A recent JAMA article (the full text is available on line) from Great Britain sheds some light on possible mechanism (s) involved in the development of venous thrombosis in long haul air travelers. ( Long haul is usually taken to mean 8 hours or more.)It does this by failing to demonstrate evidence for an increased tendency of the blood to clot after doing extensive testing for relevant blood coagulation system changes including tests for platelet activation,fibrinolysis,and coagulation factor activation.
The issue-in part- has been if the reported association between long haul flights and venous thromboembolism (VTE) is due to prolonged sitting -at times in cramped conditions- or does the air travel induce some increased tendency to clot perhaps related to hypoxia and or decreased atmospheric pressure ( hypobaria) and , of course, both could be operative.
This study simulated air travel using a hypobaric chamber to expose some 73 healthy volunteers to hypobaric hypoxia for 8 hours Persons with either the prothrombin gene mutation (G200210A) or Factor V Leiden were excluded. The study design compared sitting for 8 hours with and without the impositions of hypoxic hypobaric conditions. No significant differences in the various tests of the clotting system were found.
It seems that every talk on VTE has ( or at least used to have) an obligatory introductory section mentioning Virchow's triad of possible culprits predisposing to thrombosis; 1) damage to the vessel wall, 2) alterations in blood flow (stasis or turbulent flow) and 3) increased tendency of the blood to clot. This paper goes at least part of the way to shift the blame in economy class syndrome to the second category. This should encourage travelers to move around as much as possible on the long flights and might be a disincentive to take a sleeping pill and sleep for much of the flight. The rest of the current advice package-at least for persons at no known increased VTE risk is to keep well hydrated and avoid alcohol. Persons recognized to be at higher risk should use graduated pressure stockings and consider low molecular weight heparin pre flight depending on the level of perceived increased risk (such as those with a history of VTE).
It should be noted that persons with Factor V Leiden and or the prothrombin mutation represent a different situation.The editorial in the same issue of JAMA noted that some evidence of hypercoagulability has been noted in persons with those two inherited conditions with exposure to flight conditions. Whether they should be offered low molecular weight heparin is an issue about which experts differ.