No effect of isolated long‐term supine immobilization or profound prolonged hypoxia on blood coagulation

Background Long‐distance air travel is associated with an increased risk of venous thrombosis. The most obvious factor that can explain air travel–related thrombosis is prolonged seated immobilization. In addition, hypobaric hypoxia has been shown to affect coagulation, and the lowered atmospheric pressures present in the cabin during the flight may therefore play an etiologic role. Because immobilization and hypoxic conditions are usually present simultaneously in airplanes or hypobaric chambers, their separate effects on the coagulation system or on thrombosis risk have not been studied extensively. Objectives To investigate the separate effects of long‐term immobilization and profound prolonged hypoxia on blood coagulation. Patients and Methods We performed two studies in collaboration with European Space Agency/European Space Research and Technology Centre. In the first study, 24 healthy, non‐smoking, adult women underwent 60 days of −6° head‐down bed rest. In the second study, we took blood samples from 25 healthy men who participated during their stay in the Concordia station in Antarctica, where, due to the atmospheric conditions, continuous severe hypobaric hypoxia is present. In both studies, we measured markers of blood coagulation at baseline and at several time points during the exposures. Results and Conclusions We observed no increase in coagulation markers during immobilization or in the hypobaric environment, compared with baseline measurements. Our results indicate that neither immobilization nor hypoxia per se affects blood coagulation. These results implicate that a combination of risk factors is necessary to induce the coagulation system during air travel.