Reducing Intracranial Pressure by Reducing Central Venous Pressure: Assessment of potential countermeasures to spaceflight associated neuro-ocular syndrome.

Spaceflight-associated neuro-ocular syndrome (SANS) involves unilateral or bilateral optic disc edema, widening of the optic nerve sheath, and posterior globe flattening. Due to posterior globe flattening, it is hypothesized that microgravity causes a disproportionate change in intracranial pressure (ICP) relative to intraocular pressure. Countermeasures capable of reducing ICP include thigh cuffs and breathing against inspiratory resistance. Due to the coupling of central venous (CVP) and intracranial pressure, we hypothesized that both ICP and CVP will be reduced during both countermeasures. In four male participants (32±13 yrs) who were previously implanted with Ommaya reservoirs for treatment of unrelated clinical conditions, ICP was measured invasively through these ports. Subjects were healthy at the time of testing. CVP was measured invasively by a peripherally inserted central catheter. Participants breathed through an Impedance Threshold Device (ITD, -7 cm.H2O) to generate negative intrathoracic pressure for five-mins, and subsequently, wore bilateral thigh cuffs at 30-mmHg for two-mins. Breathing through an ITD reduced both CVP (6±2 vs 3±1 mmHg; P=0.02) and ICP (16±3 vs 12±1 mmHg; P=0.04) compared to the supine posture, which was not observed during the free breathing condition (CVP, 6±2 vs 6±2 mmHg; P=0.87 and ICP, 15±3 vs 15±4 mmHg; P=0.68). Inflation of the thigh cuffs to 30-mmHg caused no meaningful reduction in CVP in all four individuals (5±4 vs 5±4 mmHg; P=0.1), coincident with a minimal reduction in ICP (15±3 vs 14±4 mmHg; P=0.13). The application of inspiratory resistance breathing resulted in reductions in both ICP and CVP, likely due to intrathoracic unloading.