LAUSR

How probable are arrhythmias in spaceflight? The International Space Station (ISS) has been in Low-Earth orbit (LEO) for more than 20 years. During this time, there have been no reports of pronounced arrhythmias in space crew members. Does this mean that there were no arrhythmias among the ISS crew members who were in LEO? Or were they not reported? On the one hand, the level of medical control in the selection and further screening of crews for space missions is high in all space agencies. On the other hand, routine activities on the ISS require significant skills and experience, and this leads to an increase in the age of space crew members, both males, and females. And this is a serious arrhythmogenic factor (Tank, 2005; Wittnich et al., 2013; Platts et al., 2014; Koenig and Thayer, 2016; Yoo and Fu, 2020). Not to mention that, undoubtedly, human cardiovascular physiology is not adapted to exist outside gravity (Shen and Frishman, 2019) and outside the magnetic field of Earth with increased levels of heavy ionizing radiation (Afshinnekoo et al., 2020). It is known that since the late 1950s, 17 cases of atrial arrhythmias have been identified among 317 active and retired astronauts. Overall, the prevalence of arrhythmias, the most common of which was atrial fibrillation, was 5%, which is comparable to the prevalence of arrhythmias in the general population, but the age of astronauts is much younger (41–45 years). All of the above increases the likelihood of arrhythmias during long-term space flight (SF), as medical care will be limited (Khine et al., 2018). In addition, in this study, 48-h high-resolution Holter monitoring data were obtained before, during the flight and on the day of landing, and magnetic resonance imaging data before and after the 6-month SF. According to the results of this study, the volume of the left atrium temporarily increased after 6 months of SF (12 ± 18 mL; p = 0.03), while the function of the atria did not change, also 1 astronaut had a significant increase in supraventricular ectopic contractions, but none of them developed atrial fibrillation. Correspondingly, 6 months of stay in SF can cause temporary changes in the structure of the left atrium, which increase the risk of atrial fibrillation, but no episodes of atrial fibrillation were detected. A Delp et al. (2016) study reported a 4–5 times higher risk of cardio-vascular disease (CVD) in Apollo astronauts compared to astronauts who never traveled beyond LEO. Therefore, due to the increase in the duration and range of planned SFs, the probability of cardiovascular systemmaladaptation and dysfunction increases as the influence of SF factors intensifies. In the absence of Earth’s gravity there is a shift of fluid in cranial direction (about 2 L of fluid moves out of the legs) due to the decrease of intrathoracic pressure. Central venous pressure decreases within 1 day of being in microgravity, which is associated with a decrease in compression on the veins by muscles and internal organs (Hughson et al., 2018). Left ventricular end-diastolic volume increases concurrently with a decrease in central venous pressure (Buckey et al., 1996). Arterial pressure in microgravity is uniform throughout the body and thereby reduces cardiac strain and the physiological need for arterial pressure regulation mechanisms. Baroreceptor stimulation as a consequence of cardiac remodeling and increased cerebrovascular pressure due to redistributed fluid may affect neural and endocrine regulatory loops. This may lead to inhibition of the renin-angiotensin-aldosterone OPEN ACCESS