Health & Fit:Research in astronauts sheds light on rare fainting disorder

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An intervention used to treat astronauts has relevance for people on the ground with medical conditions that cause repeated fainting, researchers say.

Astronauts newly returned to Earth commonly have episodes of lightheadedness and fainting. But researchers have found in the past that daily exercise in space and intravenous saline solution to boost blood volume upon landing seem to prevent these episodes under test conditions.

In the new study, 12 crew members from the International Space Station had their heart function and blood pressure assessed before, during and after a six-month stint in space, researchers report in Circulation.

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The assessments showed changes in heart rhythms, and the nervous system signals that control them, that are similar to what's seen in certain disorders that cause blood pressure to drop and people to faint.

But the daily exercise in space, and the extra fluid infusion when then returned, allowed them to proceed with their normal daily activities without lightheadedness or fainting.

The longer one spends in a gravity-free environment, the greater the risk of fainting during normal activities upon return to regular conditions, senior study author Dr. Benjamin Levine told Reuters Health.

Similar to getting up after extended bed rest, returning to gravity brings about changes to blood flow and sometimes causes orthostatic hypotension, or a temporary drop in blood pressure upon standing up, he explained.

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"This problem has bedeviled the space program for a long time, but this condition is something ordinary people often experience as well," said Levine, a professor at UT Southwestern Medical Center in Dallas.

One example, he noted, is a rare condition called postural orthostatic tachycardia syndrome (POTS), in which the heart shrinks and can no longer maintain normal blood pressure. Patients with POTS are prone to lightheadedness, fainting and experience uncomfortable, rapid increases in their heart rate.

"Astronauts don't develop POTS, but they develop POTS-like physiology," he said.

"Understanding why an adaptation to the absence of gravity makes people more likely to faint is what (helped us) understand why that happens to people who don't even go into space," Levine said.

In 2011, Levine, who has studied astronaut rehabilitation since the early 1990s, treated a 27-year-old POTS patient, Julie Christoph. He advised her to follow a medication-free regimen that included a tailored exercise program to condition the heart and a high-salt diet (instead of intravenous saline, as the astronauts receive).

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At the time, Christoph was "basically bedridden," she told Reuters Health by email, but after three months of exercise and higher salt intake, she felt better, and after six months, she saw a marked improvement.

Five years after he treated Cristoph, Levine and his colleagues published a report on more than 100 patients with POTS who had followed the same regimen. Three-quarters of them were in remission at the end of the treatment.

Although Levine's latest astronaut study considered data from just 12 people, it's promising, said Dr. Prashant Vaishnava, a cardiologist at the Mount Sinai Hospital in New York City who wasn't involved in the study. But adapting the astronauts' regimen to real-world situations could be challenging, he cautioned.

"To expect two hours of (exercise) training from real-world patients or ourselves may be unrealistic," Vaishnava noted.

The data show many interesting aspects of the autonomic nervous system's response to microgravity exposure and prolonged deconditioning, which would have been difficult to study otherwise, said Dr. Tae Chung of Johns Hopkins Medicine in Baltimore, Maryland, who leads the Johns Hopkins POTS clinic.

The data potentially could have large impact on the treatment of POTS and other dysautonomia - autonomic nervous system disorders - added Chung, who was not involved in the study.

SOURCE: https://bit.ly/2GXi6yX Circulation, online July 19, 2019.

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