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In-Flight Blood Clots: Theory Grounded

Low Oxygen on Long Flights Doesn't Seem to Cause Clots, Study Shows
WebMD Health News
Reviewed by Louise Chang, MD

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.

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