A deep venous thrombosis (DVT) is a blood clot (thrombus) that forms inside deep veins in your legs or pelvis. The clot blocks blood flow and causes pressure to build up in the vein. Part of the clot can break away and move through your bloodstream to your lungs. If the clot blocks one or more of the blood vessels in your lungs, it is called a pulmonary embolism.
DVT is a common problem. Most of these clots occur when blood flow in the veins of the legs is slowed. This is usually as a result of inactivity.
Ordinarily, as you walk around, your leg muscles squeeze your veins and keep blood flowing back to the heart. But if you are inactive for many hours, blood flow in the veins of your legs may slow so much that clots form. Long periods of inactivity can occur during a long airplane flight or while recovering from an operation or stroke, for example.
Certain people are more likely to get blood clots. These include:
People with some medical problems, including cancers and inherited abnormalities of the blood-clotting system
People on certain medications, such as birth control pills and hormone therapy
People who are very overweight
People with heart failure
Anyone who develops DVT is at risk of developing a pulmonary embolism.
A pulmonary embolism can lead to a sudden and sometimes very dramatic decrease in blood flow through the lungs. The decrease in blood flow can reduce the amount of blood flowing to your heart and the rest of your body. This can cause a drop in blood pressure and lead to fainting spells and even sudden death.
The blood flow decreases in part because the blood clot blocks blood flow. In addition, the blockage damages the walls of the lung's blood vessels. The damage releases chemicals that cause blood vessels to narrow.