Deep veins, in which a DVT occurs, can not be seen or felt under the skin. Poor blood circulation in the deep veins Conditions that increase the tendency of blood to clot Injury to a deep vein from trauma or surgery causes release of substances that activate the clotting system. Clotting is needed to prevent excessive bleeding when blood vessels are injured. When clotting is no longer needed other clotting factors help dissolve clots. When this balance is disturbed either excessive bleeding or clotting of blood that serves no useful purpose may occur.
Persons affected are generally older.
A DVT can be lifethreatening if a large enough blood clot breaks loose and travels to the heart or lungs. Pulmonary emboli can disrupt blood flow to the lungs by directly blocking blood flow to part of the lung and by triggering certain reflexes that can cause the heat to suddenly stop beating. Many, although not all, pulmonary emboli are caused by embolization of DVTs. Another complication of DVT, particularly recurrent DVTs, is postphlebitic syndrome. Chronic stretching and injury to deep veins may cause blood to pool and stagnate in the lower legs.