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

Weightlessness in space induces initially an increase in stroke volume and cardiac output, accompanied by unchanged or slightly reduced blood pressure. It is unclear whether these changes persist throughout months of flight. 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. 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. 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.