What Is It?

A deep venous thrombosis (commonly called DVT) is a blood clot (also called a 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. If part of the clot breaks away, it may 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 medical problem. About 1 out of every 2,000 people develops this type of blood clot each year. Most of these clots occur when blood flow in the veins of the legs is slowed, 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 - such as during a long airplane flight or while recovering from an operation or stroke - blood flow in the veins of your legs may slow so much that clots form.

Certain people are at increased risk of blood clots, including:

• 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

• Pregnant women

• People who are very overweight

• People with heart failure

Anyone who develops DVT is at risk of developing a pulmonary embolism. More than 500,000 Americans develop this life-threatening problem each year.

A pulmonary embolism condition can lead to a sudden and sometimes very dramatic decrease in blood flow through the lungs. The decrease in flow can reduce the amount of blood flowing to your heart and the rest of your body, which can cause a drop in blood pressure and lead to fainting spells and even sudden death.

The blood flow decreases not only because a blood clot is blocking blood flow, but the blockage damages the walls of the lung's blood vessels (pulmonary arteries). The damage releases chemicals that cause multiple blood vessels -- even those that don't contain blood clots -- to constrict (clamp down).