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 objectives of this three-part study were 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 evaluated the following specific aims:
For Aim 1, the effectiveness of the selected countermeasures, LBNP, VTC, and ITD, to reverse headward fluid shift was evaluated. Ten subjects visited NASA’s Johnson Space Center (JSC) Cardiovascular and Vision Laboratory three times for data collection. During each visit, subjects were 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 included: LBNP, VTC and ITD. Combined countermeasure and subsequent maintained conditions included: 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 consisted of a block of measurements implemented in the same order each time, lasting approximately 45 minutes. The measurement block began with five minutes of stabilization followed by 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 was 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 examined the effects of one countermeasure using quantitative magnetic resonance images (qMRI). Baseline data was collected in the supine position without the countermeasure, and then again with the countermeasure. The countermeasure was applied 10 minutes prior to the start of data collection. This approach was to help determine the effect countermeasures have on CSF, white matter and gray matter volume, aqueductal CSF pulsatility, and global cerebral perfusion.
For Aim 3, co-investigators from the University of Texas Southwestern developed a LBNP device that was able be worn for eight hours during sleep. Subjects participated in a cross-over design study with a 3 day supine bedrest with no countermeasure, and then a 3-day supine bedrest with 8 hours of LBNP during sleep every night. The countermeasure was developed to be an easy-to-use sleep sac that can generate -20 mmHg LBNP that can be worn at night.
Acute use of LBNP partially reversed the posture induced headward fluid shift by reducing IJV area and pressure, as well as non-invasive ICP. Most measures returned to supine baseline values after release of the individual countermeasures during the 45 min recovery phase, demonstrating that the countermeasures are maximally effective while actively applied. However, the use of combined countermeasures did not reverse the headward fluid shift to a greater degree than with LBNP as an individual countermeasure. LBNP, and to a lesser extent VTC and ITD, represents a promising mechanical countermeasure to reverse the headward fluid shift during spaceflight and warrants further investigation during longer-duration ground-based analog and spaceflight studies to determine effectiveness as a SANS countermeasure.
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