In people who have deep vein thrombosis, a blood clot may break loose from an affected vein in the leg and travel through the bloodstream. A clot that breaks loose is called an embolus. The embolus travels toward the heart and passes through the right atrium and ventricle and into one of the pulmonary arteries, which carry blood to the lungs. The clot may lodge in an artery in a lung and block blood flow, resulting in pulmonary embolism. Pulmonary embolism may be life threatening, depending on the size of the blocked artery. To prevent pulmonary embolism, doctors usually use drugs that limit blood clotting. However, for some people, doctors may recommend that a filter (formerly called an umbrella) be temporarily or permanently placed in the inferior vena cava. This filter device typically is recommended when drugs that limit clotting (anticoagulants, or blood thinners) cannot be used, for example, when a person is also having bleeding. The filter can trap emboli before they reach the heart but allow blood to flow through freely. Emboli that are trapped sometimes dissolve on their own. However, filters do not completely eliminate the risk of emboli. Sometimes other veins from the legs enlarge, allowing blood and emboli to bypass the filter. Also, filters can break loose or become blocked by a clot. Filters are much less effective at preventing pulmonary embolism than treatment with anticoagulants.