Exposure to long periods of weightlessness in space can cause an astronaut's heart to change shape, increasing the risk of cardiovascular problems.
Researchers analysed data collected from 12 astronauts while working on the International Space Station, as well as before and after. The results show the heart takes on a more spherical shape by a factor of 9.4%.
The American College of Cardiology's 63rd Annual Scientific Session presented the findings, which show astronauts have a heightened risk of having a heart attack.
James Thomas, lead scientist for ultrasound at Nasa and the senior author of the study, said: "The heart doesn't work as hard in space, which can cause a loss of muscle mass. That can have serious consequences after the return to Earth, so we're looking into whether there are measures that can be taken to prevent or counteract that loss."
Yet according to NBC news, the effects are temporary and the heart returns to its normal shape once the astronaut is back on Earth.
In order to continue with the mission to sent astronauts to Mars in the 2030s, scientists are keen to further their understanding of how a spaceflight of 18 months or more could affect astronauts' heart health.
Nasa and its Russian counterparts are planning to keep two astronauts on the International Space Station for an entire year in 2015, as part of the study.
As reported in Nature World News, Thomas added that models predicted the changes the team observed in the astronauts: "It gives us confidence that we can move ahead and start using these models for more clinically important applications on Earth, such as to predict what happens to the heart under different stresses."
For the study, astronauts were trained by the research team to capture images of their hearts using ultrasound machines installed on the ISS.
Other technologies regarding astronaut health are also being developed, including a miniature robot which can act as a surgeon in outer space.
The robot, which is roughly the size of a fist, is a product of Virtual Incision in Lincoln, Nebraska. It is due to have its first zero-gravity test in a few months, with the hope that the equipment will accompany astronauts on long deep-space missions.
Dubbed the "miniature surgeon", the robot can remove an appendix, remove parts of a diseased colon or perforate a gastric ulcer. It slides into the body through an incision in the belly button, once the abdominal cavity has been filled with inert gas to make room for it to work.
Shane Farritor, a member of the research team, told New Scientist that the robot will only be used on long missions, when someone is more likely to experience physical trauma: "It must be an emergency if you could consider surgery in space."