Sitting, Not Pressure, May Increase VTE Risk in Flight

BY JOHN R. BELL

The low-air-pressure, hypoxic environment experienced in air travel is not likely a cause of the increased risk for venous thromboembolism associated with long-distance flight, according to Dr. William D. Toff of the University of Leicester (England) and his colleagues.

THE CHANGES WERE ASCRIBED TO CIRCADIAN RHYTHM AND PROLONGED SITTING RATHER THAN TO LOWERED ATMOSPHERIC PRESSURE.

Their 2-year single-blinded crossover study compared the effects of a simulated long-haul flight--prolonged sitting in a hypobaric, hypoxic environment--with the effects of prolonged sitting in a normobaric, normoxic control environment. Study participants in both groups showed significant changes in measures of several blood markers associated with thrombolysis, but these changes were not significantly different between the two exposure environments and were ascribed to circadian rhythm and the act of prolonged sitting rather than to lowered atmospheric pressure (JAMA 2006;295:2251-61).

A total of 73 participants were screened for factor V Leiden and prothrombin G202190 mutations (the most common causes of thrombophilia) and stratified into three groups according to their risk of thromboembolism: a younger group (49 participants; age range 18-40 years; mean age 24 years) not taking oral contraceptives; a smaller group of oral-contraceptive users (12 participants; age range 18-40 years; mean age 24 years), and an older group of men and women (12 participants; age, at least 50 years; mean age 57 years).

The researchers then randomly assigned all participants to one of two exposure groups, which differed only in the order of exposure. One group first sat for 8 hours in a bariatric chamber pressurized to create an environment of hypobaric hypoxia equal to an altitude of roughly 8,000 feet (the lowest cabin pressure permitted by airline regulations) and 1 week later, they sat for another 8 hours in the chamber under normobaric normoxia. The second group underwent the same exposure but in the reverse order. The participants were allowed to stand up and move for 5 minutes every hour, could drink nonalcoholic beverages, and were given a light lunch and snacks.

As expected, participants in all three risk groups experienced lower arterial oxygen saturation during the low-pressure portion of the study. However, statistically significant changes were seen in markers of coagulation activation and fibrinolysis not only during the hypobaric exposure, but also during the normobaric (control) session. The investigators thus concluded such changes were associated with long-term sitting and natural circadian patterns, rather than with air pressure.

They did not comment on whether the changed blood marker levels, corresponding with increased risk for venous thromboembolism, are in themselves cause for concern. However, they called these changes "minor." They conceded that for some, genetic risk factors might interact with hypoxia, increasing the risk of thromboembolism.

In an accompanying editorial, Dr. Peter Bärtsch of the University of Heidelberg (Germany) concurred with the authors that mild hypoxia and prolonged sitting pose little risk to most individuals, but he added that "the small numbers of older participants and individuals taking contraceptives preclude drawing reliable conclusions about these groups" (JAMA 2006;295:2297-9).

The U.K. Department for Transport, the U.K. Department of Health, and the European Commission funded the study.