNP) and reduce ICP not only in space, but also on Earth. For example, a major Earth-based benefit to society will be the application of LBNP in a clinical environment. Alongside pharmacological interventions, placing patients with intracranial hypertension in the semi-recumbent position is standard practice to lower ICP. This slight hydrostatic gradient reduces ICP, whilst maintaining arterial blood pressure and thus cerebral perfusion pressure. LBNP may provide a more controlled and robust intervention within a hospital and field-based environment. Indeed, pathological pressure waves are often observed during sleep when patients are in the supine position. Nocturnal LBNP may provide a novel method to lower mean ICP and improve intracranial stability during this critical period. Given the robust observation that simply placing the head on a pillow lowers ICP, the combination of low level LBNP and head elevation may prove optimal. If a practical and comfortable device is developed, LBNP could also be used to reduce the incidence of chronic headache in patients with pseudotumor cerebri (raised ICP), as well as patients with traumatic brain injury.
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