Plasma volume, vascular measures (area and diameter of head and neck vessels among other ultrasound measures), and ocular measures (intraocular pressure, optical coherence tomography, and ocular ultrasound) were collected on all subjects. Non-invasive internal jugular venous pressure was collected using ultrasound.

RESULTS:
For the majority of cardiovascular and ocular variables studied, there was no significant effect of 14 days of bed rest. The bed rest-induced decrease in plasma volume did not appear to differ from plasma volume losses that occurred during previous NASA bed rest studies using standard dietary practices. Arterial diameters and flow were not different from pre- to post-bed rest at rest during the whole body tilting protocol (-20, -10, 0, +10, +20 degrees of tilt). intraocular pressure responded to posture changes, but did not change pre- to post-bed rest. Similarly, axial length (via ultrasound) and retinal nerve fiber layer (via Optical Coherence Tomography) were not different after bed rest. In contrast, internal jugular vein area (via ultrasound) was lower after bed rest in the head-down tilt postures, suggesting a decrease in venous compliance. This effect was transient and recovered to pre-bed rest levels after three days of normal ambulation. Unfortunately, the investigators were unable to recruit a sufficient number of older subjects to test the interactions between aging and bed rest on the parameters studied. These results suggest that a high salt diet more closely matching that of astronauts during space flight does not increase the likelihood of developing signs of VIIP alone after 14 days of head-down tilt bed rest.