What Is a Pulmonary Embolism?

Medically Reviewed

A pulmonary embolism is a blockage caused by a clot in one or more of the arteries in or leading to the lungs. In almost every case, the clot originates in a deep vein in the legs, arms, or pelvis—a condition known as deep vein thrombosis.

The clot can then break loose and travel to the lungs (called an embolus). Depending on its size, the blood clot obstructs a large or small pulmonary artery and blocks the flow of blood through that vessel.

Pulmonary embolism and deep vein thrombosis are often underdiagnosed and their true incidence is not clear. Estimates suggest that 300,000 to 900,000 people in the United States are affected each year by deep vein thrombosis/pulmonary embolism, and 10 percent to 30 percent will die within one month of diagnosis.

Fortunately, steps can be taken to prevent the development of 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. If this condition develops, it can lead to the development of right-sided heart failure.

Pulmonary embolism causes

The vast majority of pulmonary emboli arise as a result of deep vein thrombosis. The condition is most common among older people. A frequent cause of deep vein thrombosis 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 deep vein thrombosis. People are at high risk for deep vein thrombosis following any major surgery, but especially after knee- or hip-replacement surgery.

A tendency for the blood to coagulate can predispose a person to deep vein thrombosis. 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.

Pulmonary embolism symptoms

Pulmonary emboli may produce sudden and severe shortness of breath, rapid breathing, and chest pain. Massive emboli, which often result in death within a few minutes, may also be accompanied by a feeling of impending doom, profuse sweating, loss of consciousness, shock, and a bluish color in the lips and fingertips.

Peripheral emboli, which also occur abruptly and without warning, are associated with chest pain, shortness of breath, and, later, coughing up blood. People with multiple small emboli may have no symptoms for many months until they develop pulmonary hypertension and right-sided heart failure, which can cause fatigue, swelling of the ankles, weight loss, and shortness of breath.

Deep vein thrombosis can cause pain and swelling in the affected body part, and the area may be tender and hot to the touch. Because the condition usually occurs deep within the leg, however, about half the time there are no signs or symptoms. Deep vein thrombosis is often discovered only when a doctor looks for it in someone who is at high risk for it.

Pulmonary embolism diagnosis

Although a doctor may suspect the presence of pulmonary embolism when symptoms occur in someone with typical signs of deep vein thrombosis, there often is not enough evidence to make a definitive diagnosis.

Experts agree that a spiral computed tomography (CT) scan with contrast dye injection is the most effective noninvasive test for pulmonary embolism. Studies suggest that this test can accurately diagnose large and medium-sized pulmonary emboli (but not very small ones).

A ventilation/perfusion scan (VQ scan) is sometimes used to identify old blood clots in the lung as it is more sensitive than CT scan for these types of clots. This is typically obtained when there is concern of pulmonary hypertension related to chronic blood clots in the lung.

Previously, the most common diagnostic test was an angiogram of the pulmonary arteries—an X-ray following the injection of a contrast material into the pulmonary artery. But this invasive test is being used less frequently, because the spiral CT is most often accurate enough to guide therapy reliably.

Research indicates that extending a chest CT scan to include the legs can help doctors more accurately diagnose a pulmonary embolism because this can detect deep vein thrombosis.

Pulmonary embolism prevention

The only effective way to prevent pulmonary emboli is to prevent deep vein thrombosis from arising in the first place and to be sure it is recognized and treated vigorously if it does occur.

Steps to prevent deep vein thrombosis include taking frequent walks when flying or riding on a long trip, getting out of bed as soon as possible after surgery, and using anticoagulants and other therapies (for example, pneumatic compression stockings) when possible if you are bedridden or have had an operation.

Studies have shown that wearing compression stockings on long airplane trips can reduce the risk of blood clots.

Individuals who have a family history of deep vein thrombosis should take special precautions because they may have inherited an abnormal tendency for their blood to clot. Individuals who have had deep vein thrombosis in the past are at increased risk for a recurrence.

Pulmonary embolism treatment

Supportive treatment for a person with a large pulmonary embolus includes pain relief and oxygen. Thrombolytic (clot-busting) drugs may dissolve the clot and restore blood flow through the blocked pulmonary artery.

Surgical removal of the blood clot may, however, be required in individuals with a large, life-threatening clot or in those whose condition deteriorates during medical treatment.

Thrombolytic therapy and surgery are generally reserved for people with massive clots and shock. Typically, people with a pulmonary embolus are treated with anticoagulants—heparin and warfarin (Coumadin)—in both the short and long term.

Intravenous heparin is started immediately and continued for at least three to five days, and oral warfarin is continued for three to six months (with close monitoring). However, individual circumstances may require longer duration of oral anticoagulants such as warfarin. Newer anticoagulants are also available.

Unfortunately, anticoagulants are not effective for the prevention of further emboli in some people and can be dangerous in individuals who are at high risk for bleeding in the head (people with a brain aneurysm, for example) or people with active gastrointestinal bleeding.

In these instances, a sieve-like filtering device may be placed in the inferior vena cava (the vein that returns blood from the lower body to the heart) to prevent blood clots from entering the heart and lungs.

Although filters reduce the immediate risk of having an embolus lodge in the lungs, they do not reduce the long-term risk of developing pulmonary emboli.

Consequently, continued use of anticoagulants is imperative, because clots can still reach the lungs through new, collateral blood vessels that slowly develop around the filter.