[Pathophysiology of immobilization]

Deep venous thrombosis is initiated primarily in the pockets of the valves of the veins of the lower limbs and the main pelvic veins. In addition to a genetic predisposition there are several acquired conditions associated with deep venous thrombosis such as major surgery, trauma, cancer, pregnancy, and immobilization. While major diseases as well as hormonal changes have been shown to cause changes in blood coagulation and fibrinolytic factors, the impact of immobilization per se is much less clear. It has been shown that in the absence of intermittent pulsatile flow the blood within the valve pockets became rapidly hypoxic when undisturbed during streamline flow (i.e. when "static"). Hypoxia on the other hand has been shown to cause procoagulatory changes of the vascular endothelium (e.g. production of platelet activating factor [PAF], expression of tissue factor), adhesion and activation of leukocytes and expression of tissue factor on their surface, as well as the activation of platelets. Together with reduced removal and/or inactivation of active clotting factors these mechanisms might contribute to the development of deep venous thrombosis during immobilization.