Cardiovascular Responses to Mild Lower Body Negative Pressure during Spaceflight.

R5373 --> 603.16 --> Introduction: Weightlessness during spaceflight causes a chronic cephalad fluid shift that has been hypothesized to underlie numerous risks associated with spaceflight, including the development of ocular structural and functional changes and in rare cases venous thrombosis. Acute use of 25 mmHg lower body negative pressure (LBNP) during spaceflight can partially reverse this headward fluid shift and thus, may represent a promising countermeasure. However, there are limited data on how the cardiovascular system responds to sustained use of mild LBNP during spaceflight when cardiovascular deconditioning and adaptation may cause crewmembers to be more susceptible to orthostatic stressors than they would have been on Earth. Purpose: The purpose of this study was to quantify the heart rate (HR) and blood pressure response throughout ~60 minutes of exposure to 25 mmHg LBNP during spaceflight and determine if this response was augmented at later time points during 6‐month spaceflight missions. Hypothesis: We hypothesized that crewmembers would experience an increase in HR during LBNP that would be sufficient to maintain mean arterial pressure (MAP) and this would not be augmented later in spaceflight. Methods: Brachial arterial pressure and HR were measured before and throughout ~60 min of 25 mmHg LBNP after ~45 days (FD45) and ~150 days (FD150) of spaceflight in 12 crewmembers. Total LBNP session times varied slightly due to logistical constraints. MAP and HR changes from baseline with LBNP exposure were analyzed with splines models. Marginal means were used for estimation and statistical comparisons across time and between flight days. Results: Before LBNP use, HR was 59 beats per min (BPM 95% CI: 56 – 62) and MAP was 93 mmHg (95% CI: 90 – 96). During LBNP exposure on FD45, HR increased by 9 BPM (95% CI: +7 – +10) at 15 min of exposure, and increased further to +10 BPM (95% CI: +8 – +12) at 30 min, and +11 BPM (95% CI: +9 – +14) at 45 min. On FD150, HR increased by 8 BPM (95% CI: +5 – +11) at 15 min of exposure but plateaued thereafter. MAP decreased by 5 mmHg (95% CI: ‐8 – ‐2) at 15 min and remained lower thereafter. MAP responses during ~60 min of LBNP exposure were not statistically different between FD45 and FD150. A total of 46 sessions of LBNP were successfully completed, and only a single case of mild hypotensive symptoms was reported, and this session was completed. Conclusion: These data suggest that the cardiovascular system can accommodate up to 60 min of sustained use of mild levels of LBNP during long‐duration spaceflight. There was a single report of mild hypotensive symptoms during use of LBNP by an individual who had low blood pressure prior to the start of the LBNP session. Thus, the absolute arterial blood pressure should be taken into consideration when determining if an LBNP session will be a potential challenge to the cardiovascular system during spaceflight. [ABSTRACT FROM AUTHOR]