Despite ongoing research efforts to elucidate the etiology of the Spaceflight Associated Neuro-ocular Syndrome (SANS) and develop potential countermeasures, visual impairment remains a critical risk factor for long-duration human space flight. A growing body of evidence suggests that microgravity-induced cephalad fluids and redistribution of blood volume, especially within the intracranial space, may play a major role in the development of SANS. The goal of this study was to assess the physiological changes related to fluid shifts during a ground-based space flight analog and to assess artificial gravity (AG) via short-arm human centrifugation as a countermeasure against cephalad fluid shifting and ultimately for SANS.
This experiment had the following specific aims:
1) Determine the physiological effects of gravitational vectors through various body positions including head-up tilt, supine and head-down tilt on body fluid distribution as well as cerebral hemodynamic and cardiovascular physiology using novel non-invasive technologies.
2) Determine the effects of short-arm centrifugation on cerebral hemodynamics and full body fluid distribution.
3) Utilize multiple experimental platforms to quantify the ability of AG to reinstate supine and upright fluid distributions as a countermeasure to cephalad fluid shifting.
Several of the proposed non-invasive devices have not yet been tested in an extreme environment setting, such as during hypergravity exposure. The eventual use of non-invasive technologies for space flight missions will require them to be both easily operated and able to withstand varying environmental stressors. This research project allowed for a feasibility analysis of the implemented non-invasive technologies during hypergravity including technological limitations, compatibility, and optimization of signals, which may give insights into their suitability for use in space flight in the current design.
Results from human testing on the short-arm human centrifuge demonstrated that centrifugation up to 2g at the feet produced significant fluid shifting to the lower limbs. Notably, near-infrared spectroscopy (NIRS) was utilized to continuously measure cerebral perfusion, blood oxygenation, and hemodynamics as well as blood volume in multiple body segments (head, chest, upper thigh, and calf). Significant shifting of blood volume from the upper body (head and chest) to the lower body (thigh and calf) was seen immediately upon ramp-up of the short-arm human centrifuge. The shifting of blood reached a peak after about 1.5 min of 2g centrifugation (at the feet) and this shift was sustained for the duration of the 2g centrifugation. Immediately upon ramp-down of the centrifuge, blood volume began to shift back towards the chest and head and upon cessation of the centrifuge, the blood volume returned to pre-centrifugation baseline. In addition, internal jugular vein (IJV) dimensions were monitored continuously during centrifugation. The IJVs are the primary outflow pathway of the cerebral venous system in the supine posture on Earth, and venous congestion in space has been hypothesized to be a major factor in SANS. Thus, assessment of the IJV provides a direct measure of cerebrovascular outflow status. Similar to the changes in blood volume seen with NIRS, the IJV displayed immediate and significant changes in cross-sectional area during centrifugation. From supine rest, the area of the IJV decreased 56% during 0.5-g exposure, 96% during 1-g exposure, and 94% during 2-g exposure. This change in IJV area was sustained during the entire duration of centrifugation exposure. Similar to the blood volume distribution measured with NIRS, IJV area immediately returned to pre-centrifugation baseline upon cessation of the short-arm human centrifuge.
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