Influence of microgravity on astronauts' sympathetic and vagal responses to Valsalva's manoeuvre

Abstract

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.

Challenges, concerns and common problems: physiological consequences of spinal cord injury and microgravity

“…Furthermore, the resting position (operational point) of astronauts on this relationship was lower after the space missions than before. 30 A more recent investigation confirmed these findings, showing that the range, maximum gain, and operational point of sigmoid carotid distending 11 Bottom: Valsalva maneuvers performed 72 days before the Neurolab space shuttle mission and on mission day 12. 30 Note the lack of sympathetic activity in the SCI subject, while there is a greater augmentation of muscle sympathetic nerve activity during straining in space than on Earth.…”

“…30 Note the lack of sympathetic activity in the SCI subject, while there is a greater augmentation of Figure 1 Left: Blood pressure and heart rate responses to orthostatic stress in a healthy male control subject (a) and a man with chronic cervical SCI (C5 AIS B; b). The SCI subject had complete destruction of descending autonomic pathways as assessed by the absence of sympathetic skin responses.…”

Probing the arterial baroreflex: is there a ‘spontaneous’ baroreflex?

“…Similarly, vasodilation induced by modest steady state infusions of nitroprusside does not reduce arterial pressures in young healthy humans, and, in some subjects, can even increase pressure via effective baroreflex-mediated tachycardia and sympathoexcitation [24]. Moreover, the response to hypogravity may be analogous to these situations in that hypovolemia is compensated by increased sympathetic neural outflow, maintaining arterial pressure at normal levels [10]. Thus, in some cases, baroreflex-mediated adjustments can be so effective that without either baroreceptive artery dimensions or recordings of sympathetic nerve activity there may be no evidence of arterial baroreflex engagement.…”

Cardiovascular autonomic control after short-duration spaceflights

“…In space, most studies agree on diminished vagal baroreflex gain together with an increase in vagal heart rate control [13,24], which is probably due to an increase in stroke volume (SV) in the first days of spaceflight [31,33]. At the same time however, muscle sympathetic nerve activity (MSNA) has been shown to be higher in microgravity, as demonstrated with lower body negative pressure (LBNP) experiments, Valsalva manoeuvres, handgrip and cold pressor tests [13,22,17,15].…”