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BOSTON -- Air travel can put frequent or casual flyers at 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 were body mass index from 25 kg/m2 to 30 or greater than 30, he reported.

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, Dr. MacCallum said. 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]).

The researchers conducted a community-based, case-control study looking at VTE 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.

Individuals were contacted by mail with consent forms and questionnaires with prepaid return envelopes. They were also sent a reminder letter and information sheet. Nonresponders to the first mailing received a second copy of the consent form and questionnaire.

Along with standard demographic information, the questionnaire asked whether subjects currently had a warfarin prescription, had a history of DVT or PE, had surgery within the past 2 years (if yes, with dates, type of surgery, and length of stay), and had traveled on an airplane in the past 2 years (if yes, with details on dates, departure and destination sites, length of stay, return route, and class of travel).

The investigators calculated odds ratios (ORs) using conditional logistic regression modeling to compensate for matching or unequal numbers of controls. The analysis factored in differing durations of air travel, and looked at risk in both the short term (4 weeks after completing travel) and the long term (12 weeks post flight). They also controlled for other VTE risk factors in a multivariate model. They received replies from 638 cases (55%) and 3,162 controls (58%). After exclusions for various reasons, they arrived at 550 cases and 1,971 controls for the final sample.

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 authors plan to conduct additional analyses to explore the relationship between air travel and other risk factors, although they are working with fairly small samples, he acknowledged.

When asked to comment Dr. Ali AbuRahma, professor of surgery, West Virginia University, stated: "Immobility has been a major pillar for DVT since the description of Virchow's triad over a century ago. It's no wonder that patients who travel for more than several hours with no mobility can be at a three times higher risk for DVT in this study. ... .I would like to see some prospective studies on this subject to finally clarify the 'economy class syndrome,' however this would not be an easy task." Dr. AbuRahma concluded.

AIR TRAVEL WAS A MILD RISK FACTOR FOR VTE IN THE SUBSEQUENT 4 WEEKS, COMPARABLE TO THE EFFECT OF LOW-RISK SURGERY.