Abstract
Head-down tilt (HDT) bed rest elicits changes in cardiac circadian rhythms, generating possible adverse health outcomes such as increased arrhythmic risk. Our aim was to study the impact of HDT duration on the circadian rhythms of heart beat (RR) and ventricular repolarization (QTend) duration intervals from 24-h Holter ECG recordings acquired in 63 subjects during six different HDT bed rest campaigns of different duration (two 5-day, two 21-day, and two 60-day). Circadian rhythms of RR and QTend intervals series were evaluated by Cosinor analysis, resulting in a value of midline (MESOR), oscillation amplitude (OA) and acrophase (φ). In addition, the QTc (with Bazett correction) was computed, and day-time, night-time, maximum and minimum RR, QTend and QTc intervals were calculated. Statistical analysis was conducted, comparing: (1) the effects at 5 (HDT5), 21 (HDT21) and 58 (HDT58) days of HDT with baseline (PRE); (2) trends in recovery period at post-HDT epochs (R) in 5-day, 21-day, and 60-day HDT separately vs. PRE; (3) differences at R + 0 due to bed rest duration; (4) changes between the last HDT acquisition and the respective R + 0 in 5-day, 21-day, and 60-day HDT. During HDT, major changes were observed at HDT5, with increased RR and QTend intervals’ MESOR, mostly related to day-time lengthening and increased minima, while the QTc shortened. Afterward, a progressive trend toward baseline values was observed with HDT progression. Additionally, the φ anticipated, and the OA was reduced during HDT, decreasing system’s ability to react to incoming stimuli. Consequently, the restoration of the orthostatic position elicited the shortening of RR and QTend intervals together with QTc prolongation, notwithstanding the period spent in HDT. However, the magnitude of post-HDT changes, as well as the difference between the last HDT day and R + 0, showed a trend to increase with increasing HDT duration, and 5/7 days were not sufficient for recovering after 60-day HDT. Additionally, the φ postponed and the OA significantly increased at R + 0 compared to PRE after 5-day and 60-day HDT, possibly increasing the arrhythmic risk. These results provide evidence that continuous monitoring of astronauts’ circadian rhythms, and further investigations on possible measures for counteracting the observed modifications, will be key for future missions including long periods of weightlessness and gravity transitions, for preserving astronauts’ health and mission success.
Highlights
Long duration spaceflights present a number of physiological and psychological stressors that deeply challenge astronauts’ health and performance in space
Results are presented as median [25th percentile; 75th percentile], and detailed information about statistical analyses results are reported in the tables
The cumulative results confirmed our previous findings observed on data of a single campaign only (Solbiati et al, 2020), showing that the characteristics of RR and QTend circadian rhythms were affected by Head-Down (−6◦) Tilt (HDT) in terms of increased RR and QTend midline and shortened QTc, reduced amplitude of day/night oscillations and slightly anticipated circadian acrophase, with larger changes visible already after the first 5 days of HDT bed rest
Summary
Long duration spaceflights present a number of physiological and psychological stressors that deeply challenge astronauts’ health and performance in space. From the Latin words “circa diem,” meaning “approximately a day,” circadian rhythms are nonrandom fluctuations having a period of approximately 24 h found in almost every physiological processes (Richards and Gumz, 2013) These rhythms are autonomous, and persisting in absence of external synchronizers, but they entrain to environmental and social cues. Together with the light-dark cycle, a number of factors contribute in maintaining a correct entrainment of circadian rhythms on Earth, such as working and feeding hours, wake/sleep and activity/rest cycles, social interaction, ambient temperature (McKenna et al, 2017), as well as gravity (Fuller et al, 1994) During spaceflight, these conditions are removed: the light-dark cycle is altered, the pull of gravity is reduced, and the regular alternation between standing and supine position is eliminated. Circadian rhythm impairment has been correlated on Earth to long-term health problems, including sleep disorders, obesity, diabetes, depression, seasonal affective disorder and aging (Logan and McClung, 2019), with the severity of the consequences increasing with the level of circadian disruption (Gazendam et al, 2013)
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