Astronauts who spent up to six months aboard the International Space Station (ISS) experienced no loss of muscle mass or function in their ventricles – the pumping chambers of the heart – largely due to extensive exercise regimens, a new study led by UT Southwestern Medical Center researchers showed. The findings, published in the Journal of the American College of Cardiology, could have important implications for treating diseases in which gravity plays a role and could help with planning for longer missions, such as to Mars.
Benjamin Levine, M.D., Professor of Internal Medicine in the Division of Cardiology at UT Southwestern, holds a Distinguished Professorship in Exercise Sciences. He is the founding Director of the Institute for Exercise and Environmental Medicine at Texas Health Presbyterian Hospital Dallas, where he also holds the S. Finley Ewing Chair for Wellness and the Harry S. Moss Heart Chair for Cardiovascular Research.
“Our study shows that, remarkably, what we are doing in space to preserve heart function and morphology is pretty effective,” said senior author Benjamin Levine, M.D., Professor of Internal Medicine in the Division of Cardiology at UT Southwestern who holds a Distinguished Professorship in Exercise Sciences. He is the founding Director of the Institute for Exercise and Environmental Medicine at Texas Health Presbyterian Hospital Dallas, where he also holds the S. Finley Ewing Chair for Wellness and the Harry S. Moss Heart Chair for Cardiovascular Research.
Since the National Aeronautics and Space Administration (NASA) began sending humans into space in the early 1960s, researchers have known that astronauts returning to Earth typically experience a dramatic loss in blood pressure. Although several factors are responsible, Dr. Levine explained, a key cause is remodeling that the heart experiences in response to microgravity conditions. Much like individuals on bed rest, astronauts don’t work as hard in space as on Earth because they aren’t counteracting the effects of gravity. As a result, their heart muscle mass decreases by an average of 1% per week, and the volume of blood that the heart holds also falls. Both factors significantly affect cardiac function.
Although astronauts on the ISS exercise about two hours each day, it has been unknown whether this training could counteract the effects of extended time in zero gravity.
To answer this question, Dr. Levine and his colleagues collected data on 13 astronauts who took missions on the ISS between 2009 and 2013 that lasted an average of 155 days. The researchers measured blood pressure and calculated stroke volume (blood pumped per beat) and cardiac output (blood flow per minute) several times before, during, and after each astronaut’s mission. They also performed cardiac MRI scans to assess heart anatomy about two months before spaceflight, three days after their return to Earth, and again about three weeks later. Both before and during their missions, the astronauts completed daily exercise programs that incorporated strength training and aerobic components.
Results showed that the astronauts’ blood pressure decreased significantly during spaceflight compared to on Earth. Similarly, the work their hearts performed also decreased by about 12%. However, both left and right ventricles showed no decrease in muscle mass, and the amount of blood pumped out of the heart remained about the same as it had been preflight.
“There’s nothing magical about space and microgravity. The heart is quite plastic and responds to changes in physical activity,” Dr. Levine noted. “It’s reassuring that the training astronauts are doing in space can protect their hearts from the risks inherent to spaceflight, even on extended missions.”
Dr. Levine notes that previously published work has shown a dilation of the atria in these same astronauts, raising the possibility that they could be at risk for atrial fibrillation during longer duration missions. This issue is the focus of his research team’s current studies on the ISS.
Similarly, he added, specific exercise regimens help patients with postural orthostatic tachycardia syndrome – a condition that causes symptoms such as increased heart rate, dizziness, and fatigue when patients transition from lying down to standing up – as well as a variety of other conditions affected by gravity.
Other UTSW researchers who contributed to this study include Shigeki Shibata, M.D., Ph.D., Instructor of Internal Medicine; Shuaib Abdullah, M.D., Associate Professor of Internal Medicine; and Denis Wakeham, Ph.D., Postdoctoral Researcher.
This study was funded by a NASA grant (NNJ04HH01A).