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

Abstract

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 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.

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.

Figure 1. Mean ± sem of systolic (SAP), diastolic (DAP) and mean (MAP) 24 h ambulatory brachial arterial pressures recorded automatically at hourly (8–23 h) or bi-hourly (23–8 h) intervals (mean ± sem of N = 8) on the ISS and before and after flight

Measurements were conducted with portable oscillometric equipment between 85 and 192 days of flight on the ISS (in-flight, filled circles) and 322 to 71 days before (pre-flight, triangles) and 58 to 209 days after (post-flight, diamonds). The means of the 24 h SAP, DAP and MAP values on the space station were significantly lower than the same values during the pre- and post-flight 24-h monitoring periods.

Figure 2. Individual and mean ± sem systolic (SAP) and diastolic (DAP) arterial pressures and heart rate (HR) measured on the ISS and before and after flight during ambulatory conditions when awake (day); in connection with seated cardiac output rebreathing measurements (CO day); and during the sleep period (night)

SAP, DAP and HR were measured by portable equipment in a brachial artery with an oscillation technique and during CO (day) using the same equipment activated by the subject and then in the seated posture. *Significant change (P < 0.05) from pre- and post-flight values at same time period [day, CO (day) or night]. ^†Significant change (P < 0.05) from day in same panel. ^aSignificant change (P < 0.05) from only post-flight value (day).

Figure 3. Individual and mean ± sem mean arterial pressure (MAP), cardiac output (CO) and systemic vascular resistance (SVR) on the ISS and before and after flight

MAP was recorded by portable oscillometric equipment in the resting and on the ground seated posture at the time of CO measurements. SVR was calculated by MAP/CO. L–, days before launch; MET, days of mission elapsed time; R+, days after landing. *Significant change (P < 0.05) from all pre- and post-flight values.