Do you fly frequently coast to coast for meetings? Heading out soon on an international flight to an exotic vacation half way around the world? There’s something you should know about traveling for hours on end that could turn your trip into a nightmare -- something far worse than a boorish, snoring seatmate or unappetizing flight cuisine.

Researchers have documented that long haul flights (lasting about 10 hours or more) triple the risk of developing a deep vein thrombosis, or DVT, which results when blood thickens, clumps and forms a clot in a vein deep in the body. If a loose clot (called an embolus) breaks off and floats through the bloodstream, it can travel to the lungs and block blood flow. This potentially deadly condition is called a pulmonary embolism (PE).

Economy-class syndrome

DVTs that develop during or even weeks after long plane rides are often dubbed "economy-class syndrome" or "coach-class syndrome," because seating and leg room are especially tight for passengers in economy class – and being immobile in a cramped position ups the risk of developing a blood clot. The reason? Prolonged sitting and pressure on your calves from the fairly hard airline seat can lead to stagnant blood flow through veins.

However, first-class and business-class passengers also develop DVT, so this problem isn’t totally the result of sitting still in for many hours in the smallest airplane seats. Other factors on long flights known to contribute to DVT risk include:

• Decreased air pressure and the release of nitric oxide in the plane’s cabin.
• Dehydration as a result of low humidity inside the plane.
• Dehydration from drinking alcohol and caffeine, which have diuretic effects, and not drinking enough water.

In addition, there are specific conditions that can raise your risk of a DVT during a flight. For example, a study by Leiden University Medical Center researchers in the Netherlands published in the journal PloS Medicine found that women taking birth control pills are three times more likely to have a flight-related DVT than those not taking the contraceptive meds. What’s more, people over six feet tall are two and a half times more likely to get a DVT compared to their shorter counterparts -- and airline passengers who are overweight have twice the risk of normal weight folks when it comes to experiencing a blood clot on a long flight.

Other risks and warning signs

"There’s a fine balance in the body when it comes to clotting. You always need to be able to form a clot when you cut yourself in order to stop bleeding. At the same time there may be individual risk factors that predispose some people to get clots more than others. And sometimes when individuals who carry this higher risk for clotting are situations such as long flights, that risk can be amplified," Dr. Marc Passman, University of Alabama associate professor of vascular surgery, tells Synergy.

"Other high risk situations for DVT include being laid up in the hospital after a serious trauma or after a major operation. Notably, some orthopedic procedures, major pelvic and abdominal operations carry an elevated risk," Dr. Passman adds.

According to the American Venous Forum (http://veinforum.org/), DVT may not cause symptoms until the blockage severely interrupts blood flow. When that happens, you could experience these symptoms:

• Pain
• Sudden swelling in one leg
• Enlargement of the superficial veins
• Reddish-blue discoloration
• Skin that is warm to the touch

If you experience any of these, it’s crucial not to dismiss them as signs of simple muscle cramps or injuries and assume they will get better. Instead, get medical help right away. The reason? Some people find out they have DVT only after the clot has broken free in the leg and traveled to their lung, creating a pulmonary embolism. Signs of a life-threatening PE include shortness of breath and chest pain when you take a deep breath.

It was this DVT complication that took the life of NBC correspondent David Bloom in spring of 2003. Covering the war in Iraq, he had experienced leg pain from sitting in a cramped position for hours on end while riding in a tank and tragically assumed what was really a blood clot was a muscle cramp behind his knee.

Preventing clots before they happen

The best way to prevent a DVT from long flights or any other situation that raises your risk, is to follow some common sense strategies. "On a flight, move those legs. Pump them back and forth a bit and walk around the aisles at least once an hour on the plane -- a couple of times an hour is better," Dr. Passman tells Synergy. "Even if you are on a long road trip by car, it’s important to stop every couple of hours and walk around."

Avoid dehydration by going easy on alcohol and coffee. On the other hand drink a lot of water. That, says Dr. Passman, is important because dehydration tends to promote stagnant blood flow.

Another practical prevention measure: wear medical grade compression support hose during your long trips. "They effectively promote emptying of the venous system while you are sitting and that augments the calf muscle function to pump blood out of the leg and help prevent stagnation of blood flow," adds Dr. Passman, who works with the American Venous Forum to help educate the public about DVTs.

If you have a personal family history of blood clots, Dr. Passman says that your family doctor can perform tests to find out if you have blood that coagulates too easily. If so, you may be prescribed a blood thinning drug to take before long flights. You can also check out the American Venous Forum web site for information on their free program that offers DVT risk screening nationwide.

Sherry Baker is a writer from Atlanta, Georgia. She last wrote Bad Medicine for Synergy. Sherry can be reached at featuredstories@adamcorp.com