3D echocardiography gauges heart mass loss in astronauts

Ultrasound is no stranger to space travel. Typically, however, the imaging modality has been used to evaluate musculoskeletal problems associated with near-weightless conditions. A new effort launched by the National Aeronautics and Space Administration uses live 3D ultrasound and advanced postprocessing to evaluate the effects of microgravity on astronauts' hearts.

Astronauts commonly are thought to lose heart mass during prolonged flight. Two-D echocardiography measurements reveal a 5% decrease, which usually reverses within three days of being back on Earth. Researchers are interested in learning the cause of these changes. Possible explanations include heart atrophy caused by weightlessness, dehydration from space travel, or error caused by the geometric assumptions used in 2D echo.

"I suspect the formula for left ventricular mass calculated from a 2D echocardiogram might be overly sensitive to dehydration and consequently give a false reading to someone who's dehydrated," said David S. Martin, chief ultrasound technologist for the NASA Cardiovascular Laboratory at the Johnson Space Center in Houston.

A 3D calculation of left ventricular mass does not have the same geometric assumptions that a 2D scan has. Consequently, 3D echocardiography should provide a more accurate number, Martin said.

Martin's team has already conducted pre- and postflight scans of two crew members from each of the last two space shuttle missions. Results are pending. The researchers perform live 3D echo with a Philips iE33 echocardiography system that allows them to capture a full-volume image of the beating heart in less than a minute. Later, they extract measurements of heart mass, ejection fraction, blood flow, strain rate, and cardiac wall motion with Philips' QLAB quantification software.

There's a narrow window of opportunity to perform an echo exam on the returning astronauts, as any changes due to weightlessness apparently are short term, Martin said. In addition, other scientists are standing in line to prod and probe the astronauts.

The 3D images will be compared with 2D scans from a modified Philips HDI 5000 ultrasound system that was installed in the International Space Station in 2001. Because of budget constraints, NASA has no plans to modify the iE33 for a permanent home aboard the space station, Martin said.

Loss of left ventricular mass coupled with dehydration could put a significant strain on astronauts' hearts. In the case of an emergency upon landing, astronauts might need to lift heavy components or run quickly, both actions that would be jeopardized with heart mass loss. Understanding the origins of LV loss could help researchers devise treatments or protocols while the astronauts are in space to delay or halt the process, Martin said.

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