SummaryBackground There is an increased risk of venous thrombosis after air travel, but the underlying mechanism is unclear. Our aim was to ascertain whether flying leads to a hypercoagulable state.Methods We did a crossover study in 71 healthy volunteers (15 men, 56 women), in whom we measured markers of activation of coagulation and fibrinolysis before, during, and after an 8-h flight. The same individuals participated in two control exposure situations (8-h movie marathon and daily life) to separate the effect of air travel on the coagulation system from those of immobilisation and circadian rhythm. To study the effect of risk factors for thrombosis, we included participants with the factor V Leiden mutation (n=11), those who took oral contraceptives (n=15), or both (n=15), as well as 30 individuals with no specific risk factors.Findings After the flight, median concentrations of thrombin-antithrombin (TAT) complex increased by 30·1% (95% CI 11·2-63·2), but decreased by 2·1% (Ϫ11·2 to 14) after the cinema and by 7·9% (Ϫ16·2 to Ϫ1·2) after the daily life situation. We recorded a high response in TAT levels in 17% (11 of 66) of individuals after air travel (3% [2 of 68] for movie marathon; 1% [1 of 70] in daily life). These findings were most evident in the group with the factor V Leiden mutation who used oral contraceptives. We noted a high response in all variables (prothrombin fragment 1 and 2, TAT, and D-dimer) in four of 63 (6·3%) volunteers after the flight, but in no-one after either of the control situations.Interpretation Activation of coagulation occurs in some individuals after an 8-h flight, indicating an additional mechanism to immobilisation underlying air travel related thrombosis. hypoxia controlled for effects of immobilisation at ground level and for circadian rhythm. We drew blood before, during, and after each exposure, at the same time of day. For the first exposure situation, we chartered a Boeing 757 for a non-stop day flight of 8 h from and to Schiphol airport, Amsterdam. We instructed volunteers not to smoke, use drugs, drink alcohol, or take any prophylactic measures to prevent thrombosis-eg, heparin or aspirin use, or wearing of elastic stockings during the flight-to remain seated as much as possible, and to keep a structured record of fluid intake. 2-3 weeks after the flight, all participants sat for 8 h (for practical reasons two sessions with half the participants in each) in a cinema. We imposed the same restrictions during this second exposure situation as in the first with respect to moving, drugs, smoking, and drinking, and again asked participants to keep a record of their fluid intake. For the final exposure situation, 2 weeks after the movie marathon, we asked participants to live their lives as they normally would for a day, recording fluid intake. We asked participants not to drink alcohol, take drugs, or smoke, but did not restrict movement. We asked participants to keep their food and fluid intake constant during the three exposures.Experienced technicians did the blood ...

In the past decade, numerous publications on the association between venous thrombosis (VT) and travel have been published. Relative and absolute risks of VT after travel, and particularly after travel by air, have been studied in case-control and observational follow-up studies, whereas the effect of prophylaxis has been studied through intervention trials of asymptomatic clots. The mechanism responsible for the association between travel and VT was addressed in pathophysiologic studies. Here, we systematically reviewed the epidemiologic and pathophysiologic studies about the association between travel and VT. We conclude that long-distance travel increases the risk of VT approximately two to fourfold. The absolute risk of a symptomatic event within 4 weeks of flights longer than 4 h is 1 ⁄ 4600 flights. The risk of severe pulmonary embolism (PE) occurring immediately after air travel increases with duration of travel, up to 4.8 per million in flights longer than 12 h. The mechanism responsible for the increased risk of VT after (air) travel has insufficiently been studied to draw solid conclusions, but one controlled-study showed evidence for an additional mechanism to immobilization that could lead to coagulation activation after air travel.

Summary In a case–control study including 11 033 participants (The Multiple Environmental and Genetic Assessment of risk factors for venous thrombosis study) on risk factors of venous thrombosis, we studied the effect of flight‐related behaviour on the risk of venous thrombosis after air travel. Patients and control subjects received a questionnaire on risk factors for venous thrombosis, including recent travel history and details of their last flight. From this population, 80 patients and 108 control subjects were selected who had recently (<8 weeks) travelled for more than 4 h by aeroplane. Window seating compared to aisle seating increased the risk twofold [odds ratio (OR) 2·2; 95% confidence interval (CI): 1·1–4·4], particularly in those who were obese (OR 6·1; 95% CI: 0·5–76·2). Anxiety (OR 2·5; 95% CI: 0·9–7·0) and sleeping (OR 1·5; 95% CI: 0·7–3·1) may increase the risk slightly. The risk was not affected by alcohol consumption (OR 1·1; 95% CI: 0·5–2·4). Flying business class may lower the risk (OR 0·7; 95% CI: 0·2–1·8). We did not find a protective effect for several measures currently part of standard advice from airlines and clinicians, i.e. drinking non‐alcoholic beverages, exercising or wearing stockings. The effect of behavioural factors during flying on the risk of venous thrombosis after air travel is limited. Current advice on prevention of travel‐related thrombosis may have to be reconsidered.

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