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

Key points 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 6months on the International Space Station, which is more than during shorter flights. Twenty-four hour ambulatory brachial blood pressure is reduced by 8-10mmHg 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. 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 1-2h intervals with portable equipment in eight male astronauts: once before launch, once between 85 and 192days in space on the International Space Station and, finally, once at least 2months after flight. During the same 24h, 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 24h average systolic, diastolic and mean arterial pressures (mean +/- se) in space were reduced by 8 +/- 2mmHg (P=0.01; ANOVA), 9 +/- 2mmHg (P<0.001) and 10 +/- 3mmHg (P=0.006), respectively. The nightly blood pressure dip of 8 +/- 3mmHg (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.