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\ub0C) or in thermoneutral water (34.5-35.5\ub0C). 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\ub0 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 (\u3b11-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 \u3b11 significantly decreased in HOWI (-18.6%, p