Moderate sodium restriction does not alter lower body negative pressure tolerance.

Hypothesis: Space travel with exposure to microgravity leads to a significant reduction in orthostatic tolerance on return to Earth, for which countermeasures are only partially successful. The purpose of this study was to examine the effect of moderate dietary sodium restriction on tolerance to LBNP.

Methods: Eight healthy men, age 25.

The heart is not necessarily empty at syncope.

Background: Although extensively investigated, the mechanism(s) of post-spaceflight orthostatic intolerance has not been elucidated. Several researchers have proposed that the "trigger" for syncope is an empty ventricle, initiated when a hypercontractile state, possibly due to a sudden surge in epinephrine, causes the walls of the left ventricle to touch leading to a profound sympatho-inhibition and intense vagal stimulation.

Hypothesis: A markedly reduced left ventricular end systolic volume (LVESV) achieved during progressive, presyncopal-limited lower body negative pressure (LBNP) is the trigger for syncope.

Validation of impedance cardiography during lower body negative pressure.

Background: Both echocardiographic and impedance cardiographic techniques have been used individually for the determination of stroke volume (SV) during lower body negative pressure (LBNP). Impedance cardiography has not been validated during LBNP.

Hypothesis: The purpose of this study was to determine both the absolute values for SV and the change in SV for each stage of LBNP using both impedance and echocardiographic techniques during staged LBNP to presyncope.