During long-duration space flight, changes to ocular structures and visual function occur which is hypothesized to be the result of the chronic headward fluid shift that is induced by weightlessness. A variety of countermeasures aimed at reversing the headward fluid shift have been tested in ground-based studies. However, no countermeasure has been implemented during spaceflight because it is unknown which countermeasure, single or in combination with another, would be the most effective. Lower body negative pressure (LBNP), venoconstrictive thigh cuffs (VTC), and an impedance threshold device (ITD) breathing are countermeasures that partially reverse headward fluid shift. LBNP counteracts headward fluid shift by drawing the fluid from the upper body and shifting it into the legs. VTC reduces blood volume from the central circulation and ITD lowers intrathoracic pressure and augments venous return.
Although LBNP, VTC, and ITD have been shown previously to partially reverse the headward fluid shift, no single countermeasure has successfully reversed values to those observed in the upright position. LBNP, VTC, and ITD individually represent specific and unique physiological strategies to reduce headward fluid shift. When these countermeasures are combined, a flexible and sustained strategy can be developed to increase the magnitude and duration of effects.
The objective of this three-part study is to determine if an individual countermeasure or a combination of countermeasures can reduce the headward fluid shift induced when moving from the upright to the supine posture and determine the effectiveness of using a countermeasure during sleep. This experiment has the following specific aims:
For Aim 1, the effectiveness of the selected countermeasures, LBNP, VTC, and ITD, to reverse headward fluid shift will be evaluated. Ten subjects will visit NASA’s Johnson Space Center (JSC) Cardiovascular and Vision Laboratory three times for data collection. During each visit, subjects will be studied in the seated and supine position (baseline) and in the supine position during individual countermeasure application, during recovery after the individual countermeasure is released, during combined countermeasures and while maintaining one passive countermeasure after the combined countermeasure approach. Individual countermeasure application include: LBNP, VTC and ITD. Combined countermeasure and subsequent maintained conditions include: LBNP+VTC, maintaining VTC after LBNP+VTC, LBNP+ITD, maintaining ITD after LBNP+ITD, ITD+VTC, maintaining VTC after ITD+VTC, and LBNP+VTC+ITD, and maintaining VTC+ITD after LBNP+VTC+ITD.
Each data collection session consists of a block of measurements that will be implemented in the same order each time, lasting approximately 45 minutes. The measurement block begins with five minutes of stabilization then ultrasound imagining of the bilateral internal jugular veins (cross-sectional area), cardiac ultrasound, otoacoustic emissions (OAE) measurements (non-invasive intracranial pressure), ocular ultrasound (left eye), bilateral internal jugular vein pressure (IJVP), intraocular pressure (IOP) measurement (right eye), and bilateral optical coherence tomography (OCT) imaging. This block of measurements is taken during each condition (baseline, countermeasures and recovery/maintain conditions).
For Aim 2, co-investigators from the University of California San Diego and the University of Texas Health Science Center will examine the effects of one countermeasure using quantitative magnetic resonance images (qMRI). Baseline data will be collected in the supine position without the countermeasure, and then again with the countermeasure. The countermeasure will be applied 10 minutes prior to the start of data collection. This will help determine the effect countermeasures have on CSF, white matter and gray matter volume, aqueductal CSF pulsatility, and global cerebral perfusion. MRI scanning time will be minimized by only utilizing the most effective countermeasure from Aim 1.
For Aim 3, co-investigators from the University of Texas Southwestern will develop a LBNP device that can be worn for eight hours during sleep. Subjects will participate in a cross-over design study with a 3 day supine bedrest with no countermeasure, and then a 3-day supine bedrest with 8h of LBNP during sleep every night. The countermeasure will be developed to be an easy to use sleep sac that can generate -20 mmHg LBNP that can be worn at night.
The experiment is in progress. Results will be available at a later date.
VCCM ARM 1:
Day 1: Single Countermeasure Exposure and Recovery (LBNP, VTC, ITD)
Day 2: Combination of Countermeasures (LBNP + VTC; LBNP + ITD; VTC + ITD; LBNP + VTC + ITD)
Day 3: Combination of Countermeasures (LBNP + VTC; LBNP + ITD; VTC + ITD; LBNP + VTC + ITD)
VCCM ARM 2: Cerebral MRI measurements with and without countermeasure(s) selected in Aim 1
VCCM ARM 3: Ocular and cardiovascular measures during two separate bedrest studies:
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