Effects of six hours daily lower body negative pressure on orthostatic tolerance and cardiac performance during 30 days strict head-down tilt bedrest

Abstract

Introduction: Orthostatic intolerance commonly occurs in astronauts returning to earth. Head-down tilt bedrest (HDTBR), which models cardiovascular adaptation to weightlessness, decreases orthostatic tolerance by 34–60% without any countermeasure. We hypothesized that daily six hours lower-body-negative-pressure (LBNP, - 25 mmHg) ameliorates orthostatic tolerance, plasma volume, and cardiovascular deconditioning during HDTBR. Methods: We submitted 23 healthy persons (12 women, 34.5 ± 9 years, 23.9 ± 2.8 kg/m²) to 30 days of strict HDTBR (SANS-CM study). Subjects were assigned to 6 h upright seating (positive control, n = 11) or - 25 mmHg LBNP (n = 12) per day. We measured left ventricular outflow tract diameter (LVOT) and LVOT-stroke volume by pulsed wave doppler echocardiography during 15 min of 80 head-up tilt testing (HUT) with incremental LBNP until presyncope before and after HDTBR. We determined plasma volume with CO-rebreathing two days before and at HDTBR day 27. Results: With HDTBR, orthostatic tolerance decreased 289 ± 89 s (- 23%) in the seated and 284 ± 95 s (- 22%) in the LBNP group (p\ 0.001 vs. baseline, p = 0.968 between groups). Plasma volume decreased 569 ± 114 ml in the seated and 604 ± 104 ml in the LBNP group (p\0.001 vs. baseline, p = 0.813 between groups). While supine stroke volume decreased 8 ± 1 ml in the seated and 9 ± 4 ml in the LBNP group (p\0.001 vs. baseline, p = 0.874 between groups), supine cardiac output did not change in either group. Both groups showed similar reductions in upright stroke volume following HDTBR, however, stroke volume at presyncope did not change with HDTBR. Conclusions: Six hours daily moderate intensity LBNP or seating did not fully attenuate orthostatic intolerance, plasma volume loss, or cardiovascular deconditioning during 30 days HDTBR. However, both interventions better maintained orthostatic tolerance compared with previous 30–60 days HDTBR studies without countermeasures.