Comparison of the short-term acute cardiovascular response between Head-Down (-6 degrees) and Horizontal Bed Rest

Prolonged weightlessness exposure and inactivity induce reduced functional capacity in multiple body systems, generating cardiovascular deconditioning. Horizontal (BR) and Head-Down (-6°, HDBR) bed rest are ground-based analogue models used to simulate the effects of spaceflight on human body through immobilization and elimination of head-to-foot gravitational stimuli. Our aim was to compare the acute effects of 5-day BR and HDBR on cardiac circadian rhythms, studying possible differences between the two models. Ten healthy males were enrolled at the Hospital of Izola (Slovenia) for a 10-day BR study. For each subject, 12-lead 24-hour Holter ECG (1000 Hz, Mortara Instrument Inc.) was acquired before BR (BDC) and on the 5th day of BR (BR5). As comparison, the BDC and BR5 24-hour previously acquired ECG signals of 63 healthy males enrolled in the non-intervention groups from six HDBR campaigns (of 5, 21 and 60 days, performed at MEDES or at DLR) were used. From beat-to-beat RR and QTend series, day and night median values were computed, and circadian rhythms were evaluated by Cosinor analysis, resulting in a value of 24-hour midline (MESOR), oscillation amplitude (OA, half variation within a nightday cycle), and acrophase (?, temporal value of maximal amplitude). Statistical analysis tested the effects of 5 days of BR and HDBR vs BDC, and compared the effects of these two maneuvers (Two-factorial linear mixed models, p<0.05; post-hoc Wilcoxon test for paired samples and Mann-Whitney test for independent samples), while the ZeroAmplitude test (p<0.05) assessed the presence of circadianity. An increase in the RR and QTend MESOR was similarly elicited by both BR and HDBR, mainly due to a daytime lengthening, caused by reduced physical activity. However, RR remained increased at night in the HDBR group, suggesting possible autonomic impairment induced by the tilted position. The OA considerably reduced in both models, decreasing the system's ability to respond to incoming stimuli and increasing the arrhythmic risk. Also, a trend of anticipation, though not significant, was observed in ?RR and ?QTend. In conclusion, this study evidenced that BR and HDBR, after 5 days, produce comparable changes in the circadian rhythms of cardiac electrical activity, with possible autonomic system deconditioning in HDBR. These findings also expand the knowledge on possible Earth-medicine bed rest-induced deleterious health impact, suggesting the need for identifying potential countermeasures to prevent or reduce the undesired changes for maintaining correct homeostasis in bedridden patients, preventing possible aggravation of patient's condition and improving post-hospitalization recovery.