Fluid Shifts Before, During and After Prolonged Space Flight and Their Association with Intracranial Pressure and Visual Impairment (Fluid Shifts) - 03.19.15
More than half of American astronauts experience vision changes and anatomical alterations to parts of their eyes during and after long-duration spaceflight. It is hypothesized that the headward fluid shift that occurs during space flight leads to increased pressure in the brain, which may push on the back of the eye, causing it to change shape. The Fluid Shifts Before, During, and After Prolonged Space Flight and Their Association with Intracranial Pressure and Visual Impairment (Fluid Shifts) investigation measures how much fluid shifts from the lower body to the upper body, in or out of cells and blood vessels, and determines the impact these shifts have on fluid pressure in the head, changes in vision and eye structures. Science Results for Everyone
Information Pending Experiment Details
OpNom: Fluid Shifts
NASA Johnson Space Center, Human Research Program, Houston, TX, United States
Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)
Human Exploration and Operations Mission Directorate (HEOMD)
Scientific Discovery, Space Exploration
ISS Expedition Duration
March 2015 - Ongoing
Previous ISS Missions
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- More than half of American astronauts have developed ocular refraction changes after long duration space flights. Findings have also included structural changes of the eye, as well as signs of elevated intracranial pressure. While short duration space flight is also characterized by vision disturbances, these are generally transient and do not appear to have lasting impacts on the structure or function of the eye.
- Changes in vision and eye structure are thought to be the result of prolonged exposure to space flight-induced headward fluid shifts and elevated intracranial pressure. However, this hypothesis has never been systematically tested. The purpose of this investigation is to characterize the space flight-induced fluid shift, including intra- and extravascular shifts, intra- and extracellular shifts, changes in total body water and lower vs. upper body shifts. Noninvasive techniques are used to assess arterial and venous dimensions and flow parameters, ocular pressure and structure, and changes in intracranial pressure.
- Lower body negative pressure is to be investigated for its ability to mitigate some of the effects of the space flight-induced fluid shift.
- Results from this investigation can help to define the causes of the ocular structure and vision changes associated with long duration space flight, and assist in the development of countermeasures.
More than half of American astronauts have developed ocular refraction change after long duration space flight on the International Space Station (ISS). Findings have also included structural changes of the eye (papilledema, globe flattening, choroidal folds) and the optic nerve (sheath dilation, tortuosity and kinking), as well as imaging signs and lumbar puncture data indicative of elevated intracranial pressure (ICP). This pattern is referred to as the visual impairment and intracranial pressure (VIIP) syndrome. While short duration space flight is also characterized by vision disturbances, these are generally transient and do not appear to have lasting impacts on the structure or function of the eye. Changes in vision, eye and adnexa morphology, are hypothesized to be the result of space flight-induced cephalad fluid shifts and elevated intracranial pressure. This hypothesis, however, has not been systematically tested. In earlier anecdotal publications, ICP elevation in long-duration space flight has been inferred, but without association with structural or functional changes of the eye. Furthermore, while fluid shifts and compartmentalization during short duration space flight (Space Shuttle missions) have been studied, the fluid distribution patterns and their effects on intracranial pressure or the structure and function of the sensory organs in the course of long-duration space flight are not well known.
The purpose of this study is to characterize fluid distribution and compartmentalization associated with long-duration spaceflight, and to correlate these findings with vision changes and other elements of the VIIP syndrome. We also seek to determine whether the magnitude of fluid shifts during spaceflight, as well as the VIIP-related effects of those shifts, is predicted by the crewmember’s preflight conditions and responses to acute hemodynamic manipulations (such as head-down tilt). Lastly, the investigation team evaluates the patterns of fluid distribution in ISS astronauts during acute reversal of fluid shifts through application of lower body negative pressure (LBNP) interventions to characterize and explain general and individual responses.
