Bursting into space: alterations of sympathetic control by space travel.
OBJECTIVE
Astronauts return to Earth with reduced red cell masses and hypovolaemia. Not surprisingly, when they stand, their heart rates may speed inordinately, their blood pressures may fall, and some may experience frank syncope. We studied autonomic function in six male astronauts (average +/- SEM age: 40 +/- 2 years) before, during, and after the 16-day Neurolab space shuttle mission.


METHODS
We recorded electrocardiograms, finger photoplethysmographic arterial pressures, respiration, peroneal nerve muscle sympathetic activity, plasma noradrenaline and noradrenaline kinetics, and cardiac output, and we calculated stroke volume and total peripheral resistance. We perturbed autonomic function before and during spaceflight with graded Valsalva manoeuvres and lower body suction, and before and after the mission with passive upright tilt.


RESULTS
In-flight baseline sympathetic nerve activity was increased above pre-flight levels (by 10-33%) in three subjects, in whom noradrenaline spillover and clearance also were increased. Valsalva straining provoked greater reductions of arterial pressure, and proportionally greater sympathetic responses in space than on Earth. Lower body suction elicited greater increases of sympathetic nerve activity, plasma noradrenaline, and noradrenaline spillover in space than on Earth. After the Neurolab mission, left ventricular stroke volume was lower and heart rate was higher during tilt, than before spaceflight. No astronaut experienced orthostatic hypotension or pre-syncope during 10 min of post-flight tilting.


CONCLUSIONS
We conclude that baseline sympathetic outflow, however measured, is higher in space than on earth, and that augmented sympathetic nerve responses to Valsalva straining, lower body suction, and post-flight upright tilt represent normal adjustments to greater haemodynamic stresses associated with hypovolaemia.
Eckberg
D L
DL
Department of Medicine and Physiology, Hunter Holmes McGuire Department of Veterans Affairs Medical Center and Medical College of Virginia at Virginia Commonwealth University, Richmond, VA, USA.
D000328:Adult; D017704:Baroreflex / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D001810:Blood Volume / Q000502:physiology; D018475:Head-Down Tilt / Q000502:physiology; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D007024:Hypotension, Orthostatic / Q000503:physiopathology; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000502:physiology; D009638:Norepinephrine / Q000378:metabolism; D010543:Peroneal Nerve / Q000502:physiology; D013026:Space Flight; D016683:Supine Position / Q000502:physiology; D013564:Sympathetic Nervous System / Q000502:physiology*; D013575:Syncope / Q000503:physiopathology; D014636:Valsalva Maneuver / Q000502:physiology
D009638:Norepinephrine