Effect of hypobaric hypoxia, simulating conditions during long-haul air travel, on coagulation, fibrinolysis, platelet function, and endothelial activation

Abstract

Conclusion: Hypobaric hypoxia at the levels encountered during long-haul air travel does not result in prothrombotic conditions in healthy individuals at low risk of venous thromboembolism. Summary: There is a proposed link between long-haul air travel and venous thromboembolism (VTE). The mechanism by which long-haul air travel may result in VTE is unknown. It is postulated reduced cabin pressure and reduced oxygen tension may result in an increase risk of VTE compared with seated immobility. The authors sought to determine whether levels of hypobaric hypoxia encountered during air travel could activate hemostasis. This was a single, blind, crossover study performed in a hyperbaric chamber. The authors assessed the effect of 8 hours of seated exposure to hypobaric hypoxia in 73 healthy volunteers. The study was conducted in the United Kingdom from September 2003 to November 2005. The patients were screened before study enrollment for factor V Leiden mutation and the prothrombin gene 20210A mutation. If such a mutation was present, they were excluded. Activation of hemostasis was tested by blood draws before and after induction of hypobaric hypoxia. Study subjects were exposed alternatively (>1 week apart) to hypobaric hypoxia at conditions similar to those of commercial air travel and to normal baric, normal hypoxia conditions equivalent to atmospheric conditions at approximately 70 meters above sea level. Changes in coagulation activity, fibrinolysis, platelet activation, and endothelial cell activation were then compared under the two testing conditions. Changes were observed in some hemostatic markers during normal baric exposure. This was attributed to prolonged sitting and circadian variation. Including analysis for thrombin-antithrombin complex, prothrombin fragment, D-dimer, and tests of endogenous thrombin potential, there were no significant differences between changes in the hypobaric and normal baric exposures. Comment: It is important to recognize who was studied and who was not studied in this investigation. Patients taking oral contraceptive pills were included, as were patients >50 years of age. However, individuals with factor V Leiden and prothrombin gene mutation were excluded, as were those with a history of VTE. Thus, the patients at highest risk for VTE associated with long-haul air travel were not studied. The study addresses potential changes in coagulation induced by hypobaric hypoxic. It does not address the clinical question of potential coagulation changes in individuals at most risk for long-haul air travel associated VTE.