Cardiovascular System in Parabolic Flight and Spaceflights

Human Spaceflight: Alterations of the cardiovascular system during parabolic flights and spaceflights
The purpose of this research is to identify the changes occurring during parabolic flights and spaceflights, where there’s weightlessness. The importance of the cardiovascular system in space, is recognised as well as some of its fundamentals based on past researches. In addition, since parabolic flights are a way of experimenting physiological alterations in the human body, instead of actual spaceflights, the procedure needed for the airbus to reach microgravity conditions is indicated as well. Findings, such as low plasma volume, circulatory pressure, central venous pressure, stroke volume and also the heart rate of the cardiovascular system are stated from past investigations. Also countermeasures, such as exercise and diet are also briefly discussed.
Introduction
Microgravity is the phenomena where objects or people experience weightlessness. Astronauts and objects face microgravity in space, where the gravity is very small (micro) and they float (free fall). Even though astronauts are relatively heavy, they can move easily inside or outside the spacecraft (Wall, 2015). Under microgravity circumstances, the physiology of the cardiovascular system changes and it reacts unlikely relative to the gravity of the Earth leading to body alterations such as redistribution of blood, cardiac arrhythmia and orthostatic hypotension (Zhu, Wang, and Liu, 2015).These changes may occur pre-flight, in flight or post-flight and they may impact the astronaut’s health. Moreover these changes can affect either healthy astronauts or astronauts with past heart diseases. Due to the environment, the body of the astronaut learns how to adapt under the new conditions and works relatively quickly.

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In order to investigate and analyse the changes of the human physiology, various microgravity based researches were conducted, not only by spaceflights but also by parabolic flights and bed rest studies. Measurements are taken in three stages of the astronaut’s body, pre-flight, in-flight and post-flight, known as the long duration since astronauts are sent to space missions while these measurements are taken. Although, for more data, investigators managed to create microgravity condition for 20-30 seconds, using parabolic flights, known as the short-term duration, which is clearly a cheaper way to collect data. Another way to study the adaptation of human physiology in space is bed rest studies, where volunteers spend up to 2 months in a bed, with their head end at an angle of 6° beneath the horizontal axis. All volunteers eat, shower and exercise while they are in bed.
The cardiovascular system
In order to analyse the cardiovascular system in space, some fundamentals of the heart should be noted. A healthy cardiovascular system is essential for astronauts going to space, since the heart functions differently in microgravity and it is responsible for many main functions of the body. The physiology of the cardiovascular system in space, therefore will be altered and this can impact the function of the system. Transporting nutrients (e.g. oxygen O2, food) to the tissues of the body, waste removal (e.g. carbon dioxide CO2, by-products) and controlling heat distribution between the body core and the skin (temperature) are some main function of the cardiovascular system (Evans, 2012). Heart is one of the muscles in our bodies which is constantly in action and it is part of the cardiovascular system. This system also includes arteries, veins and capillaries, all known as blood vessels. Additionally, O2 and CO2 are delivered and collected, respectively, to and from various organs, through blood vessels pumped by the heart.  Furthermore, the cardiovascular system is responsible for the blood pumped towards the heart, due to the muscles of the legs (Evans, 2012).
The cardiovascular system in weightlessness
When an astronaut is bare in space, the cardiovascular system learns how to function in such an environment.  The cardiovascular system changes in microgravity, since the downward force of gravity does not exist anymore, as it existed on Earth’s environment. Therefore, due to the lack of the gravitational force, blood and body fluids are not uniformly distributed in the body, but more importantly in the legs, where all these fluids shift upwards, towards the head, resulting for astronauts to have “puffy faces” and less leg circumference (bird legs), as shown in Figure 1. Fluid shift in the body, leads to the increase of the size of the heart, initially, in order to handle the increase of the blood flow. This occurs during the first day of exposure in microgravity. In addition, due to the upward direction of the fluids, astronauts do not feel as thirsty, resulting to the reduction of the fluid levels after the first day and the heart shrinks (Lujan, Bartner, and White, 1994).
Figure 1: Illustration of fluid shift level. The fluids are distributed uniformly, pre-flight (left), fluids shift, during flight (“bird legs” and “puffy faces”)(middle) and post flight, the pressure is lower in the upper body, due to gravity, causing faintness to the human. (Watenpaugh and Hargens, 1996)
Parabolic flights and the cardiovascular system
Airbus A300 ‘Zero G’ is the aircraft used by the French company Novespace for simulation of microgravity through parabolic flights, between 1997 and 2014 as shown in Figure 2. Agencies such as the European Space Agency (ESA) and the German Aerospace Centre, performed researches using this airbus in the stated period of time, but by 2015 the new Airbus A310 ‘Zero G’ replaced it.

Figure 2: The Airbus A300 ‘ZERO-G’ as it is flying in an incline of 40° to reach 0g. (Pletser, et al., 2015)
These aircrafts, were built for researches due to testing results before or after space missions, by achieving parabolic flights under weightlessness for 20 seconds (Pletser, et al. 2015). More specifically, the airplane from a steady horizontal altitude, pulls up at an angle approximately 40° in a period of 20s, resulting to an acceleration between 1.8 g and 2 g and therefore, the engines start to slow down, which leads to microgravity conditions inside the aircraft as it reaches the peak of the parabola. Finally, the aircraft generates an acceleration of 1.8 g to 2 g, while flying back down with roughly 40° again for 20s and then before returning to its initial steady altitude, repeats the manoeuvre from the beginning, as shown in Figure 3 (ESA, 2004). In addition, parabolic flights can investigate how the cardiovascular system of the human body reacts under 0-g conditions, within this period of time by spending relatively less money than actual spaceflights.

