Deep Vein Thrombosis (DVT) occurs when a blood clot (thrombus) develops in one or more of the deep veins of the legs, pelvis, or arms. Blood clots can form in either superficial veins (known as superficial thrombophlebitis) and/or in the deep veins. Blood clots in superficial veins rarely cause serious problems; however, clots in the deep veins require immediate medical attention.

Veins located deep in the center of the leg are surrounded by strong muscles. When they contract, they force deoxygenated blood back to the lungs and heart. In a healthy vessel, one-way valves prevent the backflow of blood between the contractions. Through repeated contractions, the blood is ultimately returned from the legs to the heart. The development of clot in the legs can result in serious morbidity and can even be fatal. Clots in the legs can break off and travel through the bloodstream to the heart and lungs as a pulmonary embolism (PE). Large pulmonary emboli can cause critical strain on the heart, resulting in cardiac arrest. Beyond the risk of migration, thrombus in the legs can cause severe swelling and pain in the leg, which can be permanent. In rare cases, the clot itself can threaten the viability of the affected limb.