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Last updated on April 17, 2014 at 21:23 EDT

The Beating Heart, Minus Gravity

July 10, 2009

We’ve all seen video of astronauts drifting and gliding gracefully around inside the International Space Station like fish in a fishbowl. It looks so relaxing. But as enjoyable as it appears to be, there’s a down side to all that freefalling (1).

“When astronauts land back on Earth after a long time in space, not only is their vestibular system mixed up and their kinesthetic sense thrown off,” says Dr. Benjamin Levine of the University of Texas Southwestern Medical Center, “but also their bones and muscles have deteriorated.”

In space, even more than on Earth, it’s “use it or lose it.” The human body and all its parts need to work to remain vital. Bones must bear weight to keep their density and strength. Muscles need to push or pull against resistance to stay in shape; without work they waste away.

Is this also true of our most critical muscle ““ the human heart?

NASA is launching a new study called Integrated Cardiovascular (2) to find out.

“We know that astronauts lose heart mass and exercise capacity when they’re in microgravity for a long time,” says Johnson Space Center’s Julie Robinson, ISS program scientist. “We suspect that this could lead to impaired heart function, which could cause low blood pressure and even fainting when astronauts get back to gravity. But we need detailed information. In the future, astronauts will spend longer and longer in space, and even live and work on the moon and Mars. We want to know exactly how space-living will affect their hearts and heart function.”

Dr. Levine is a principal investigator for the experiment along with Dr. Michael Bungo of the University of Texas Health Science Center at Houston. They’ve enlisted the support of several other cardiovascular experts (3) to conduct this research ““ the most comprehensive and advanced study of its kind to date.

“We’re investigating how, how much, and how fast deterioration occurs in the heart during long duration space travel,” says Levine.

The space station crew, which has recently increased to six members, will help Levine and his team find answers by serving as subjects for Integrated Cardiovascular. The experiment will last for more than 2 years — long enough to gather plenty of data on 12 different astronauts before, during, and after their stints in space.

“We’re incorporating the most sophisticated tools (4) ever used in such an experiment to look at the heart and its chambers and valves,” says Levine. “This is the first investigation ever to use advanced echo-Doppler techniques to follow the structure and function of the heart during long periods in space and confirm findings by using advanced magnetic resonance imaging tools on the ground. For example, we’re using an echocardiogram to determine how heart muscle atrophy influences the way the heart relaxes and fills, and an MRI to quantify this atrophy precisely, and determine whether it scars or gets infiltrated by fat.”

An echocardiogram uses high-pitched sound waves that are picked up as they reflect off different portions of the heart. These echoes are turned into a moving picture, allowing researchers to watch a movie of the heart in action as blood flows through the heart. By looking at such movies before, during, and after spaceflight, the team can discern mechanical changes that happen in a person’s heart after he or she is away from Earth’s gravity for a long time. With the MRI, they can look at detailed computer images of the heart tissues to pinpoint exactly what kind of atrophy occurs.

“We’re answering questions like ‘is the deterioration simply in size, like Arnold Schwarzenegger’s muscle loss if he stopped lifting weights, or does the heart scar, do cells die?’”

The team is also studying the effects of heart atrophy on crewmembers’ ability to exercise and on the likelihood of their developing unusual heart rhythms both on the space station and after returning to Earth. In addition, the researchers will look closely at other cardiovascular issues, such as how blood pressure responds to the reintroduction of gravity at the levels experienced on Earth, the moon, and Mars.

“All of the results will help us fine-tune exercise protocols for the space station crew,” says Robinson. “We’ll also learn what to look at in astronauts’ hearts before we send them to, say, Mars. We’ll identify a set of risk factors that can help flight surgeons determine the best candidates for long space missions.”

Levine adds, “We may, however, show that the heart does just fine in space, and that the strategies now used to keep astronauts in shape are adequate to keep the heart functioning normally and in good health. If so, flight surgeons can turn their attention instead to other potentially critical problems such as bone loss or radiation exposure.”

