Influence of microgravity on astronauts' sympathetic and vagal responses to Valsalva's manoeuvre.
When astronauts return to Earth and stand, their heart rates may speed inordinately, their blood pressures may fall, and some may experience frank syncope. We studied brief autonomic and haemodynamic transients provoked by graded Valsalva manoeuvres in astronauts on Earth and in space, and tested the hypothesis that exposure to microgravity impairs sympathetic as well as vagal baroreflex responses. We recorded the electrocardiogram, finger photoplethysmographic arterial pressure, respiration and peroneal nerve muscle sympathetic activity in four healthy male astronauts (aged 38-44 years) before, during and after the 16 day Neurolab space shuttle mission. Astronauts performed two 15 s Valsalva manoeuvres at each pressure, 15 and 30 mmHg, in random order. Although no astronaut experienced presyncope after the mission, microgravity provoked major changes. For example, the average systolic pressure reduction during 30 mmHg straining was 27 mmHg pre-flight and 49 mmHg in flight. Increases in muscle sympathetic nerve activity during straining were also much greater in space than on Earth. For example, mean normalized sympathetic activity increased 445% during 30 mmHg straining on earth and 792% in space. However, sympathetic baroreflex gain, taken as the integrated sympathetic response divided by the maximum diastolic pressure reduction during straining, was the same in space and on Earth. In contrast, vagal baroreflex gain, particularly during arterial pressure reductions, was diminished in space. This and earlier research suggest that exposure of healthy humans to microgravity augments arterial pressure and sympathetic responses to Valsalva straining and differentially reduces vagal, but not sympathetic baroreflex gain.
Cox
James F
JF
Department of Medicine and Physiology, Hunter Holmes McGuire Veterans Affairs Medical Center and Medical College of Virginia at Virginia Commonwealth University, Richmond, Virginia, USA 23249.
Tahvanainen
Kari U O
KU
Kuusela
Tom A
TA
Levine
Benjamin D
BD
Cooke
William H
WH
Mano
Tadaaki
T
Iwase
Satoshi
S
Saito
Mitsuru
M
Sugiyama
Yoshiki
Y
Ertl
Andrew C
AC
Biaggioni
Italo
I
Diedrich
André
A
Robertson
Rose Marie
RM
Zuckerman
Julie H
JH
Lane
Lynda D
LD
Ray
Chester A
CA
White
Ronald J
RJ
Pawelczyk
James A
JA
Buckey
Jay C
JC
Baisch
Friedhelm J
FJ
Blomqvist
C Gunnar
CG
Robertson
David
D
Eckberg
Dwain L
DL
D000328:Adult; D018480:Astronauts; D017704:Baroreflex / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000294:innervation; D013564:Sympathetic Nervous System / Q000502:physiology*; D014630:Vagus Nerve / Q000502:physiology*; D014636:Valsalva Maneuver / Q000502:physiology*; D014893:Weightlessness