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Updated May 5, 2006


Strong recent studies are described below. The work of these researchers forms a whole that is greater that the sum of its parts. Schobersberger finds clotting factors rising significantly while anti-clotting factors decline. Bendz' findings were similar, but in an environment where immobility was ruled out as the cause of the changes in blood chemistry. And Hamada found increased blood viscosity in the lower legs. These finding were in all of the test subjects, not just a few with some abnormality of blood chemistry. Thus it is not surprising that Belcaro finds blood clots in large numbers of air travelers.

Rodney Hughes et al did follow-up studies after air travel finding 1% of travelers developing DVT after long flights. Hughes had excluded people with risk factors for DVT such as recent injury or surgery. Of 878 air travelers, 9 developed DVT. Four of the nine were wearing compression strockings, contradicting previous studies finding compression stockings effective in preventing DVT during air travel. The Lancet, December 20, 2003.

Wolfgang Schobersberger et al measured molecular markers for coagulation and fibrinolysis in 20 volunteers (10 subjects with a low and 10 with a moderate risk for venous thromboembolism) during and after a nine hour flight. They also did functional measurements of coagulation using activated thrombelastography.

Results: Thrombelastographic measurements revealed activation of coagulation in all passengers, as well as an increased activity of FVII and FVIII as well as suppressed fibrinolysis. There was no evidence of a pronounced thrombin and fibrin formation. there were no differences between the low risk group and the moderate risk group concerning coagulation changes. Conclusion: Long-haul flights induce a certain activation of the coagulation system. This activated coagulation could be a risk factor for venous thromboembolism during long-haul flights mainly when other risk factors are present. (See abstract at Thrombosis Research.)

A previous study by Bendz found similar changes in blood chemistry in a hypobaric chamber with room to move about showing that altitude, not immobility, was the cause of increased coagulability.

Gianni Belcaro's LONFLIT series are the largest and most important studies of economy class syndrome. In most of these, researchers did ultrasound scans of air travelers before and after long flights, to see how many developed clots larger than .5 cm. Over 2,500 have been tested, divided into groups to test various hypotheses:

LONFLIT I. Incidence in low-risk vs. high-risk groups: 13 of 389 developed DVT in the high-risk group. None in the low-risk group.

LONFLIT II Effectiveness of compression stockings: people who wore compression stocking were about twenty times less likely to develop DVT than people in the control group, where 4.5% developed DVT.

LONFLIT III Effectiveness of aspirin and heparin as preventive measures: aspirin had very little effect and some undesirable side effects. Heparin was very effective and is recommended for people with risk factors who must fly. (But aspirin is still effective for arterial clotting and still recommended if you can tolerate it.

LONFLIT IV, Business 2003, found 4.5% of frequent business-class travelers developing one or more clots per year.

LONFLIT __, on short (2-3 hour) flights this not yet published study found the usual 4-5% of passengers developing DVT.

In Japan, Hamada et al tested forty young men on a nine-hour flight. Twenty drank plenty of water, one cup per hour, as many experts recommend. They developed significantly increased blood viscosity (thickness) in the lower legs and increased urine output. The other twenty young men drank one cup per hour of an electrolytic beverage. They did not develop increased blood viscosity and urine output. See JAMA, developed a two-year case registry finding a large majority of victims are young, 80% under 60, and athletic, usually endurance-type athletes like marathoners. A substantial minority of cases arise from relatively short flights of about three hours. Michael Reynolds presented a paper on these findings at an angiology conference in Paris in January, 2003.

In Norway, Bendz et al put 20 young men in a hypobaric chamber simulating aircraft cabin altitude and found substantial hour-by-hour increases in blood clotting factors in all of the young men. This means that everyone in an airliner is at risk, not just a few with some unusual susceptibility. (The Lancet, 11/11/00)

In England, John Scurr , a consulting vascular surgeon, did ultrasound scans of 100 travelers before and after vacation flights, and found that 10% of them developed potentially injurious blood clots. In another group wearing compression stocking, no blood clots were detected. Scurr had carefully excluded anyone with any of the known risk factors such as heart disease. (The Lancet, 5/12/01)

At the World Congress of the International Angiology Union in April 2002, Dr. Omer Iqbal presented a paper saying it is a mistake to focus only on venous clotting. As coagulability rises, the risk of arterial clotting also rises, causing heart attacks and strokes. For example, warfarin is frequently prescribed for people subject to atrial fibrillation because of the tendency to form stroke-causing clots. The rise in coagulability demonstrated by Bendz (above) increases the risk of stroke for these people. Aspirin, ineffective for preventing venous clotting, is effective for preventing arterial clotting, which, unlike venous clotting, is more likely to involve aggregation of platelets.