ICP During Daily Life in Healthy Adults: What Does Microgravity Add to the Mix?

Abstract

PURPOSEDue to visual changes seen in astronauts, the primary objective is to make the first direct, invasive measurements of intracranial pressure (ICP) during changes in hydrostatic gradients experienced during daily life and induced by simulated and real microgravity.METHODSSubjects are four healthy adults who previously had an Ommaya reservoir implanted, which allows continuous pressure measurements to be made. Central venous pressure (CVP) is measured by a peripherally inserted central catheter. Hemodynamics are obtained upright, supine, then after 5 min, 3 hrs and 24 hrs of 6° head‐down tilt (HDT). Measurements are repeated during real microgravity (parabolic flight).RESULTSICP increased by transitioning from the sitting to supine position (4±2 vs 15±2 mmHg); ICP increased further with HDT (17±3, Δ2 mmHg), but returned towards normal values after 3 hrs (16 mmHg) and 24 hrs (14 mmHg). In comparison to the supine position, CVP tended to decrease over time during HDT (5 min, Δ1 mmHg; 3 hrs, Δ2 mmHg; 24 hrs, Δ1 mmHg). ICP increased during the 1.8g (Gx) phase of the parabola (from supine 17±2 to 25±2). However, ICP fell dramatically during 0g (to 13±4, Δ12 mmHg). The magnitude of the reduction in ICP is the combination of removing the 1.8 Gx and the subsequent fall in CVP (1g, 5±4; 1.8g, 7±5; 0g, 4±3).imageConclusionsElevated ICP is unlikely to be the culprit for visual impairment in space. Alternatively, visual impairments may be simply due to consistently “elevated” ICP relative to the upright posture, due to the inability to “stand‐up” in space.