Deep vein thrombosis remedies up in the air

More legroom, aspirin, and compression
stockings may not help

By Owen Dyer

New Zealanders, like Canadians, do a great deal of flying and many of them take active measures to avert the danger of deep vein thrombosis (DVT) or other venous thromboembolism. So New Zealand was the perfect setting for a prospective study into the dangers of long-haul or frequent flight. The findings of the New Zealand Air Traveller's Thrombosis study, published in The Lancet appear to justify the nervousness that many passengers feel when squeezing into the seat of the modern airliner.

A previous study in The Lancet suggested that no fewer than 10% of long-distance air travellers suffer symptomless DVT during or after flight. This study set out to measure the frequency of symptomatic DVT, and other symptomatic events involving thromboembolism.

The researchers recruited 878 passengers aged 18 to 70, judged at low to moderate risk of clotting problems. All took flights of at least four hours duration and flew a total of at least 10 hours during the six-week study period, logging an average 39 hours in the air per passenger. Many of these frequent flyers were aware of the dangers of DVT -- 17% wore compression stockings and 31% took aspirin to reduce thrombosis risk.

Passengers were given a D-dimer blood test before and after flying. This test, which measures fibrin degradation products, is a standard measure of thrombosis. Those whose D-dimer count rose significantly and those who reported symptoms indicative of thrombosis were investigated further, using bilateral compression ultrasonography and CT pulmonary angiography.

Nine passengers developed venous thromboembolism, of which five suffered DVT and four developed pulmonary embolism. That suggests a 1% rate of venous thromboembolism even in this low-to-moderate risk group.

The really startling finding was the apparent total lack of protection conferred by aspirin and compression stockings. In fact, people who took these precautions appeared more likely to develop a thrombus than those who did not. Of the nine subjects who showed evidence of clotting problems, five used aspirin and four wore compression stockings.

The authors are reluctant to draw hasty conclusions from a sample of just nine people, particularly since six of them had pre-existing clinical risk factors. Co-author Professor Richard Beasley, of the Medical Research Institute of New Zealand, cautiously suggested that "the role of prophylactic measures in air travel-related venous thromboembolism needs further investigation."

Most travellers assume that the extra legroom afforded by business-class seats offers some protection, but that widespread belief also failed to stand up to scrutiny. Two of the nine symptomatic subjects were business-class travellers. The high-paying customers at the front of the plane were proportionally just as heavily represented in the venous thromboembolism group as the multitudes crammed in the back.

Professor Beasley commented, "the term economy-class syndrome is now redundant, with a better term being air-traveller's thrombosis.