Astronaut Exercise Curbs Cardiac Atrophy From Prolonged Microgravity

August 17, 2023

The heart doesn't have to work as hard under conditions of weightlessness experienced by crew members living weeks and months aboard the International Space Station (ISS). Frequent, intense exercise sessions are known to mitigate hemodynamic adaptations to prolonged microgravity that can otherwise lead to orthostatic hypotension and other problems once astronauts return to earth.

Now there is direct evidence from MRI scans that such exercise during ISS missions, which for some astronauts can last 6 months or longer, protects the heart partly by offsetting some of microgravity's adaptive effects on cardiac structure and function.

The benefit appears great enough to help convince researchers that some version of the current exercise protocols will be sufficient to similarly protect astronauts during the much longer Mars missions planned for coming years.

The team assessed ventricular mass and volumes by MRI in nine male and four female astronauts about 2 months before and 3 days after stays in orbit aboard the ISS lasting 4 to 6 months. Their hemodynamics were measured or were estimated in flight.

All the astronauts performed endurance and resistance exercise for up to 2 hours daily on equipment designed for microgravity during the missions included in the study, which took place from 2009 to 2013.

Cardiac work (derived from estimated ventricular stroke volume and mean aortic pressure) fell a mean of 12% during stays on the ISS compared to preflight assessments. But after their missions, overall, the astronauts showed far less than the degree of cardiac atrophy that had been predicted by Earth-based studies of prolonged bedrest, the investigators reported August 14 in the Journal of the American College of Cardiology.

It's likely, they say, that cardiac atrophic changes were at least partly averted by ventricular loading heightened by the astronauts' lengthy, almost daily in-flight exercise sessions.

The findings "suggest the current exercise countermeasures used on the ISS are effective at maintaining ventricular morphology, despite the reduction in overall cardiac work during space flight," write the authors, led by Shigeki Shibata, MD, PhD, University of Texas Southwestern Medical Center, Dallas.

"I think we've shown that if the astronauts can sustain regular exercise, that they can preserve their cardiovascular structure and function," senior author Benjamin D. Levine, MD, told | Medscape Cardiology.

On average, their cardiac workload decreased, but "they did enough in-flight short-duration, high-intensity training that many of them were able to preserve their cardiac structure even over six months."

Changes in ventricular mass varied among the 13 astronauts, who started their ISS missions at different levels of physical fitness, Levine observed.

Some who habitually exercised more intensely before their mission compared to their prescribed exercise regimen aboard the ISS showed modest cardiac atrophy on post-flight MRI.

"Our fittest astronaut had about an 11% loss of cardiac muscle mass," Levine said. But the predicted loss was about 22%, so his exercise routine aboard the ISS was still considered protective against ventricular shrinkage.

In contrast, ventricular mass tended to go up for those unaccustomed to the frequency and intensity of their onboard exercise regimen, who achieved greater cardiac workloads in space than they would have on the ground.

"Our least-fit astronaut told us she had never done as much exercise in her life as she did in space. And her heart actually got bigger."

Those examples, Levine said, illustrate how there's "no magic" to cardiac structural and functional changes during space flight. "It's a hemodynamic phenomenon. The heart responds to the load that's placed on it." It shrinks if the load is consistently decreased and increases in mass with greater loads.

The astronauts aboard the ISS performed at least 90 minutes of exercise 6 days a week; 30 to 45 minutes was reserved for aerobic exercise, and the rest for resistance training, the report states. They worked out on equipment specially designed for use in microgravity.

Total cardiac work was assessed 1 to 3 weeks before space flight and 15 days prior to the astronauts' departure from the ISS. Twenty-four-hour blood pressure was measured, and other hemodynamic markers were estimated using validated techniques.

Pre- and post-flight cardiac MRI scans revealed late gadolinium enhancement (LGE) in one "endurance-trained astronaut," but none showed new LGE signs after months in microgravity.

Mean left ventricular (LV) and right ventricular (RV) mass were not significantly different before and after time on the ISS, nor were there significant mean changes in LV or RV stroke volume, ejection fraction, or end-diastolic or end-systolic volumes.

Relative mean change in LV mass was "strongly and positively" correlated with relative mean changes in total cardiac output and total cardiac work, notes the report.

The findings suggest that "cardiac mass and volume will be maintained during future longer-duration space flight missions," such as those planned to Mars, "as long as appropriate exercise equipment is available and no injuries or logistical issues occur that would prevent regular exercise training," the authors conclude.

"That's one of the things I worry about ― what if they get sick or have an injury? What if the machines break down and they can't exercise?" Levine said.

"Then I think they're going to be susceptible to a lot more deconditioning effects. That probably won't matter much in flight, or even on Mars, where there's only a third of [Earth's] gravity. But it'll make a big difference when they come back down to Earth."

An accompanying editorial says the current report "demonstrates the commitment that is being made to define and counter the effects of space travel on the human body ― to the extent that the exercise protocols in this study were personalized."

It goes on to describe other current and future challenges to practicing medicine and preserving health on space missions. In addition to cardiovascular deconditioning, notes Carl L. Tommaso, MD, Baylor Scott and White Medical Center, Temple, Texas, skeletal muscle atrophy, poor bone mineralization, vestibular disturbances, anxiety, other psychological issues, injuries, medical emergencies, and other problems that are inevitable on future space flights will require "unique solutions."

The study was funded by a grant from the US National Aeronautics and Space Administration. Shibata, Levine, their co-authors, and Tommaso report tno relevant relationships.

J Am Coll Cardiol. Published online August 14, 2023. Full text, Editorial

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