March 9, 2006 -- For some people on long flights, sitting still for hours might not be their only in-flight risk of developing leg clots.
In The Lancet, researchers in the Netherlands found that some people were at higher risk for leg clots on a long flight than on land. But immobility -- a risk factor for leg clots -- didn't seem to be the only reason for that, the study shows.
Low oxygen levels on planes might be a reason, but that's not certain, write the researchers. They included Anja Schreijer, MD, of the vascular medicine department at Leiden University in the Netherlands.
Schreijer's team isn't recommending that anyone avoid long flights. No one in their study developed leg clots, regardless of their risk factors.
About the Clots
Schreijer's team studied risk of deep vein thrombosis (DVT), a potentially dangerous type of blood clot that develops in the deep veins of the legs, torso, or arms.
Such clots can grow, break off, and travel through the bloodstream to the lungs, causing a life-threatening condition called pulmonary embolism.
DVT risk rises if people are seated for a long time. That's why travelers are encouraged to move around every now and then on long flights. People are at risk for DVT in lower extremities during periods of immobility. Walking causes contraction of leg muscles, which gets the blood in the veins moving and thus helps prevent clot formation.for your legs have also been shown to cut in-flight clot risk.
Of course, DVT isn't limited to air travelers, and it's not just due to immobility. DVT can be caused by conditions that slow blood flow or thicken blood. It is also associated with injury, pregnancy, use of hormones (such as estrogen or birth control pills), genetic disorders, damaged valves in veins, and cancer.
Research on Land and in the Air
Schreijer and colleagues studied 15 men and 56 women on land and in the air.
Some participants were at higher risk of clots, including women taking oral contraceptives and people with a certain gene mutation. Others didn't have any known clotting risk.
The researchers took blood samples from participants before, during, and after regular daily activities, a movie marathon, and an eight-hour flight.
Using the blood samples, the researchers checked levels of a chemical marker of clotting. They found higher levels of that marker during the flight than on land, even while participants became couch potatoes during the movie marathon.
None of the participants actually developed leg clots on land or in the sky.
Who Was at Risk?
The higher levels of the clotting chemical marker were strongest in women taking oral contraceptives who also had a genetic mutation that boosted their clotting risk, the researchers report.
Perhaps participants' levels of the clotting marker were rising before the flight, the researchers note. They add that participants were young, healthy, and not anxious on the flight.
If something besides immobility raises in-flight clotting risk, their study doesn't pinpoint what it is.
A journal editorial notes "much debate" in recent years about an association between clotting and long-distance air travel. People at high risk of clotting may want to explore ways to limit their DVT risk on long flights, writes editorialist Hans Stricker, MD, of the Ospedale La Carita in Locarno, Switzerland.