Cardiac Echo Makes Waves in Final Frontier

NATIONAL HARBOR, Md. -- In the microgravity environment of space, the heart undergoes changes in shape and volume, according to a presentation at the annual American Society of Echocardiography (ASE) meeting.
"The hearts of astronauts who spend longer durations in space become more spherical," noted James D. Thomas, MD, the head of the core lab analysis for the International Space Station. Thomas is also medical director of the Heart and Vascular Institute at the Cleveland Clinic and the outgoing ASE president.
One of the first revelations about the cardiovascular system in space was that it undergoes an initial volume redistribution as the blood moves from the legs to the thorax, he said. "The heart expands in volume, but then there is a diuresis that occurs and the volume goes down over the course of a few weeks."
Another early lesson was the occurrence of orthostatic hypotension when astronauts return to Earth.
Upon atmospheric reentry, nearly three-quarters of the astronauts' blood volume is below the heart. When they stand, it increases the sympathetic nerve activation, said Benjamin D. Levine, MD, from the University of Texas Southwestern Medical Center in Houston.
"Everyone has limits to vasomotor reserve. We can test them on a tilt table. Those with a certain amount of reserve can stay upright with hypovolemia," Levine said.
Most of the data about hearts in space come from short duration flights. But the recently launched Integrated Cardiovascular (IVC) study will examine the longer-term effects of microgravity on astronauts who spend up to 6 months on the International Space Station.
One such study aims to quantify the extent, time course, and clinical significance of cardiac atrophy associated with long-duration spaceflight. Levine, the lead author, hopes to identify the atrophy mechanisms and the functional consequences of extended periods of time in space.
Astronauts have routinely been subjected to echocardiography exams since the HDI 5000 (Philips Medical Systems) ultrasound system was delivered to the International Space Station in 2001. That unit, which needed repair only once, finally died last year. It was replaced by a smaller system that was part of the space shuttle Atlantis' final mission payload.
NASA has spent millions of dollars developing new technologies to assess cardiac function in space and those advances are making their way into mainstream care, Thomas told MedPage Today.
Echo techniques such as strain and strain rate, which measure the flexibility and flexibility in time of myocardial tissue, were developed by NASA researchers.
"These tools are very applicable for heart failure patients and those with ischemic disease, but they also should help guide us on ways to handle astronauts on longer duration flight," Thomas said.
Interestingly, one of the best ways to counterbalance the negative cardiovascular effects of microgravity is exercise, Thomas said. "The same that is true on Earth is true in space," he quipped.
"Astronauts already need to exercise a couple hours each day to maintain cardiovascular fitness in space. One of the efforts we hope to come out of our work with the International Space Station will be to define the best type of exercise," Thomas said, adding that it will probably be a mixture of aerobic and isometric exercises.