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May 16, 2006 -- A new study clips the wings of one theory about why blood clots sometimes form during long flights.
The theory holds that lower oxygen levels on long flights make blood clots more likely. The new study, published in The Journal of the American Medical Association, challenges that notion.
The study was conducted in the U.K. It included 73 healthy adults who didn't have genetic conditions that raise the risk of clotting and hadn't recently taken blood thinners, which cut clotting risk.
Participants reported to a Royal Air Force facility for two experiments, held a week apart. They sat in three groups in special chambers that simulated the conditions of an eight-hour flight.
During one session, the chamber had lower oxygen levels similar to that on airplanes flying at 8,000 feet. During the other session, oxygen levels were like those on the ground at sea level.
Fake Flight
The study tried to copy conditions on a lengthy flight -- without jet lag, passports, and plane tickets.
Blood clots on such flights are rare, but they can happen, causing DVT (deep vein thrombosis, or clots in the legs' deep veins) or pulmonary embolismpulmonary embolism (a clot that travels through the bloodstream to the lungs).
Such clots can also happen with long stretches of idleness on the ground, such as during long car or train trips or prolonged bed rest.
In the new study, participants were allowed to get up and move around for up to five minutes every hour. They were also allowed to drink alcohol-free beverages and were served a light meal and snack during their "flights" to nowhere.
Checking for Clotting Risk
The researchers -- who included William Toff, MD, of England's University of Leicester -- checked several biochemical markers of clotting in participants' blood samples taken before and after each session.
Toff's team found no significant differences between those markers during the sessions. "No significant differences" means that any small differences may have been due to chance.
Participants had been asked to report any clotting symptoms within a week of each session. None did so.
The study included 12 women who were taking oral contraceptives (which raise clotting risk) and another group of 12 people aged 50 and older. Those groups' test results didn't show higher clotting risk, but with such few participants, it's hard to be sure of that finding, the researchers note.
It's not clear if the results apply to people with other health conditions. The study was funded in part by the U.K.'s Department for Transport.
Another recent study, done in the Netherlands, showed that low oxygen levels on long flights might increase clotting risk in some fliers.
Surprising Finding
The finding that low oxygen and prolonged sitting didn't raise clotting risk was "surprising," writes editorialist Peter Bartsch, MD. He works in Germany at the Universitatsklinikum Heidelberg and wasn't involved in Toff's study.
The Dutch and British studies weren't exactly like a real flight, Bartsch notes, noting the alcohol ban and hourly activity allowed in the experiment.
Bartsch writes that "the small number of older participants and individuals taking oral contraceptives preclude drawing reliable conclusions about these groups." He also asks if lower oxygen levels might interact with other risk factors, such as advanced age and a history of DVT.
The results might not apply to clotting risk in mountain climbers at high altitudes, Bartsch also notes.
Avoiding in-Flight Clots
In a University of Leicester news release, Toff commented on his study.
"Although we found no evidence that the low pressure and low oxygen activate blood clotting, we know from many other studies that prolonged sitting, such as during long journeys by air, road or rail, does increase the risk of deep vein thrombosis (DVT). That link is quite clearly established," Toff says.
"The overall risk of thrombosis after a long-haul flight is estimated to be about 1 in 2,000," he continues. "For people with known risk factors, the risk may be higher but for those without other risk factors it is likely to be very low and should be kept in perspective."
Toff recommends taking "sensible precautions" on long flights, such as regularly doing leg exercises, getting up and walking around the plane's cabin from time to time, wearing special compression stockings, and (if recommended by a doctor) taking anticlotting drugs.
SOURCES: Toff, W. The Journal of the American Medical Association, May 17, 2006; vol 295: pp 2251-2261. WebMD Medical News: "More Risks for Leg Blood Clots." Bartsch, P. The Journal of the American Medical Association, May 17, 2006; vol 295: pp 2297-2299. News release, University of Leicester. News release, JAMA/Archives.