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Dizzy in Space - Orthostatic Hypotension

Astronauts returning to Earth sometimes feel light-headed. It's been a problem since the earliest days of human space exploration, but now doctors may have a solution.

by Doug Hullander and Dr Tony Phillips

Landing a spaceship is a terrible time to feel dizzy, yet that's what happens to some astronauts. Their legs become heavy and their heads light even as the planet below expands to fill the windshield. It's an unwelcome side-effect of returning home.

Researchers have learned that the sensation is caused, in part, by orthostatic hypotension - "in other words, a temporary drop in blood pressure," explains NASA Chief Medical Officer Rich Williams. On Earth you can feel it by standing or sitting up too fast. Gravity has much the same effect on astronauts returning from a long spell in space: Blood rushes down and the space travelers become, literally, lightheaded.

Susceptibility is highly individual. Some astronauts are hardly affected while others feel very dizzy: About 20% of short-duration and 83% of long-duration space travelers experience the symptoms during re-entry or after they land.

"Cosmonauts who spent a long time onboard Mir commonly had to be carried away in stretchers when they came home," recalls Williams. Fortunately, their Soyuz return capsules did not require a pilot to land, so it didn't matter much. Shuttle pilots, on the other hand, must perform complex re-entry procedures. To them it matters a great deal.

Orthostatic hypotension can strike Earth-dwellers for many reasons: Weak hearts might not pump enough blood, for example. Certain medications or even a hot shower can dilate blood vessels and cause blood pressure to drop. Women - especially pregnant women - are more likely to suffer from it than men. "Some patients with this condition are afraid to leave home or even get out of bed," writes neurologist Phillip Low of the Mayo Clinic.


Fluid shifts caused by space flight. From "The Bone"(Vol. 11 No.2 1997.6) Medical View Co., Ltd.

Astronauts experience orthostatic hypotension because of the way human bodies respond to gravity, explains Richard Cohen of the Harvard-MIT Division of Health Sciences and Technology. (Cohen leads the Cardiovascular Alterations Team at the National Space Biomedical Research Institute, or "NSBRI.") On Earth gravity pulls blood toward the lower body. But in space - either in free-fall or far from a source of gravity - blood that normally pools in the legs collects in the upper body instead. That's why astronauts have puffy-looking faces and spindly "chicken legs."

Astronauts don't feel orthostatic hypotension while they're traveling through space, but they do begin to feel it during re-entry (when g-forces mimic gravity) and after landing. Blood returns to the lower body and blood pressure to the head is suddenly reduced. Hence the dizziness. (The sensation can continue for a while after landing, too.)

Grey's Anatomy

Muscles in the veins of the legs help force back uphill toward the heart

It's a classic case of "use it or lose it." Veins in human legs contain tiny muscles that contract when the veins fill with blood. Their function is to send blood uphill toward the heart and so maintain blood pressure. But in space there is no "uphill," so those tiny muscles in the veins are less-used - a normal adaptation to weightlessness.

During re-entry those muscles are needed again, but they have temporarily "forgotten" how to contract. They fail to push blood back toward the heart and brain. "This effect is more severe after prolonged space flights," notes Cohen.

For many years astronauts have tried to counteract orthostatic hypotension by drinking lots of salt water, which increases the volume of bodily fluids.(There is a general loss of body fluids during space missions.) Astronauts also wear "G-suits" - rubberised full-body suits that can be inflated with air. This action squeezes the extremities and raises blood pressure.

Such countermeasures are only partially effective. "Almost all returning astronauts experience changes in gait and balance," continues Williams. Nevertheless, "most are able to walk around just fine. A small number experience orthostatic changes that render them quite dizzy."

An anti-dizzy pill would be helpful, but until recently there was no such thing.

Enter Midodrine: Midodrine is the first drug approved by the United States Food and Drug Administration to treat orthostatic hypotension. It constricts blood vessels and so increases blood pressure. "By increasing blood pressure when patients need it, Midodrine can help people lead a more normal life," writes Low.

Cohen thinks it might help astronauts, too.

Expedition Four

ISS crew members will soon participate in tests of Midodrine. Pictured here is the crew of the ISS Expedition Four

Cohen works with Janice Neck, head of the Cardiovascular Lab at NASA's Johnson Space Centre, and Gordon Williams, a doctor at the Brigham and Woman's' hospital in Boston, to study adverse effects of space flight on the human cardiovascular system. Following animal studies and computer simulations (performed by members of the NSBRI Cardiovascular Alterations Team), they conducted bed-rest testing of real humans - a situation that simulates the effect of space travel on the cardiovascular system. Astronauts, they concluded, would likely benefit from the drug.

An important advantage to Midodrine, says Cohen, is that it can be administered just before re-entry or even after landing. The benefits are immediate. Astronauts wouldn't have to take it throughout their mission when it might interfere with their body's own (and welcome) adaptations to zero-g.

"A flight study protocol has been approved to test the drug's effects on space shuttle astronauts and ISS crew members," says Cohen.

Perhaps soon astronauts returning home from space will feel lightheaded - but only due to elation. Orthostatic hypotension will have nothing to do with it.


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First Science 2014