Photograph of a woman's swollen and gangrenous right foot, resulting from the reduction in blood supply caused by a deep vein thrombosis (DVT) in her right leg. Deep vein thromboses are provoked by situations that encourage sluggish blood flow and which increase the tendency of the blood to coagulate; these include prolonged bed rest, surgery, heart failure and pregnancy. The main danger is that a part of the clot may become detached and cause a blockage in another part of the circulation - as in the case of a pulmonary embolism, where a blood clot is carried in the circulation to lodge in the pulmonary artery.