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What is the main cause of pulmonary embolism?

Pulmonary embolism (PE) is a blockage in one of the pulmonary arteries in the lungs. It is usually caused by blood clots that travel from the veins in the legs or pelvis and lodge in the lungs. PE can be life-threatening if not treated quickly, as it can damage the lungs and strain the heart.

What causes blood clots that lead to PE?

Blood clots form through a process called coagulation, which is the body’s normal response to stop bleeding from an injury. In some cases, blood clots can form even when a person is not injured. These clots form due to one or more risk factors that cause the blood to clot abnormally.

The main risk factors for developing a blood clot that can lead to PE are:

  • Prolonged immobility – Being immobile for long periods of time, such as when recovering from surgery, traveling long distances, or being bedridden. This allows blood to pool and clot in the veins.
  • Injury to veins – Damage to leg veins from surgery, fracture, or trauma can cause blood clots to form.
  • Pregnancy – Pregnancy increases blood clotting risk due to higher estrogen levels and pressure on veins from the uterus.
  • Birth control pills & hormones – Contraceptives with estrogen increase clotting factors.
  • Inherited clotting disorders – Genetic mutations that lead to problems with blood clot regulation.
  • Cancer – Cancer and cancer treatment can increase risk.
  • Obesity – Being overweight stresses the veins.
  • Smoking
  • Older age – Risk increases over age 40.

These risk factors cause the blood to clot abnormally and turn into deep vein thrombosis (DVT) in the legs or pelvis. DVT is a blood clot that forms in a deep vein, usually in the leg. Parts of the clot can break off and travel through the heart to the lungs, blocking a pulmonary artery. This blockage is called a PE.

Key Statistics on Pulmonary Embolism

Some key statistics on pulmonary embolism:

  • Around 600,000 Americans are affected each year
  • It is the 3rd most common cause of hospital-related death
  • 30-50% of PEs are “silent” and asymptomatic when they occur
  • 20-50% of PE cases are fatal if not treated promptly
  • The recurrence rate is 17% after stopping anticoagulant therapy
  • More than one-third of those who have a DVT will develop a PE

Signs and Symptoms

Some common signs and symptoms of PE include:

  • Shortness of breath – Occurs suddenly and worsens with exertion
  • Chest pain – Sharp or stabbing pain that may get worse with deep breathing
  • Rapid heart rate
  • Lightheadedness or passing out
  • Coughing up blood
  • Swelling, pain, warmth, and discoloration in a leg with DVT

In many cases, the PE may not cause any symptoms at all. When symptoms do occur, they can range from mild to severe depending on the degree of blockage in the lungs.


If PE is suspected, the doctor will evaluate any risk factors and symptoms. Diagnostic tests may include:

  • Blood tests – To look for signs of clotting problems
  • ECG – To check for signs of strain on the heart
  • Chest X-ray – May reveal abnormalities but not always
  • CT pulmonary angiogram – Helps visualize clots in the pulmonary arteries
  • Ventilation/perfusion (V/Q) scan – Uses radioactive material to check blood flow and air flow
  • Venous ultrasound – Checks for DVT in the legs
  • Pulmonary angiogram – Uses dye injection and X-rays to visualize arteries

A CT angiogram is the most common and accurate test to diagnose PE. Additional tests may be done to determine the cause and severity.


Treatment focuses on preventing further clotting and reversing existing clots to improve symptoms and prevent recurrence. Options may include:

  • Anticoagulants – Blood thinners like heparin and warfarin to stop clot extension
  • Thrombolytics – Clot-busting drugs to dissolve clots
  • Thrombectomy – Surgical removal of large clots
  • Oxygen – To relieve breathing difficulties
  • Pain medication

Most patients will need to take anticoagulant medications long-term to prevent new clots. Compression stockings may also be recommended.


Complications that can occur with pulmonary embolism include:

  • Recurrence – Happens in about 30% of cases after stopping treatment
  • Chronic thromboembolic pulmonary hypertension – High blood pressure in the lungs due to clot damage
  • Heart strain and right ventricular dysfunction – PE strains the heart
  • Bleeding due to anticoagulant use
  • Chronic edema and swelling in the legs
  • Pulmonary infarction – Tissue death from lack of blood flow
  • Death – From right heart failure or lack of oxygen


For those at high risk, prevention is key to avoiding pulmonary embolism. Preventive measures include:

  • Early mobilization after surgery
  • Compression stockings or pneumatic devices with prolonged immobility
  • Avoiding long travel if at high risk
  • Staying hydrated on long flights
  • Following dosage instructions carefully for birth control pills
  • Maintaining a healthy weight
  • Quitting smoking
  • Blood thinners if at high genetic risk


In summary, the main cause of pulmonary embolism is blood clots, usually forming in the leg veins, that travel to the lungs. Major risk factors include prolonged immobility, injury or surgery, inherited clotting disorders, pregnancy, birth control use, cancer, and obesity. Preventive measures like hydration, early mobility, compression stockings, and blood thinners in high-risk groups can help avoid PE. With prompt treatment, many patients recover fully.