Pulmonary Embolism - Definition, Causes, and Treatments

How Do Pulmonary Emboli Occur and When are They an Emergency?

diagram of a pulmonary embolus in a pulmonary artery
Pulmonary Embolus. A.D.A.M. Medical Encyclopedia

Pulmonary Embolism - Definition

A pulmonary embolism is a blockage in an artery leading to the lungs, most often caused by a blood clot. They most commonly occur when a clot lower down in one of the legs, a deep vein thrombosis (DVT) breaks loose in the leg or pelvis and travels through the bloodstream to the lungs.

Why Do Blood Clots in the Legs Travel to the Lungs?

Pulmonary emboli are usually caused by a blood clot breaking off in the legs, so the underlying cause of pulmonary emboli is what causes blood clots.

We don't know exactly why some clots break off and travel and others do not.

People sometimes wonder why clots occur in the veins and not in the arteries. Why do clots travel to the lungs rather than somewhere else? The reason is that blood moves more slowly through veins than arteries and can have the opportunity to "pool" and clot.

To understand why these clots end up in the lungs it helps to talk a little about the anatomy. The veins in the legs come together and blood returns to the right side of the heart through a big vein called the inferior vena cava. From the right heart, the blood then travels to the lungs via the pulmonary arteries to renew its supply of oxygen. Going upstream from the veins in the legs, all of the blood vessels (including the heart) are larger. When the blood enters the lungs, however, the vessels become progressively smaller, and this is where the clots then become "trapped" in one of the pulmonary arteries leading to pulmonary infarction (death) of part of a lung.

Where in the Lungs do Pulmonary Emboli Occur?

Clots may become trapped nearly anywhere in the lungs depending on the size of the clot. If it is a small clot (or a series of small clots) they may become lodged in smaller blood vessels and produce only minor symptoms. In fact, some people with pulmonary emboli look back to realize that they had probably been "throwing" smaller clots for some time.


If the clot is large and becomes lodged in a large blood vessel in the lungs, the result can be catastrophic. A large clot interferes with the ability of the lungs to oxygenate blood subsequently starving the brain and the rest of the body of oxygen.

It can be confusing if your doctor talks about the clot being lodged in a pulmonary artery. Pulmonary arteries are the only arteries in the body that carry deoxygenated (blue blood.)

Risk factors

Risk factors for pulmonary emboli are the risk factors for DVT's (deep venous thrombosis) or blood clots and include:

  • Immobility - Being immobile such as during bedrest and hospitalization, during long car rides, or plane flights risk developing blood clots and pulmonary emboli. This was made known to the public in recent years when a reporter died suddenly from a pulmonary embolus, presumably related to frequent plane flights. In fact, more than 2 flights a month carries an increased risk of these emboli.
  • Conditions which increase the chances of blood clotting, including many medical conditions.

Common Symptoms

Symptoms may first be related to a blood clot in the leg and include:

  • Pain, redness, tenderness, or swelling in your calves or groin

Symptoms of a pulmonary embolus may include:

  • Sudden severe chest pain
  • Shortness of breath
  • Cough and/or coughing up blood
  • Heart arrhythmias such as atrial fibrillation
  • Lightheadedness
  • Unconsciousness
  • Blue lips and extremities (cyanosis)
  • Low blood pressure, increased heart rate, increased respiratory rate

Medical Emergency

Pulmonary embolism is a medical emergency that can be fatal if left untreated. If you have any concern whatsoever that you may have a pulmonary embolism, call 911 immediately.

If you're not having symptoms, learn about what you can do to prevent blood clots. Keep in mind that clots can occur even in young, healthy adults and are a leading cause of death in the United States.  


Several different tests may be done to look for the presence of emboli. The most common is a 3 step approach to diagnosing pulmonary emboli using:

  • Ventilation-perfusion scan
  • D-dimer
  • Pulmonary angiogram

Treatments for Pulmonary Emboli

The treatments of a pulmonary embolus or emboli will depend on the severity and extent of the clot. If symptoms are not severe, blood thinners may be started while you are kept at rest to avoid dislodging any further clots. For large, severe clots thrombolytics may be used (medications like those used to open up heart vessels during a heart attack.)


If someone survives a pulmonary embolus there may still be long-term effects. The clot may permanently damage the lungs.  There may also be brain damage and damage to other organs due to lack of oxygen related to the clot in the lungs.


National Library of Medicine. MedlinePlus. Pulmonary Embolism Updated 07/16/16. https://medlineplus.gov/pulmonaryembolism.html

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