BOSTON — Air travel can put frequent or casual flyers at significantly increased risk for a venous thromboembolic event for up to a month after the end of a trip, British investigators reported at a meeting of the International Society on Thrombosis and Haemostasis.
Flying for more than 4 hours at a stretch—or a total flying time of more than 12 hours in the past 4 weeks—was associated with a two- to nearly threefold greater risk for VTE, compared with nontraveling controls, reported Dr. Peter K. MacCallum of Barts and The London at the University of London.
“In this community-based case-control study, we found that air travel was a mild risk factor for venous thrombosis in the subsequent 4 weeks. The risk seen at 4 weeks was no longer apparent at the 12-week time-frame, so the dose response and the declining risk with the passage of time tend to support a causal relationship between air travel and subsequent thrombosis,” Dr. MacCallum said.
The size of the air-travel effect on VTE risk was comparable to that of low-risk surgery. Other factors associated with increased risk for VTE were body mass index from 25 kg/m
Cases series linking air travel to VTE risk date to the 1950s, and by 1977 the phenomenon had earned the nickname “Economy class syndrome.” Over the last decade, researchers have taken a more systematic approach, with case-control, observational, follow-up, intervention, and laboratory studies.
The findings echo those of a recently published meta-analysis, which suggested that air travel was associated with about a threefold risk for VTE (Ann. Intern. Med. 2009 Aug. 4 [Epub ahead of print]).
Dr. MacCallum and his colleagues conducted a community-based, case-control study looking at venous thromboembolic events among patients in 123 general practices in the United Kingdom. They identified patients who had received a prescription for warfarin over the previous 12 months, performed a record search to identify those patients who had confirmed deep vein thromboembolism/pulmonary embolism (DVT/PE), and assigned six age- and sex-matched controls for each case.
All cases and controls were contacted by mail with consent forms and questionnaires. A total of 550 cases and 1,971 controls were studied.
In univariate analysis, the only significant flight-associated risk factor for short-term VTE was total flight time longer than 12 hours (OR, 1.91; 95% confidence interval, 1.08-3.39). In multivariate analysis adjusted for BMI, surgery, and past history of VTE, the only significant risk factors for VTE within 4 weeks of flying were any flight leg longer than 4 hours (OR, 2.20; 95% CI, 1.29-3.73) and total flying time greater than 12 hours (OR, 2.75; 95% CI, 1.44-5.28). By week 12, however, neither flight leg duration nor total flight time was significantly associated with increased risk for VTE.
The funding source for the study was not disclosed. Dr. MacCallum said that he had no relevant conflict-of-interest disclosures.