Fluid shifts, vasodilatation and ambulatory blood pressure reduction during long duration spaceflight

Abstract

Key pointsr Weightlessness in space induces initially an increase in stroke volume and cardiac output, accompanied by unchanged or slightly reduced blood pressure.r It is unclear whether these changes persist throughout months of flight. r Here, we show that cardiac output and stroke volume increase by 35-41% between 3 and 6 months on the International Space Station, which is more than during shorter flights.r Twenty-four hour ambulatory brachial blood pressure is reduced by 8-10 mmHg by a decrease in systemic vascular resistance of 39%, which is not a result of the suppression of sympathetic nervous activity, and the nightly dip is maintained in space.r It remains a challenge to explore what causes the systemic vasodilatation leading to a reduction in blood pressure in space, and whether the unexpectedly high stroke volume and cardiac output can explain some vision acuity problems encountered by astronauts on the International Space Station.Abstract Acute weightlessness in space induces a fluid shift leading to central volume expansion. Simultaneously, blood pressure is either unchanged or decreased slightly. Whether these effects persist for months in space is unclear. Twenty-four hour ambulatory brachial arterial pressures were automatically recorded at 1-2 h intervals with portable equipment in eight male astronauts: once before launch, once between 85 and 192 days in space on the International Space Station and, finally, once at least 2 months after flight. During the same 24 h, cardiac output (rebreathing method) was measured two to five times (on the ground seated), and venous blood was sampled once (also seated on the ground) for determination of plasma catecholamine concentrations. The 24 h average systolic, diastolic and mean arterial pressures (mean ± SE) in space were reduced by 8 ± 2 mmHg (P = 0.01; ANOVA), 9 ± 2 mmHg (P < 0.001) and 10 ± 3 mmHg (P = 0.006), respectively. The nightly blood pressure dip of 8 ± 3 mmHg (P = 0.015) was maintained. Cardiac stroke volume and output increased by 35 ± 10% and 41 ± 9% (P < 0.001); heart rate and catecholamine concentrations were unchanged; and systemic vascular resistance was reduced by 39 ± 4% (P < 0.001). The increase in cardiac stroke volume and output is more than previously observed during short duration flights and might be a precipitator for some of the vision problems encountered by the astronauts. The spaceflight vasodilatation mechanism needs to be explored further.

Characterizing the effect of exposure to microgravity on anemia: more space is worse

“…The 10.9% Hb reduction matched with the 10% plasma volume loss previously measured in space, 1,8,10,11 suggesting that Hb regulation in space replicates earthly Hb levels. Therefore, the 10.9% Hb reduction we reported in astronauts returning from missions averaging 145 days may represent the completed human erythrocytic adaptation; the new "space normal".…”

“…[2][3][4] However, 31 astronauts had normal Hb during 180-day missions onboard the International Space Station (ISS), seemingly contradicting the hypothesis that space flight leads to anemia. 8,9 This blood volume redistribution rapidly reduces plasma volume by 10%. 5,6 The study of anemia during and after spaceflight is complicated by the diagnostic criteria for anemia that are based on Hb concentration, a quantity affected by both changes in plasma volume and changes in RBC mass.…”

“…This paradox raised the hypothesis that the human erythropoietic system adapted to the effects of space. 1,8,10,11 The shift in plasma volume is reversed upon return to Earth, where it must return to a distribution that can overcome the pull of the Earthʼs gravity. 7 Exposure to space causes an almost immediate central and cephalad redistribution of an estimated two liters of blood.…”

Long-term exposure to space’s microgravity alters the time structure of heart rate variability of astronauts

“…Previous investigations showed that the baroreflex sensitivity fluctuates along with altered blood volume distribution [1, 2, 3], which affects neural mechanisms involved in dynamic cardiovascular coordination. Several reports indicate that heart rate is maintained at preflight values [4, 5, 6] and that parasympathetic activity is reduced [4] in space. Cardiac output and stroke volume are reportedly increased in space as a result of an increase in preload to the heart induced by upper body fluid shift from the lower body segments with no major difference in sympathetic nerve activity [6].…”

“…Several reports indicate that heart rate is maintained at preflight values [4, 5, 6] and that parasympathetic activity is reduced [4] in space. Cardiac output and stroke volume are reportedly increased in space as a result of an increase in preload to the heart induced by upper body fluid shift from the lower body segments with no major difference in sympathetic nerve activity [6]. However, high sympathetic nervous activity, measured invasively by microneurography in peroneal nerves, has been simultaneously detected in space in three astronauts [7] compared to the ground-based supine posture.…”