New Bed Rest Paradigm: Strict adherence of -6°-Head Down Tilt Bed Rest

Abstract

Poster Abstract: The recent Medium-term Bed Rest Study - VaPER was conducted in 2017 as joint project between NASA and DLR in Cologne, Germany. The primary aim of this comprehensive study was to test influences of the novel paradigm for microgravity simulation combining - 6° head down tilt (HDT) with increased ambient CO2 (0.5%) on cerebral and ocular anatomy and physiology. The study focused on the Spaceflight Associated Neuro-ocular Syndrome (SANS, formerly the visual impairment and intracranial pressure, or VIIP syndrome). On the ISS, astronauts are exposed to microgravity and increased ambient CO2 (hypercapnia). Hypercapnia potently augments cerebral blood flow which may acutely increase intracranial blood volume and intracranial pressure. When added to the cephalad fluid shifts in space, hypercapnia may further raise intracranial pressure and hence SANS risk. HDT has stood the test of time as ground-based microgravity analogue for space medicine research. However, unlike in microgravity where all hydrostatic gradients are abolished, a gravitation vector is still present in HDT bed rest which may elicit different physiological responses. Indeed, -6° HDT studies have not reproduced cerebral or ophthalmic findings observed in astronauts affected by SANS. Possibly, -6° HDT alone may not create head-ward fluid shifts mimicking true microgravity conditions. Steeper HDT tilt angles are unrealistic for long-duration bed rest studies. On the other hand, all experimental bed rest studies thus far provided pillows. Consequently, we sought to establish whether the bed rest model for SANS-related research could be improved by mimicking hypercapnia on board the ISS and stricter controlled -6° HDT bed rest position. During the VaPER Study no pillows were permitted except for a thin pillow when subjects were lying on their sides. Subjects were camera-monitored 24 h per day and feedback on head position was provided during waking hours whenever needed. Test subjects showed an excellent adherence and maintained strict -6° HDT throughout the study. Adherence is of central importance for successful completion of bed rest studies. One prerequisite is careful screening before study inclusion with an emphasis on psychological aspects. The :envihab facility and the experienced staff at the DLR-Institute of Aerospace Medicine create an atmosphere of best possible well-being during confinement. Within the next NASA-ESA-DLR Longterm-Bedrest Study (ABGRESA-Study) planned in 2019 we will get more information to determine whether strict -6° HDT bed rest, hypercapnia, or both combined are required to model SANS-like findings.