Deep vein thrombosis (DVT) occurs when blood clots form in the deep veins of the legs, including veins in the calves, thigh or hip. These veins are responsible for draining oxygen-poor blood from the lower extremities. After traveling up through the legs, they feed into a large vein called the inferior vena cava that connects to the right atrium. From there, the oxygen-poor blood drains into the right ventricle and is pumped into the pulmonary arteries. The pulmonary arteries are connected to the lungs, where the oxygen-poor blood sheds its waste products and picks up new oxygen.

About half of people who have deep vein thrombosis have no symptoms, while the other half may experience pain, throbbing or warmth in their legs. The condition is dangerous because of the risk that a blood clot will dislodge and travel to the lungs as an embolus. If it lodges in the lungs and restricts blood flow, it will result in a pulmonary embolism, a dangerous condition that can be fatal. The likelihood of a pulmonary embolism is somewhat related to the location of the DVT. DVT that is located in the calves is less likely to result in a pulmonary embolism than DVT that is located in the veins of the thighs or upper legs.

Deep vein thrombosis is more common among women and people over the age of 50, although it may occur among men and younger people as well. Researchers have identified a number of risk factors for deep vein thrombosis and estimate that as many as 80 percent of patients with DVT exhibit one or more risk factors. These risk factors include:

In recent years, the use of central venous catheters has also been connected to DVT. Roughly 10 percent of DVT cases have been attributed to these devices.

In the past, DVT thrombosis had been closely associated with airline travel (the so-called “economy class syndrome”) but recently researchers have begun to question the connection between airline travel and DVT. Because the risk of DVT increases with long periods of immobility, it may be that sitting in any seat for a long period of time may be the culprit behind the increased risk of DVT.

The risk of DVT is especially high among people who have experienced previous blood clots, including another form of thrombophlebitis called superficial vein thrombosis (SVT or phlebitis). In these conditions, blood clots form on the surface veins of the legs, but there is no risk they will travel to the lungs and cause pulmonary embolism. However, there is a possibility that one of these conditions will progress to DVT.

Therapy for DVT is aimed at preventing pulmonary embolism and relieving any symptoms that may be present. Physicians may choose to use medications, compression stockings or special filters that are implanted into the veins to catch loose blood clots before they can travel to the lungs.