Pulmonary embolism is a blockage in one or more of the arteries in or leading to the lungs caused by a clot, or embolus. In almost every case, the clot originates in a deep vein in the legs, arms, or pelvis, breaks loose, and travels to the lungs. Depending on its size, the blood clot obstructs a large or small pulmonary artery and blocks the flow of blood through that vessel.

More than 100,000 to 200,000 people in the United States develop pulmonary embolism each year, and nearly one third of them die. Fortunately, steps can be taken to prevent pulmonary emboli.

Pulmonary embolism varies in its severity and effects. Most dangerous is a massive blood clot that blocks a main pulmonary artery and/or one or more of its branches. Another dangerous situation occurs when a small blood clot blocks a peripheral pulmonary artery (one near the surface of the lung).

Some individuals have multiple clots that block many medium-sized pulmonary arteries but produce no symptoms. The network of blood vessels in the lungs is large enough to tolerate considerable amounts of obstruction, however, such extensive blockage can increase resistance to blood flow through the pulmonary arteries, eventually producing pulmonary hypertension (high blood pressure in the pulmonary arteries).

Pulmonary hypertension places excessive stress on the heart's right ventricle, which pumps blood into the pulmonary arteries. This can lead to right heart failure.

Causes of Pulmonary Embolism

The vast majority of pulmonary emboli arise from deep vein thrombosis (DVT), when clots develop in a deep vein in the leg, pelvis, or arm. Each year, an estimated 200,000 to 400,000 people in the United States have an episode of DVT. The condition is most common among women and older people.

A frequent cause of DVT is stagnation of blood flow, which often occurs in bedridden people who are immobile or in healthy people who sit still for an extended period, such as on a long trip. Women taking oral contraceptives or hormone replacement therapy after menopause are at increased risk for DVT. People are at high risk for DVT after any major surgery, but especially after knee- or hip-replacement surgery.

A tendency for the blood to coagulate can predispose a person to DVT. Having cancer also can cause the blood to coagulate excessively. Injury to blood vessels by trauma, intravenous catheters or needles, or certain medications also can cause blood clots.

Publication Review By: Peter B. Terry, M.D., M.A.

Published: 19 Sep 2011

Last Modified: 19 Feb 2015