Mighty hearts in space

Space travel was one of our childhood dreams. In the unlikely event that a spaceship had landed nearby, we would have joined the crew without a second thought. Now, we are more cautious recognizing that spaceflight imposes enormous psychological and physiological stresses on our human frames. Rigorous training and extensive medicals including human centrifuge or rotating chair tests are mandatory and we would be likely to fail the tests. On launch day, astronauts are squeezed into tight seats perched on top of a vast volume of highly explosive fuel, and Earthlings can hardly imagine the subsequent severe acceleration. A trip to the International Space Station (ISS) takes about 6 h and then the astronauts fall around the earth, completing one orbit every 90 min for days, weeks, or even months. This free fall and the associated lack of g forces profoundly affects many aspects of human physiology (White & Averner, 2001). To increase the safety of astronauts and cosmonauts, space programmes in the USA and Russia heavily invested in life science research starting in the early 1960s, with the Europeans and China following on. Overall, the research seems to suggest that the human body adapts to space flight and that overt pathology is rare (Parin & Gazenko, 1963). Elaborate physical training programmes have been implemented to maintain muscle, bone and cardiovascular wellbeing. Remarkably, human cardiovascular physiology can cope not only with profound volume shifts during take-off and landing but also with chronic changes in volume distribution and cardiac loading conditions in microgravity. However, the possibility that cardiovascular adaptations to microgravity may not be fully reversible is a matter of concern. Early on in space, astronauts exhibit ‘puffy’ faces and ‘stuffy’ noses secondary to microgravity-induced cephalad fluid shifts. More chronically, plasma volume reductions, increases in calf vascular resistance (Watenpaugh et al. 2001), and loss of cardiac muscle mass (Perhonen et al. 1963) may ensue. Increases in sympathetic nerve activity have been described. In this issue of The Journal of Physiology, Norsk et al. (2015) challenge the idea that sympathetic nerve activity and peripheral vascular resistance increase in space. The authors applied state-of-the-art methodology including ambulatory blood pressure and pulmonary blood flow measurements in eight middle-aged male astronauts before space flight, following several months in space, and approximately 2 months after return to earth. In space, cardiac output increased 41% whereas 24 h mean blood pressure decreased 10 mmHg. However, the study had important limitations. For example, measurements on earth were obtained in a seated position making it a little difficult to interpret the findings. Furthermore, the authors reported exceedingly high plasma noradrenaline (norepinephrine) levels. The sample size would be considered small on earth. In space, the study would qualify as a large-scale trial taking years to complete. Despite these issues, attenuated total peripheral resistance in the face of unchanged plasma noradrenaline levels is unexpected and thought provoking. The mechanisms producing vasodilatation independently of sympathetic nervous system activity could have a bearing on human medical research on earth. The observation that cardiac output is kept so high in space is equally important. Is cardiac output increased to meet the metabolic demands of peripheral tissues or is an Earthling's heart too mighty for microgravity conditions? For example, increased carbon dioxide concentrations onboard ISS may have affected cardiac output. On earth, human heart and vasculature are tuned to provide sufficient blood oxygen and nutrient supply within a normal blood pressure range. With cardiac and vascular adaptation to microgravity the delicate balance may go out of tune resulting in ‘luxury perfusion’. Perhaps intensity and timing of the adaptation differ between peripheral and central vascular compartments. Data on blood pressure measurements at the level of digital and brachial arteries support this idea. We cannot exclude that chronically increased cardiac output has adverse health effects; for example, excessive retinal perfusion could contribute to vision problems in astronauts following long-term space flights. Despite all the known risks, space flight continues to attract and inspire people regardless of age. Millions of viewers were fascinated by video clips showing the Canadian astronaut Chris Hadfield playing a guitar while freely floating onboard ISS. Space flights of 1 year duration are planned for the year 2015 and suborbital commercial space flights may become reality in the near future. These developments provide new opportunities to resolve some of the cardiovascular mysteries in space, including the mighty heart phenomenon. However, there are also challenges ahead in terms of medical control and safety. People of an older age, not meeting current medical requirements for space flights may, nevertheless, have the motivation and the financial means for commercial suborbital flights. Evaluation of the risk–benefit relationship of an adventure providing the unique opportunity of seeing the blue planet from space becomes more complex in the light of the findings by Norsk et al. None declared. Jens Tank is supported by a research grant of the German Space Agency (DLR) 50WB1117.

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