Sitting, not flying, may cause clots

Reduced air pressure and oxygen levels do not appear to promote the formation of deadly blood clots during long commercial flights, an ailment sometimes called “economy class syndrome.”

The findings seem to bolster the widely held belief that clots develop in otherwise healthy people mainly because they are sitting in cramped quarters that slow blood flow, especially in the legs, not because of cabin environment.

Researchers at England’s University of Leicester said they ruled out cabin pressure and oxygen changes as contributors to the problem by testing 73 healthy volunteers. The participants were placed in seats for eight hours in a chamber where the air pressure and oxygen levels mimicked those experienced during commercial air travel. They were allowed to stand up and move about for five minutes every hour.

The same group was retested at ground-level pressure and oxygen levels. Blood was drawn before and after each of the tests.

The researchers said they found no significant difference between the two tests on clot formation; on the breakdown of small, naturally occurring clots; in the activation of platelets -- cells in the blood that clump together when stimulated to promote clot formation; or in the action of endothelial cells that line the interior surface of blood vessels.

The study was published in the May 17 issue of the Journal of the American Medical Assn.