Autonomic cardiovascular control following 30 days strict head down tilt bedrest with six hours of daily lower body negative pressure training

Abstract

Background: Autonomic cardiovascular regulation is impaired both after bedrest and after space flight. Impaired autonomic control likely limits cardiovascular adaptation to physiological stresses such as standing. We hypothesized that 6 h daily lower body negative pressure (LBNP) helps maintain heart rate (HR), heart rate variability (HRV) and baroreflex sensitivity (BRS) following strict head-down tilt bedrest, an established terrestrial space-analogue. Methods: We included 23 healthy persons (12 women, 34.5 ± 9 years, 23.9 ± 2.8 kg/m²) in the 30 days SANS_CM strict head down tilt bedrest study. Subjects were randomly assigned to 6 h daily LBNP training with moderate intensity (LBNP, - 25 mmHg) or 6 h daily sitting (seated, positive control). We measured HR and finger blood pressure before bedrest and at the end of bedrest during head-up tilt testing. We assessed HRV in the time and frequency domains, systolic blood pressure variability in the low frequency range (LF-SBP), and baroreflex sensitivity using cross spectral analysis (BRS-LF). We analyzed stationary time series during the last 5 min supine and during minutes 2–6 at 80 degrees head-up tilt. Results: Supine HR increased during bedrest (LBNP group: 67 ± 9 vs. 74 ± 11; seated: 66 ± 10 vs. 71 ± 12 bpm; p \0.001) and the orthostatic HR response was enhanced (HR upright: 90 ± 10 vs. 112 ± 13 bpm; 86 ± 12 vs. 109 ± 13 bpm; p\ 0.0001). HRV in the time domain was reduced after bedrest while supine (rmssd: 37 ± 24 vs. 27 ± 14 ms; 50 ± 44 vs. 30 ± 20 ms; p\ 0.017) and upright (15 ± 5 vs. 8 ± 4 ms and 28 ± 21 vs. 14 ± 10 ms; p \0.001). HRV in the frequency domain (total power, LF- and HF power) tended to decrease while supine and standing but the bedrest effect did not reach significance. Supine and upright LF/HF ratios did not differ after bedrest. The tilt induced increase of LF-SBP tended to be enhanced after bedrest (p = 0.0547). Baroreflex mediated vagal HR control was reduced after bedrest at rest (p = 0.011) and during tilt (p \0.0001). No interactions between groups and conditions were detected. Conclusions: Six hours of daily moderate intensity LBNP training or seating do not completely preserve autonomic cardiovascular autonomic control during 30 days strict head-down tilt bedrest. Additional countermeasures, such as physical exercise, may be required.