Dry immersion

The observation of vision problems experienced by some astronauts during long duration spaceflights onboard the International Space Station (ISS) has led to the hypothesis that cephalic fluid shifts induced by the microgravity environment result in an increased intraocular pressure (IOP) and intracranial pressure (ICP). A recent study has provided support for this hypothesis demonstrating an increase in ICP and IOP with cephalic fluid shifts induced by head down tilt and a reduction in ICP and IOP when fluid was shifted away from the head using lower body negative pressure (Macias et al., 2015). A seven-day head down tilt study also noted cephalic fluid shifts indicated by a significant enlargement of the jugular vein (JV) which was accompanied by an enlargement of the eye fundus vein and the presence of edema at the eye fundus level but not an increase in IOP (Besnard et al., 2002).
 

Dry immersion (DI) has been proposed as method of mimicking the effects of microgravity exposure on Earth (Shulzhenko, 1977) with various durations of use (10 hours to 28 days). During DI, the subject is seated in a semi-recumbent position inside of a water tank (Figure 1). Bags are used to separate the subject from the water such that the subject is not in contact with the water. The subject is submerged up to the neck level with the water pressure on the body promoting the transfer of interstitial fluid into the vascular system and a total shift of fluid towards the cephalic area (Navasiolova et al., 2011; Miki et al., 1989). During the first four hours of immersion, cardiac stroke volume has been found to increase with peripheral vascular resistance, heart rate and diastolic blood pressure all decreasing (Navasiolova et al., 2011). Plasma volume has been found to be significantly reduced after one day of DI with further reductions seen after three days (Shulzhenko et al 1980). However, there is currently no published data on the central or peripheral venous flow or pressure changes during the first hours of DI.

The purpose of our last study was to investigate changes in venous volume and flow during the initial hours of DI and to relate these potential changes to changes in ICP. As DI is expected to result in a fluid shift towards the head, we hypothesized that both the JV and the portal vein (PV) would increase in volume similar to what has been observed with spaceflight (Arbeille et al., 2015). Additionally, it was hypothesized that the increase in JV volume would result in an increase in ICP and, consequently, an increase in MCVv. Finally, it was speculated that thyroid volume would also be increased providing further indication of a cephalic fluid shift. (Paper in press)