This experiment examines a variety of physiologic variables in 10 long-duration ISS crewmembers before, during and after long duration space flight. Measures include: (1) fluid compartmentalization (total body water by D2O, extracellular fluid by NaBr, intracellular fluid by calculation, plasma volume by CO rebreathe, interstitial fluid by calculation); (2) forehead/eyelids, tibia, calcaneus tissue thickness (by ultrasound) ; (3) vascular dimensions by ultrasound (jugular veins, cerebral and carotid arteries, vertebral arteries and veins, portal vein); (4) vascular dynamics by MRI (head/neck blood flow, cerebrospinal fluid pulsatility); (5) ocular measures (optical coherence tomography, intraocular pressure, 2-dimensional ultrasound including optic nerve sheath diameter, globe flattening, and retina-choroid thickness, Doppler ultrasound of ophthalmic and retinal arteries, and veins); (6) cardiac variables by ultrasound (inferior vena cava, tricuspid flow and tissue Doppler, pulmonic valve, stroke volume, right heart dimensions and function, four-chamber views); and (7) ICP measures (tympanic membrane displacement, distortion-product otoacoustic emissions, and ICP calculated by MRI). On the ground, acute head-down tilt induces cephalad fluid shifts, whereas LBNP opposes these shifts. Controlled Mueller maneuvers manipulate cardiovascular variables. Through interventions applied before, during, and after flight, the relationship between fluid shifts and the VIIP syndrome can be fully evaluated.
Fluid Shifts investigates the causes for severe and lasting physical changes to astronaut's eyes. Because the headward fluid shift is a hypothesized contributor to these changes, reversing this fluid shift with a lower body negative pressure device is investigated as a possible intervention. Results from this study may help to develop preventative measures against lasting changes in vision and eye damage.
Results from the Fluid Shifts investigation improve scientists' understanding of how blood pressure in the brain affects eye shape and vision, which could also benefit people confined to long-term bed rest, or suffering from disease states that increase swelling and pressure in the brain.
The Fluid Shifts experiment requires 10 subjects. Dilution Measures occur at Flight Day (FD) 45 and Return minus 45 for mission durations of six months, with an additional collection at FD 150 for subjects with a mission duration of one year. The Imaging Measures baseline activity occurs within 10 days of each performance of Dilution Measures collections. The Lower Body Negative Pressure (LBNP) Imaging Measures are completed within 30 days of Dilution Measures operations. There is a requirement for real-time data downlink for remote guidance for all ultrasound, OCT, and IOP scans. Additional medical monitoring requirements are in place for the LBNP collections using the Russian Chibis. An additional crewmember acting as an on-orbit medical monitor and real-time monitoring of physiological data on the ground are required during these sessions. Also, LBNP is limited to no more than 60 minutes at a time, resulting in the split of the LBNP collections over 2 days in-flight.
Dilution Measures includes determination of fluid compartmentalization via assessment of total body water (via saliva and urine analysis) and extracellular and intracellular fluid volume (via blood and urine analysis). The Imaging Measures Baseline activities use various hardware to measure a multitude of anatomical and physiological data points:
Ultrasound measures of fluid shifts:
- Arterial and venous measures of head and neck
- Cardiac, ophthalmic, and portal vein measures
- Tissue thickness of lower and upper body
Other physiological measures:
- Intracranial Pressure (CCFP/DPOAE)
- Intraocular Pressure (Tonopen/iCare and Ultrasound)
- Ocular Structure (OCT)
- Blood Pressure / Heart Rate / Vascular Resistance
The LBNP Imaging Measures are a repeat of a subset of the Baseline data points, excluding those that are not available due to the subject wearing Chibis.
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Ground Based Results Publications
Macias BR, Liu J, Grande-Gutierrez N, Hargens AR. Intraocular and intracranial pressures during head-down tilt with lower body negative pressure. Aviation, Space, and Environmental Medicine. 2015 January 1; 86(1): 3-7. DOI: 10.3357/AMHP.4044.2015. PMID: 25565526.