Importantly, this study’s results will help researchers in developing preventive and rehabilitative regimens for people on Earth.

“The information we get from these experiments will be relevant for patients after long-term bedrest or other physical activity restrictions, as well as for patients with congestive heart failure, heart disease, and even normal aging.”

(1) Up on the space station in Earth orbit, you’re weightless. In fact, if you don’t fasten yourself onto or into something while you sleep, there’s no telling where in the space station compartment you’ll wake up. You may find yourself wedged next to an air vent. But space station astronauts only appear to be floating. They are actually in “freefall,” which means the major force acting on them is from gravity. On the station, the gravity pull comes from the Earth because it is the closest large body. The space station free-falls as it orbits the Earth. If there were no forces acting on the space station, it would travel in a straight line away from the Earth. Because the Earth pulls the ISS towards it and is traveling 7900 meters per second (26,000 feet per second) parallel to the Earth’s surface, the ISS moves around the Earth in a circle. The force of gravity on both the astronauts in the ISS and the ISS itself is about nine-tenths of what it is at the Earth’s surface. Why do you think NASA astronauts in the ISS feel weightless? You only feel weight when something pushes against you. The ISS can’t push the astronauts because both the ISS and the NASA astronauts free-fall at the same rate. (They are traveling at the same speed and in the same direction.)

(2) This experiment is supported entirely through NASA funding mechanisms utilizing grants to the University of Texas Southwestern Medical Center and the University of Texas Health Science Center at Houston and onsite civil service and contractor support at the Johnson Space Center. The study’s full name is Cardiac Atrophy and Diastolic Dysfunction During and After Long Duration Spaceflight: Functional Consequences for Orthostatic Intolerance, Exercise Capability and Risk for Cardiac Arrhythmias (Integrated Cardiovascular). More information.

(3) RESEARCH TEAM

Principal Investigators: Benjamin D. Levine, M.D., Institute for Exercise and Environmental Medicine, Presbyterian Hospital and University of Texas Southwestern Medical Center at Dallas, Dallas, TX. Michael W. Bungo, M.D., University of Texas Medical School, Houston, TX

Co-Investigator(s)/Collaborator(s): Steven H. Platts, Ph.D. Johnson Space Center, Houston, TX. Douglas R. Hamilton, M.D., Ph.D., Wyle Laboratories, Houston, TX. Smith L. Johnston, M.D., Johnson Space Center, Houston, TX

Payload Developer: Johnson Space Center, Human Research Program, Houston, TX

Sponsoring Agency: National Aeronautics and Space Administration (NASA)

(4) Magnetic resonance imaging and echocardiography will be used before and after spaceflight. Echocardiography will also be used in flight.

A special imaging technique called magnetic resonance spectroscopy will also be used to quantify the amount of fat in the subjects’ hearts.

Before and after flight, subjects will be tilted on a table at angles to approximate various levels of gravity (from levels experienced on the moon up to those experienced on Earth). During those tests, each subject’s heart rate and blood pressure will be monitored and the blood flow from their hearts will be measured by using an echocardiogram.

The reaction of the subjects’ bodies to exercise stress will be determined before and after flight by having them perform exercise while their heart rate, blood pressure, and blood flow from their hearts are measured.

Electrocardiograms will also be taken on several occasions during the study and will last up to 48 hours at a time. These recordings will be concurrent with continuous measurements of blood pressure and activity (using Actiwatches worn at the waist and ankle) to estimate the amount of work their heart is doing daily on Earth and in space.

Dauna Coulter, Science @ NASA

Image 1: Astronaut Clay Anderson floats through the Unity node of the International Space Station.

Image 2: A computer-generated diagram of the Integrated Cardiovascular investigation onboard the ISS. Image courtesy of the Johnson Space Center, Human Research Program.

Image 3: Astronaut Cady Coleman performs a remotely guided echocardiogram on a test subject utilizing Integrated Cardiovascular protocols, while Betty Chen, a training coordinator, observes.

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