Long airplane flight does not appear to increase risk of blood clots

Researchers simulating conditions of reduced cabin pressure and reduced oxygen levels, such as may be encountered during an 8-hour airplane flight, found no increase in the activation of the blood clotting system among healthy individuals, according to a study in the May 17 issue of JAMA.

Venous thromboembolism, a term used to describe deep vein thrombosis (DVT; blood clots forming in the veins) and pulmonary embolism (clots passing to the lungs where they may obstruct the blood flow), has been associated with long-haul air travel, but it has been unclear whether this is due to the effects of sitting for a long time, or whether there is a relationship with some other specific factor in the airplane environment, according to background information in the article. One hypothesis has been that hypoxia (reduced oxygen in the blood), associated with the decreased cabin pressure that occurs at altitude, produces changes in the blood that increase the risk for blood clots.

William D. Toff, M.D., from the University of Leicester, England, and colleagues conducted a study, from September 2003 to November 2005, to assess the effects of hypobaric (reduced air pressure) and hypoxia, similar to that which might be encountered during commercial air travel, on a variety of markers of activation of the hemostatic (blood clotting) system. The study included 73 healthy volunteers who spent 8 hours seated in a hypobaric chamber and were exposed to hypobaric hypoxia, similar to the conditions of reduced airplane cabin pressure that might occur during a long-haul flight (cabin pressure may be reduced to the equivalent of that at an altitude of about 8,000 feet). Blood was drawn before and after exposure to assess activation of factors associated with hemostasis (blood clotting). Similar measurements were taken of the same volunteers, on a separate occasion, before and after they spent 8 hours seated in a controlled environment, equivalent to atmospheric conditions at ground level (normobaric exposure).

The researchers found that when comparing the results between the normobaric and the hypobaric exposures, there was no significant difference in the overall change for markers of coagulation activation (clot formation), fibrinolysis (the normal breakdown of small, naturally occurring blood clots), activation of platelets (small cells in the blood that clump together when stimulated to promote clot formation), and activation of endothelial cells (the cells that line the interior surface of blood vessels).

"In conclusion, our findings do not support the hypothesis that hypobaric hypoxia of the degree that might be encountered during long-haul air travel is associated with prothrombotic alterations in the hemostatic system in healthy individuals at low risk of venous thromboembolism," the authors write.


(JAMA. 2006;295:2251-2261. Available pre-embargo to the media at www.jamamedia.org)

Editor's Note: This study was funded by the UK Department for Transport, the UK Department of Health, and the European Commission.

Editorial: How Thrombogenic is Hypoxia?

In an accompanying editorial, Peter Bärtsch, M.D., of the Universitätsklinikum Heidelberg, Germany, comments on the study that examines hypobaric hypoxia.

"The surprising finding in the studies by Toff et al and [another study] that prolonged sitting did not enhance markers of thrombin [an enzyme that causes blood to clot] formation may be due to several factors that are different from the usual setting of traveling. In the first study, participants were allowed to stand and walk for 5 minutes each hour; in both studies, narcotics and alcohol were not allowed … In addition, it has been suggested that the minor decrease of prothrombin fragment 1 + 2 [a protein in the blood that assists in clotting] observed with prolonged sitting in both studies confirming a previous report could indicate less activation of protein C, a major inhibitor of coagulation, and thus indicate a shift toward a procoagulant state."

(JAMA. 2006;295:2297-2299. Available pre-embargo to the media at www.jamamedia.org)

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