What Causes Pulmonary Embolism?

A pulmonary embolism happens when a blood clot travels through the bloodstream and gets stuck in a lung artery. This blockage stops blood from reaching part of your lung, which reduces oxygen levels in your blood and puts strain on your heart. The condition is serious and can be life-threatening, but understanding what causes it is the first step in preventing it.

What Causes Pulmonary Embolism?

The most common cause of a pulmonary embolism is a deep vein thrombosis (DVT). A DVT is a blood clot that forms in a deep vein, usually in the leg or pelvis. When part of that clot breaks loose, it travels through the veins, into the right side of the heart, and gets pumped into the lungs where it becomes lodged in a pulmonary artery.

Almost all pulmonary embolisms start this way. The clot does not form in the lung itself. It forms elsewhere, typically in the lower body, and then moves. Understanding this chain of events matters because preventing a pulmonary embolism often means preventing the DVT that starts it.

Why Do Blood Clots Form in the First Place?

Blood clotting is a normal body process. When you cut yourself, your blood thickens to form a plug that stops the bleeding. The problem begins when clots form inside veins without any injury, or when a normal clot does not dissolve as it should.

Doctors describe three main reasons blood clots form inside veins. This is sometimes called Virchow’s triad, named after the physician who described it in the 1800s. The three factors are:

• Slow blood flow: When blood moves slowly through a vein, clotting factors have time to collect and form a clot.
• Damage to the vein wall: Injury to the inside of a vein can trigger the clotting process.
• Blood that clots too easily: Some people have blood that is more prone to clotting than normal.

Most pulmonary embolisms involve at least one of these three factors. Many involve more than one.

What Situations Put You at Risk for a Clot?

Several common situations increase your risk of developing a DVT that can become a pulmonary embolism. These are well established by clinical research.

Prolonged immobility is one of the biggest risk factors. When you sit for long periods without moving your leg muscles, blood pools in your lower legs. This is why long flights, long car trips, or bed rest after surgery all raise the risk of clot formation. The risk increases with the length of immobility, though the exact threshold varies from person to person.

Surgery is another major trigger. Major operations, especially on the hips, knees, or abdomen, increase clot risk in several ways. The surgery itself damages tissue, which activates clotting. Anesthesia often leaves you immobile for a period. And the body’s natural inflammatory response to surgery makes blood more likely to clot.

Cancer is a significant risk factor as well. Some cancers release substances that make blood clot more easily. Chemotherapy and radiation treatment also increase clot risk. In fact, a pulmonary embolism can sometimes be the first sign of an undiagnosed cancer.

What Medical Conditions Increase the Risk?

Certain health conditions change how your blood behaves or how it flows. Heart disease, including heart failure, makes it harder for the heart to pump blood efficiently, which leads to slower blood flow and pooling. This raises clot risk.

Obesity increases pressure in the veins of the pelvis and legs, which slows blood flow. It also carries a higher baseline level of inflammation, which can activate clotting.

Inflammatory conditions such as rheumatoid arthritis or inflammatory bowel disease also raise clot risk. The chronic inflammation these diseases cause makes the blood more likely to clot.

Some people inherit conditions that make their blood hypercoagulable, meaning it clots too easily. These include factor V Leiden, which is the most common inherited clotting disorder, and other less common conditions. If you have one of these, you may need to be more careful about other risk factors like immobility or surgery.

Can Pregnancy Cause a Pulmonary Embolism?

Pregnancy raises the risk of blood clots significantly. This is a normal physiological response to pregnancy, not a sign that something is wrong. The body increases clotting factors during pregnancy to protect against excessive bleeding during delivery.

The risk is highest in the weeks after delivery, but it begins during pregnancy and continues for about six weeks postpartum. The growing uterus also presses on veins in the pelvis, which slows blood flow from the legs. Pulmonary embolism remains one of the leading causes of maternal death in the United States, though the absolute risk for any individual woman is still low.

Women who have had a previous clot, have a clotting disorder, or who had a cesarean section are at higher risk during pregnancy. Doctors typically screen for these risk factors early in pregnancy so they can plan preventive measures.

What Are the Symptoms of a Pulmonary Embolism?

Symptoms can vary widely. Some people have no symptoms at all. Others have sudden, severe symptoms that require emergency care.

The classic symptoms include:

• Sudden shortness of breath, especially if it comes on without exertion
• Chest pain that often feels sharp and gets worse with deep breathing or coughing
• A rapid heart rate
• Coughing, sometimes with blood-streaked sputum
• Lightheadedness or fainting

If you have sudden shortness of breath and chest pain, especially if you have any risk factors for clots, seek emergency medical care. A pulmonary embolism can be fatal if not treated quickly.

How Is a Pulmonary Embolism Diagnosed?

Doctors use a combination of tests to diagnose a pulmonary embolism. The first step is usually a blood test called a D-dimer. This test measures a substance that is present when blood clots break down. A normal D-dimer makes a pulmonary embolism unlikely. A high D-dimer is not specific, meaning many other conditions can raise it, but it signals that more testing is needed.

The definitive test is a CT pulmonary angiogram. This is a specialized CT scan that uses contrast dye to visualize the blood vessels in the lungs. It can show exactly where a clot is blocking blood flow. In some cases, a ventilation-perfusion scan, sometimes called a V/Q scan, is used instead. This test compares airflow in the lungs to blood flow and can identify areas where blood is not reaching.

Ultrasound of the legs is often used to check for a DVT. If a DVT is found in someone with pulmonary embolism symptoms, it strengthens the diagnosis and confirms the source of the clot.

Can You Prevent a Pulmonary Embolism?

Prevention focuses on reducing the risk factors you can control and using medical prevention when risk is high.

If you are going to be sitting for a long time, get up and walk every couple of hours. If you cannot walk, flex and extend your ankles repeatedly. This uses your calf muscles to pump blood back toward your heart. Staying hydrated also helps keep blood from thickening.

After surgery, doctors often use blood thinners to prevent clots. They may also use intermittent pneumatic compression devices, which are sleeves around the legs that inflate and deflate to keep blood moving. Walking soon after surgery is strongly encouraged for the same reason.

For people at high risk, such as those with a prior clot or a known clotting disorder, doctors may recommend longer-term blood thinners. This decision is made on an individual basis, weighing the risk of another clot against the risk of bleeding from the medication.