Hypovolemia Induced Orthostatic Hypotension in Presyncopal Astronauts and Normal Subjects Relates to Hypo-sympathetic Responsiveness

Spaceflight-induced hypovolemia requires astronauts to mount a hyper-sympathetic response in order to complete a tilt test on landing day. We tested the hypothesis that experimentallyinduced hypovolemia, in the absence of spaceflight, also requires a hyper-sympathetic response during tilt. We studied 17 subjects during normovolemia and hypovolemia. Plasma volume, tilt tolerance, supine and standing arterial pressure, heart rate and plasma norepinephrine (NE) levels were measured. During hypovolemia tilts, subjects who increased their norepinephrine release above that during normovolemia tilts were significantly more likely to complete the test. These results indicate that adequate norepinephrine release during tilt can overcome the hemodynamic compromises of hypovolemia. This model may be used to identify susceptible astronauts prior to flight so that prospective, individualized countermeasures can be developed.