Venogram showing deep vein emboli, the blockage of a vein in the leg by the formation of a blood clot. Clot formation in the deep veins of the legs may often be the cause of arterial obstruction in another part of the circulation. A part of the clot may break away & be carried by the circulation where it may lodge in one of the finer arterial branches. This most commonly occurs in the branches of the pulmonary artery; such a condition is known as a pulmonary embolism, which in severe cases may result in acute heart failure & sudden death.