Figure 3: This figure illustrates the manoeuvre which the aircraft (thick-black line) follows to generate microgravity conditions and demonstrates the acceleration and the microgravity level as well. (ESA,2004)
Between 2010 and 2012, Novespace undertook an experiment based on the reaction of the cardiovascular system during a parabolic flight, using the Airbus A300 ‘Zero-G’. The test presents a short duration of microgravity, where the fluids inside the body are distributed. The heart is pumped with more blood than usual resulting to an increase of the blood pressure in the ventricles of the heart. The stoke volume of the cardiovascular system remained constant but the heart rate decreased by 14 min-1. Furthermore, it was stated that astronauts were in an environment, where the body lacked sufficient oxygen supply, known as hypobaric hypoxia condition (HH) and since the study is under a parabolic flight, the gravity was shifting as well. This kind of environment influenced the cardiovascular system, where the data obtained for the plasma volume showed a decrease mostly due to HH, from -52 ml (hypobaric chamber) to -115 ml (parabolic flight) (Limper and Gauger ,2014). Another research, compared the data for humans in supine posture, under normal gravity and microgravity in parabolic flight (0G), which showed an increase in cardiac filling pressure resulting to the diameter of the left atrium to increase by 3.6 mm. At the same time the central venous pressure (CVP) decreased by 1.3 mmHg but the transmural CVP increased by 4.3 mmHg. Finally, as soon as an astronaut returns to Earth, due to the gravity, the blood flow is reduced and that can cause the astronaut to collapse (Watenpaugh and Hargens, 1996). These results were obtained by researches, in order to investigate the consequences of the cardiovascular system under weightlessness, by avoiding actual spaceflights, where these changes are only temporarily.
The cardiovascular system during spaceflights
As soon as astronauts enter space, the fluid levels in the body are not uniformly distributed as they were on Earth, which results to alterations of the cardiovascular system. As it was mentioned in parabolic flights, the astronauts are under hypobaric- hypoxia conditions, meaning that the oxygen saturation decreases (SaO2) and hence the oxygen in the blood. It has been stated that the concentration of O2 in the blood can drop down to 75%, where usually this levels should be more than 80%, but if the astronauts stays in space for longer, this concentration will increase back to 85% (Opatz and Gunga, 2014). Moreover, the mass of the heart decreases during spaceflights and therefore the heart rate is less than that on Earth. In 1996, it was reported that the heart rate would increase as the astronaut continuous to be under microgravity circumstances, during a long-term spaceflight (Charles, Frey, and Fritsch-Yelle, 1996). In weightlessness, significant effects were also realised, the cardiac output increased whereas the systolic and diastolic pressure decreased (Hamilton, Sargsyan, and Martin, 2011). Hence, stroke volume is also reduced, due to hypovolemia which is responsible for hypotension and atrophy of the heart (Levine, 1997).
Investigators postulate that plasma volume decreases from the first day and it continuous to reduce throughout the whole spaceflight by 17%. This occurs, because of the negative fluid distribution and the fluid movement towards the extravascular space and therefore the orthostatic intolerance (Alfrey, Udden, and Leach- Huntoon, 1996). A study reported by J.C Buckey et al. 1996, studied the central venous pressure (CVP) in space and stated that the CVP increases during the launch and more in the spaceflight. The left ventricular end-diastolic volume (LVEDV) was also analysed in order to figure out how it is affected by microgravity. Furthermore, it was stated that as astronauts enter space, the LVEDV and therefore the total heart volume increases significantly. While the astronaut is in space, the body adjusts to the environment resulting to the LVEDV to decrease (Buckey Jr. and Gaffney, 1996)
Countermeasures
For short duration exposure, effects are less than actual spaceflights where the duration could be more than 6 months. It is really important for astronauts to be healthy during a mission, therefore some actions should be taken in order to counteract these threats of their physiology. It has been reported that somatic stress in weightlessness effects the cardiac arrhythmia (Romanov et al., 1987). The astronauts must exercise and have a healthy diet, before and during the spaceflight, to ensure the appropriate volume for extravehicular action (Hargens, 2009). Also, the lower body negative pressure (LBNP) should be exercised regularly since it increases the plasma volume (Watenpaugh and Hargens, 1996) and in fact, aerobic exercise keeps the aerobic volume (peak of VO2) constant. For long-term exposure in microgravity, exercising machines, provided in the spacecraft can reduce the consequences of the physiology of the astronaut after returning to Earth. Although, studies have not shown the particular amount and type of exercise, that astronauts should perform, yet (Schneider and Watenpaugh, 2002).
Discussion and Conclusion
Researches within the last 20 years, examined how the cardiovascular system adapts under microgravity conditions, in order to provide astronauts with a safe working environment and physiology. Astronauts are sent to space to test experiments for the future of science, but their lives shouldn’t be at risk. Due to microgravity, several characteristics of the cardiovascular system are affected. The fluids in the body of an astronaut exposed in microgravity, shift head-wards due to the missing gravitational force. Therefore, plasma volume and mean circulatory filling pressure are decreased. Hence, there are effects on the central venous pressure (CVP) and stroke volume, which both are reduced during weightlessness. The heart rate is also declined due to these changes, in order to maintain the arterial blood pressure and metabolism. Some of these parameters can affect significantly the astronaut’s health and in rare cases may lead to tragedies, since they are long- term flights. Although, when subjects are under investigation in parabolic flights, these changes are only temporarily. Also, countermeasures, such as aerobic exercises and healthy diet, before, during and after the spaceflight are required. These actions may reduce the orthostatic hypotension of astronauts during flights but also as they return back to Earth. More experiments will be conducted in the future, where researchers will have an even better understanding of space environment and the physiology in it.
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