DEAR DOCTOR K:

My 61-year-old mother plans to take a long plane trip. Her legs usually become swollen when she flies a long distance. Should she wear elastic stockings or take any other precautions so she doesn't develop a blood clot in her legs?

DEAR READER:

Long flights increase anyone's risk of developing mild swelling in the feet, ankles and lower legs -- and of getting blood clots in the legs. Swelling is common; blood clots are uncommon.

On a long flight, unless you're in a fully reclining first-class or business-class seat, you are not lying flat. Gravity is pulling blood down into the veins of your lower legs. Also, your legs are bent at the knees. This makes it harder for blood to travel through your leg veins and back to your heart. As a result, the veins swell up.

On a long flight, you also don't walk a lot. When you walk, your leg muscles squeeze your veins, which keeps the blood flowing back to your heart. But if you are inactive for many hours, this also causes the leg veins to swell up.

When your leg veins swell, some of the fluid in the blood leaks out into the tissues of your legs, causing the swelling. Also, the flow of blood in your leg veins slows down. When blood flow slows, clots tend to form.

Blood clots that form in the veins of the legs are called deep-vein thrombosis (DVT). A DVT blocks blood flow and causes pressure to build up in the vein. This can cause long-lasting problems in the affected leg.

Even more dangerous is a pulmonary embolism. This is a clot that breaks away from the leg and travels up through the bloodstream to your lungs. A pulmonary embolism can suddenly and dramatically decrease the flow of blood in your lungs. This can result in chest pain, breathing problems, fainting spells -- even sudden death.

What can your mother do to protect herself against both leg swelling and DVT? The compression stockings you ask about gently squeeze the legs below the knee. They should help to prevent or at least reduce the swelling you mentioned. But in my judgment, compression stockings have not been shown to prevent DVT on long flights.

There are three ways to prevent DVT (be sure to pass them along to your mother): First, stay well hydrated. Drink enough non-alcoholic beverages to force you to get up to make frequent trips to the toilet.

Second, do in-seat exercises to keep the blood flowing through your legs. For example, contract and relax your calf muscles, or rapidly wiggle your feet up and down.

Finally, get up to walk every hour. I always get an aisle seat so that I can walk up and down the aisle frequently without bothering the people in the seats next to me. In the months after 9/11, I'm afraid my pacing up and down did bother people, but that seems to have passed. Now, instead of thinking I'm a terrorist, they just think I'm restlesss.

(Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.AskDoctorK.com.)

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