What medical specialists do know about instances of DVT is that "it usually involves a set of factors working together - probably a constellation of factors working together," says University of Aberdeen haematology professor Mike Greaves. He is adamant, however, that the lack of knowledge on the condition is "really quite profound". And Greaves, like other researchers, is critical of some research that already has been conducted as being "not well controlled".
Studies are needed on what the risks are, how to reduce them and the effects of cabin environmental conditions such as altitude. Lower oxygen levels, for instance, activate blood clotting.
Greaves is seeking a financial go-ahead to undertake a study at Aberdeen's barometric chamber where the aircraft cabin environment could be simulated, and pressurisation tests conducted.
Dutch haematology specialist Frits Rosendaal of Leiden University envisions a study in which airline passengers on actual long-haul flights would be invited to participate in follow-on surveys of possible thrombotic occurrences. By Rosendaal's estimate, "this would require at least 100,000 people, but considering that each jumbo jet has about 400 on board, that would be quite feasible. Obvious factors to study are differences between business and economy class and flight duration."
Recruiting long-haul passengers is a new wrinkle, and an addition to a concept Rosendaal and partners developed in the late 1990s to study DVT. Formed from retrospective and prospective segments, the study would have given an idea of relative and absolute risks as well as supplying additional information on in-flight conditions.
The DVT studies designed by Rosendaal and partners attempt to solve issues such as "the low rate of thrombosis, the low frequency of flying, and the difficulty in interpreting relative risks. There is also a public health issue involved in the latter point which is that it is difficult to translate relative risks for meaningful everyday use - even without the problem related to the short risk exposure implied in the duration of most flights - the risk is relative to the baseline risk in the absence of the exposure."
For example, Rosendaal explains: "The relative risk for a brain concussion when standing under the shower as compared to lying in bed is sky high. Still that does not stop most people from getting up in the morning, since the absolute risk remains very small. However, in bed, it is nearly zero."
The international group of medical specialists which gathered in Frankfurt in late January under the aegis of John Scurr's organisation is looking at two studies it believes should be carried out. The first would compare the physiological reactions of participants in a ground cabin mock-up and then in the air. The second, involving more than 2,000participants, would look at risk assessments. High-risk subjects would be included in the study, unlike Scurr's own recent research which excluded them.
The latter study, of which final results will be published soon in The Lancet, indicated that one in 10 people would potentially suffer blood clots after air travel, but Scurr emphasises that none of the clots experienced by study participants proved injurious. "We believe that in the vast majority of people who do develop small clots, as soon as you get off the aeroplane and start walking around, the body has the ability to dissolve it, it goes away; it doesn't cause any problems. Nobody knows about it, and that's it," he says.
In the 1970s, he adds, patients undergoing major surgery in UK hospitals stood a 1 in 3 chance of developing a blood clot in the leg. After research, patients could be identified for high, medium and low risk of developing blood clots, and preventive measures such as graduated compression stockings and blood-thinning agents could be applied. The number of patients getting clots has dropped since then. This philosophy could be used to identify and target air travellers at risk: "It's based on what we know we can do for surgical patients and extrapolating it into the air," Scurr says.
Another factor that can come into play for up to 5% of the population is a genetic blood-clotting abnormality known as Factor V Leiden, which may be responsible in part for some DVT instances involving young people. Both Scurr and Greaves are wary of suggesting that all travellers take aspirin as a DVT precaution because of the drug's propensity to cause bleeding in some people's systems.
All in all, Scurr says, "I think these matters go beyond deep-vein thrombosis. We're looking at a series of new issues brought about by things we didn't do years ago. And when we go from aeroplanes to space travel, which we undoubtedly will do, we will be encountering new medical problems, and you often can't predict what they are until they happen."
Source: Flight International
