No link between low air-pressure on long-haul flights and DVT

Researchers simulating conditions of reduced cabin pressure and reduced oxygen levels, such as may be encountered during an 8-hour aeroplane 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 Journal of the American Association.

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 aeroplane 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.

Dr William D. Toff, from the University of Leicester's Department of Cardiovascular Sciences and colleagues -including in the UK Professor Mike Greaves, Head of the University of Aberdeen's School of Medicine -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 haemostatic (blood clotting) system. The study included 73 healthy volunteers who spent 8 hours seated in a hypobaria chamber and were exposed to hypobaric hypoxia, similar to the conditions of reduced aeroplane 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 haemostasis (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 study was a collaboration between the Universities of Leicester and Aberdeen. The chamber studies were performed at the RAF Centre of Aviation Medicine at Henlow in Bedfordshire and Aberdeen. Analysis of blood samples was performed at Leicester and Aberdeen, and also by colleagues at the Academic Medical Center at the University of Amsterdam.

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).