Cardiovascular responses to daily use of lower body negative pressure during 30 days of strict head-down tilt bedrest
Authors: Lytle Jason , Laurie Steve , Marshall-Goebel Karina , Lee Stuart , Miller Annelise , Martin David , Young Millennia , Macias Brandon 
Published: May 2023

Introduction: Exposure to weightlessness causes a chronic headward fluid shift that is believed to underlie numerous risks associated with spaceflight, including the development of cephalad venous congestion, venous thromboembolism, and Spaceflight Associated Neuro-ocular Syndrome (SANS). Application of moderate levels (25 mmHg) of lower body negative pressure (LBNP) during spaceflight can partially reverse the headward fluid shift, and thus, may represent a promising countermeasure. Strict head-down tilt bed rest (HDTBR), a spaceflight analog, causes the development of some SANS findings and is used as a platform to test candidate countermeasures. Purpose: To measure the heart rate (HR) and mean arterial pressure (MAP) response during and subject tolerance to twice daily use of 3 hours of 25 mmHg LBNP during 30 days of HDTBR. Hypothesis: We hypothesized that during 3 hours of moderate LBNP MAP would be maintained, but HR would increase and that these responses would not be altered throughout 30 days of HDTBR. Methods: Twelve subjects (n = 12; 6 female) maintained strict 6˚ HDTBR for 30 days (HDT1 through HDT30) and were exposed to 2 separate 3-hour sessions of LBNP with continuous blood pressure and HR monitoring (Finapres, NOVA). Continuous data were averaged across 10-minute segments during each 3-hour LBNP period for statistical analyses to determine if MAP or HR changed within the 3-hour LBNP exposure, and whether the response to LBNP changed across all study days. Data were analyzed with generalized linear models with GEE effects to address repeated measures within subjects. Time trends over the 3 hours were modeled by the best fitting model utilizing linear or natural cubic splines of time. Results: All LBNP sessions (720 total) were completed across the 30 days of strict HDTBR, and no sessions were terminated early. In the first 10 minutes of LBNP MAP increased (+4 mmHg, p<0.01) and increased further over time (additional +3 mmHg by the final 10 minutes of LBNP exposure, p=0.02). This response did not change over the 30 days of HDTBR (p=0.67). Similarly, HR increased (+7 bpm, p<0.01) during the first 10 minutes of LBNP exposure but decreased towards pre-LBNP levels by the conclusion of LBNP (-6 bpm, p<0.0001). The HR response to LBNP did not change over 30 days of HDTBR (p=0.14). Conclusion: Twice daily use of 25 mmHg LBNP during 30 days of strict HDTBR was well-tolerated with no reported hypotensive symptoms or cardiovascular responses suggestive of impending syncope. Despite cardiovascular deconditioning expected to occur due to inactivity during HDTBR, neither the MAP or HR response to LBNP were significantly changed between HDT1 and HDT30. Prolonged use of moderate LBNP should be further investigated as a potential countermeasure for various risks associated with long-duration spaceflight. National Aeronautics and Space Administration Human Research Program Directed Research This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.