Autonomic cardiovascular control during head-out water immersion and head-down bed rest

Head out water immersion (HOWI) and head down bed rest (HDBR) are good simulators of weightlessness (1,2), but their possible distinct effects on cardiovascular autonomic control remain partially unexplained. We compared the time-course of cardiovascular autonomic control during exposure (60 min) to HOWI and HDBR. Ten young healthy males (20-40 years) were enrolled. Each subject underwent HOWI and HDBR on two separate days, at constant room temperature (23.5-26.5°C) or in thermoneutral water (34.5-35.5°C). We measured heart rate (HR)(ECG: V5 lead) and finger blood pressure (BP)(CNAP, Austria) beat-by-beat for 10 minutes in baseline (dry, sitting) and every 10 min for 60 min during HOWI and -6° HDBR. Heart rate variability (HRV) was quantified by indexes of time-domain (pNN50), frequency domain (Low Frequency [LFnu: 0.04-0.15 Hz] and High Frequency [HFnu: 0.15-0.40 Hz] powers in normalized units) and complexity (α1-DFA [Detrended Fluctuation Analysis])(3), calculated with the software Kubios HRV (Finland). During the first 10 min of exposure HR was significantly reduced in both conditions (HOWI: -10.5%; HDBR: -16.4%, p<0.05 vs baseline). SBP was significantly reduced during HOWI (-9.9%; p<0.05 vs baseline) but slightly increased during HDBR, although not significantly (+8.2%; p=0.11). HR and SBP remained stable thereafter, for the whole 60 min of exposure to both HOWI and HDBR (p=ns, last 10 min vs first 10 min of exposure for both parameters and conditions). Regarding HRV, pNN50 significantly increased (+70.6% and +58.7%, respectively, p<0.05 vs baseline) during the first 10 min of exposure to HOWI and HDBR and stabilized thereafter. LFnu tended to decrease and HFnu to increase (19.7%; p=0.08) in HOWI but not in HBDR, and did not change thereafter in both conditions. DFA α1 significantly decreased in HOWI (-18.6%, p<0.05 vs baseline) but not in HDBR, and stabilized thereafter in both conditions. Adaptation of HR was similar in acute HOWI and HDBR, whereas BP adaptation differed, suggesting different autonomic adjustments to simulated weightlessness, probably involving vascular resistances. Indeed, although the parasympathetic system seems to be activated by both conditions, the sympathetic branch appears less involved by HDBR. The stability over time of cardiovascular and HRV indexes during the HOWI and HBDR exposures suggests that both stimula can maintain the new cardiovascular set points for prolonged periods.