Longer Flights may Raise Blood Clot Risk

(AP) -- The farther people fly, the more likely they are to suffer a potentially fatal blood clot in their lungs, according to the largest study so far to the risks of long-haul flights.

The study analyzed records from the French ambulance unit for Charles de Gaulle Airport from November 1993 through December 2000. Nearly 135.3 million passengers arrived in France during that time.

Fifty-six of those picked up by an airport ambulance had a pulmonary embolism, a clot which had traveled into the lung, doctors at University Paris XIII wrote in Thursday's New England Journal of Medicine.

Out of nearly 98 million who had flown less than 3,100 miles, only one -- on a six-hour flight from Senegal -- developed a pulmonary embolism. The rate rose to 1.5 cases per million for those traveling more than 3,100 miles, including travelers from the United States to France.

For those traveling more than 6,200 miles, including flights from Australia, Peru and Vietnam, the rate rose to 4.8 per million.

The actual numbers are almost certainly higher because many people develop symptoms hours or even a week after a flight, said Dr. Stephen Borron, one of the study authors and an associate professor at both the university in Bobigny, France, and at George Washington University in Washington, D.C.

Dr. Victor Tapson, an expert from Duke University, said the numbers are probably a gross underestimation, both because of the time limit and because the study only looked at those clots which made it to the lungs.

Tapson said there probably were at least three or four times as many blood clots in the legs, known as deep vein thrombosis. "And, who knows? Maybe many, many more," he said.

A study of more than 200 people flying out of London's Heathrow Airport found no clots in people given surgical-type compression knee socks to wear on a round trip of eight hours or more each way. It found detectable clots in 12 of 116 people who wore their usual footwear. Eight did not need any treatment; four got clot-busting drugs.

Doctors agree that the biggest culprit is sitting still in one seat for hour after hour. The longest flights ranged from 12 hours to nearly 23 hours long -- it takes 101/2 hours to fly from California to de Gaulle Airport near Paris.

The vast majority of the patients -- or at least those who were asked and whose answers were written down -- said they had not moved during the flight, Borron said.

Walking, stretching

"When we walk, when we're moving around doing normal activities, the muscles in your legs put pressure on your veins to push the blood back to your heart. When you're sitting doing absolutely nothing, that muscular pump which helps with blood return is not working properly or not working at all," Borron said.

Constant pressure against the blood vessels for that long can injure their walls, also increasing the chance of a blood clot, he said.

The thinner air in airport cabins, which are kept at about the pressure found at 7,000 to 8,000 feet above sea level, is thought to make blood more likely to coagulate, Borron said.

And if people do not drink enough fluids -- or drink alcohol or caffeinated drinks -- their blood is thicker, he said.

So if you can't get up and walk up and down the aisle several times during a long flight, you should at least fidget, the study says. Don't cross your legs. Move them. Pull each knee in turn up to your chest, then stretch your legs.

Singapore Airlines' Web site describes symptoms of pulmonary embolism and suggests "in-flight aerobics" and regular walks around the cabin to avoid them. British Airways' site suggests walks and stretches, as well as foot and leg exercises described in the in-flight magazine.

United Air Lines' site includes in-seat exercises. Northwest Airlines' suggests, "Drink plenty of water. Keep your circulation going by walking up and down the isles when you can, wiggle your toes and flex your ankles."

Tapson said air travel is probably not a risk factor by itself, but one which can bring on a blood clot in someone who already has one or more risk factors. "That may be what tips the scales," he said.