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    • Introduction The Space Shuttle has been retired and the

    2019-06-20

    Introduction The Space Shuttle has been retired and the International Space Station (ISS) will be in use until 2020 (Fig. 1). Scientists and engineers are thinking about a mission to Mars in the future. Human exploration of space is at a turning point, and a number of questions about the future of space programs around the world continue to be discussed. Long spaceflights present serious physical challenges that need to be addressed.
    History of arrhythmias in astronauts
    Effects of spaceflight on the heart There is little information on the cardiovascular responses to long-duration spaceflight, especially from the ISS era. However, from past studies, we know that spaceflight has many effects on the human heart. For example, gravity affects the distribution of human bodily fluids. Entering the microgravity environment causes a large (1–2L) shift of fluid towards the head, increasing the cardiac size and stroke volume soon after launch. [13]. After the 10-day second German Spacelab Mission in 1993, the left ventricular (LV) mass of the crewmembers had decreased by 12±6.9% [14]. During the STS-107 space mission, the cardiac output was measured by using noninvasive rebreathing equipment [15]. In that study, cardiac output immediately increased by 29±7%, from 6.6±0.7 to 8.4±0.9L/min, after launch. Another study showed that when astronauts returned to Earth, they p38 inhibitor had lost a total of 10%–20% of their blood volume and exhibited increased orthostatic intolerance [16].
    Potential cardiovascular problems during long-duration spaceflights
    Possible fatal arrhythmias during long-duration spaceflights In exploratory spaceflights, the possibility of a cardiac event would be a risk on long missions, such as a Mars mission. “The 2004 NASA Bioastronautics Critical Path Roadmap identified the occurrence of serious cardiac dysrhythmias and diminished cardiac and vascular function as the primary cardiovascular risks of spaceflight” [32]. During short to moderate spaceflights, the possibility of a life-threatening event such as a cardiac arrest is about 1% per year [33]. The likelihood of a need for cardiopulmonary resuscitation (CPR) in spaceflight is very low, but still exists. Recent data suggest that SCD accounts for approximately 20% of the total mortality in the United States and other industrialized countries, with an incidence in the US of over 300,000 cases annually [34]. SCD can occur because of fatal arrhythmias or other cardiac problems. Since ventricular fibrillation (VF) and pulseless VT are the major causes of SCD, preventing VF and VT is very important for long-duration spaceflights. Exercise can be a trigger for SCD with people who have cardiovascular disease. The risk for SCD in young athletes with cardiovascular disease is 2.5 times higher than that in non-athletes. More than 90% of SCDs occur during or immediately after a training session or competition [35,36]. Astronauts usually get plenty of exercise in the ISS during their missions. The actual crewmembers of the ISS (who spend 6 months in space) spend 2–3h daily exercising (Fig. 5) [8]. Astronauts should monitor their ECGs during training in space.
    Non-fatal arrhythmias in space
    Current cardiac screening Several techniques are available for the screening of fatal and non-fatal arrhythmias (Table 1). At present, selection (select-out) evaluations consist of a cardiovascular screening examination of blood pressure (recumbent and orthostatic position), heart rate measurement, and a cardiac physical examination. This screening protocol includes 24h-Holter monitoring, resting echocardiogram, resting 12-lead ECG, and Bruce protocol treadmill stress test [42]. Usually the astronauts selected have to undergo training for a long time, and for long-duration spaceflights they will need to undergo coronary screening before their launch. Therefore, the coronary artery calcium score and high-sensitivity C-reactive protein level are also included in the protocol [43]. Select-out criteria include a clinical history of major congenital abnormalities, evidence of ischemic heart disease (IHD), persistent tachycardias, arrhythmias, conduction defects, ECG abnormalities, thrombophlebitis, deep venous incompetence, varicose veins, peripheral vascular disease, cardiac tumors, valvular disorders or significant hypertrophy, dilation of the heart, or a reduced ejection fraction. Additionally, active pericarditis, myocarditis, and endocarditis are also listed in the select-out criteria [42].