Holiday travelers will soon clog the nation's highways and inundate its airports in numbers not seen in recent years. The number of travelers over the four-day Thanksgiving holiday is expected to surpass the 31 million Americans who traveled more than 50 miles by car and the 5 million who went by plane in 2003, according to AAA spokesman Lon Anderson. "This is the first year we've seen travel returned to what it was pre-9/11," he says.

No matter what the mode of transportation, sitting motionless for long periods may put some travelers at an increased risk for deep vein thrombosis (DVT), a blood clot in a vein deep within the muscles, usually in the calf or thigh. But people can reduce their risk of getting DVT, says the American Heart Association (AHA), by taking some simple precautions on long trips.

The AHA estimates that 1 out of every 1,000 Americans develops DVT each year. "It oftentimes gives you a swollen, painful leg, usually in the calf," says Richard Stein, M.D., a cardiologist and associate chair of medicine at Beth Israel Medical Center in New York City and a spokesman for the AHA. "But it can be silent," producing no noticeable signs. "Tragic cases are when ... a piece of thrombus [blood clot] breaks off and goes into the lungs," says Stein. This complication of DVT, known as pulmonary embolism, was brought to public attention in 2003 when it caused the death of 39-year-old NBC reporter David Bloom. Bloom had spent long hours reporting the war in Iraq from the cramped quarters of a military vehicle.

Any long period of immobility — such as being bedridden from illness, recovering from surgery, or sitting for extended periods while traveling — is a risk factor for DVT and pulmonary embolism, says the National Heart, Lung, and Blood Institute (NHLBI). DVT can also develop in other instances when the blood flow in the legs is restricted and slows down. Restricted flow may occur with certain types of cancer and cancer treatment, obesity, inherited clotting disorders, pregnancy, and damage to the veins following injury or orthopedic surgery.

Clotting the blood is "nature's way of trying to prevent bleeding," says Wolf Sapirstein, M.D., a cardiologist at the Food and Drug Administration. But when nature's protective mechanism overcompensates and precautions aren't taken, there is a danger of blood clots.

Reducing the Risk While Traveling

DVT has been dubbed "economy-class syndrome," reflecting the cramped legroom in economy class airline seating. But it can happen to passengers in any seating class of an aircraft, according to the Federal Aviation Administration. It can also happen to people on long rides in cars, trains, or buses.

"People should not be afraid to travel," says Stanley Mohler, M.D., professor emeritus of aerospace medicine at the Wright State University School of Medicine in Dayton, Ohio. "They should just anticipate that they may be inclined to be immobile," he says, and take precautions. A two-hour flight wouldn't be a problem, he says, but a 12-hour flight would be "a big problem" if a person sits inactive the entire time. Children who travel don't appear to be at risk for DVT, says Mohler, because they are generally more active in their seats than adults.

In adults, "hub-and-spoke flying is also a problem," he says, referring to a series of connecting flights interspersed with long hours of waiting between flights. "It's important for passengers to keep moving their legs to help the blood flow," even when waiting in the airport terminal, says Mohler, who advises walking when possible. "When you walk, the muscles of the legs squeeze the veins and move blood to the heart."

Another way to help move blood to the heart is to wear compression stockings, which put gentle pressure on the leg muscles. Studies in healthy people have shown that wearing compression stockings minimizes the risk of developing DVT after long flights, according to the AHA. These stockings are available at medical supply stores.

Stein advises avoiding regular socks with very tight elastic bands at the top and sitting with your legs crossed for long periods of time, which constricts the veins. He also urges travelers who can't walk around frequently to exercise their legs by curling or pressing the toes down, which causes the muscles to contract and squeeze on the leg veins, helping to pump the blood along.

Airlines, also, are encouraging passengers to periodically move and stretch their legs. The Australian carrier Qantas, for example, offers leaflets with leg exercises that passengers can do in their seats. Qantas began printing warnings for DVT on its tickets following the highly publicized death of a 28-year-old woman in October 2000. The woman died from a pulmonary embolism shortly after she stepped off a 20-hour Qantas flight from Australia to England after attending the Olympic Games in Sydney.

Stein also advises drinking plenty of fluids to prevent dehydration. Dehydration causes blood vessels to narrow and blood to thicken, increasing the risk for DVT. Reducing alcohol and coffee consumption, which both contribute to dehydration, is also recommended. These steps aren't scientifically proven to prevent DVT, but they're common sense, says the AHA. As for taking aspirin to prevent DVT, "there is no real evidence that an aspirin reduces the likelihood, but it very possibly could be of value," says Stein.

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