Fluid shifts

Proposal : integrated fluid shift (NASA project)

30% of American astronauts have developed ocular refraction change after long duration space flight on the International Space Station (ISS). Recent findings have also included structural changes of the eye (papilledema, globe flattening, choroidal folds) and the optic nerve (sheath dilatation, tortuosity and kinking), as well as imaging signs and lumbar puncture data indicative of elevated intracranial pressure (ICP).

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 transiently elevated intracranial pressure.

Consult the list of participants

MIR HDT Card Art vein 2001
Vein Stag 2002
Dry immersion
V imaging Vein

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.

Several ISS crewmembers have reported consistent worsening of nasal congestion and associated symptoms in late afternoon hours, necessitating topical and systemic decongestant use (Johnston, Sargsyan, personal communications). Although several explanations have been entertained, food (salt) and water intake are likely to have provoked these symptoms through postprandial modification of fluid balance or increase in the circulating volume that manifests in the most susceptible individuals.

The purpose of the proposed work is to objectively characterize the changes in fluid distribution, including intra/extracellular and intra/extravascular fluid shifts, by applying advanced non-invasive assessment technologies before, during and after long duration space flight. Additionally, we will examine the relationship between the type and magnitude of the fluid shift with any effects on eye morphology and vision disturbances, intraocular pressure (IOP), and measures of intracranial pressure. Further, we seek to determine whether the magnitude of fluid shifts during space flight, as well as the above effects of those shifts can be predicted based upon crewmember baseline data and responses to acute head-down tilt tests performed before launch. Finally, we propose to evaluate the effect of lower body negative pressure (LBNP) on the above parameters.

To our knowledge, this is the first attempt to systematically determine the impact of the fluid distribution in microgravity on a comprehensive set of structural and functional measures including, but not limited to, those related to intracranial pressure, vision, morphology of the eye and its adnexa, and the vascular systems of the head and neck, during and after long duration space flight. The study design and methodology are based on the extensive relevant experience of the Investigators, including many successful ground-based, space flight analogue, and space flight projects and investigations.

• Hypothesis 1 : Prolonged microgravity-induced, headward volume and pressure shifts promote elevation of intracranial pressure and result in alterations in crewmembers’ vision.

• Hypothesis 2 : Regional headwards fluid shifts in-flight are documented by increased cephalad venous dimensions (jugular, temporal, intracranial veins) and flow characteristics (Intracranial vein velocity), skin and soft tissue thickness.

• Hypothesis 3 : Splanchnic venous congestion (detected by portal vein size) contributes to head-ward volume shift, but is not in communication with the veins of head and neck. Thus, the venous congestion should be different in these two compartments.

Measurements in relation with fluid redistribution and Intracranial pressure will be performed at pre, in, post flight at REST.

• Jugular, Temporal vein for evaluating intracranial and extra-cranial vein pooling.
• Portal vein for evaluating splanchnic venous stagnation.
• Soft tissue thickness at cephalic level for evaluating tissue filtration at a level close to brain.
• Intracranial vein (Middle cerebral vein) flow velocity by Doppler (with valsalva maneuver) - eye fundus echography and ocular pressure.
• Intra cranial pressure by non traumatic method.

Measurements will be performed also during pre in and postflight LBNP (at 20min at –20mmHg). Jugular, Portal, Temporal, vein will be assessed for evaluating intracranial and extracranial vein emptying.