The role of hypoxia and platelets in air travel-related venous thromboembolism.

Although somewhat controversial, there is good evidence that long-distance travel in general is a risk factor for venous thromboembolism, even in the absence of other risk factors. This is probably due to effects consequent to prolonged sitting but air travel in particular may be associated with risk factors other than this. One likely factor is hypoxia caused by the low ambient pressure of aircraft cabins. There is an association between venous thromboembolism and the hypoxia of altitude, chronic respiratory disease, neonatal hypoxia, sleep apnoea and experimentally-induced hypoxia. Platelet number and/or function are altered in all of these circumstances. Platelet aggregation is pivotal to venous thromboembolism and hypoxia alters platelet number and function. The early-onset thrombocytosis caused by hypoxia may be due to increased release of platelets from megakaryocytes and the late-onset thrombocytopaenia may be due to decreased platelet production and/or stem cell competition between erythrocytes and megakaryocytes. Hypoxia-induced platelet activation and aggregation may be due to increased circulating catecholamine levels but it is not known whether hypoxia can affect platelets directly. There is a need for further studies on the possible involvement of hypoxia-induced changes in platelet number and function in air travel-related venous thromboembolism.