An embolism is a blockage in a blood vessel caused by a traveling mass, most often a blood clot that forms in one part of the body, breaks loose, and gets stuck in a vessel somewhere else. The obstruction cuts off blood flow to the tissue beyond it, which can damage organs or, in serious cases, cause death. In the United States alone, pulmonary embolism (the most common dangerous form) contributed to over 49,000 deaths in 2020.
How an Embolism Forms
The traveling object that causes the blockage is called an embolus. In most cases, it starts as a blood clot in a deep vein, typically in the legs or pelvis. Three conditions make clots more likely to form: slow-moving blood, damage to the blood vessel wall, and blood that clots more easily than normal. When a clot breaks free from where it formed, it rides through the bloodstream until it reaches a vessel too narrow to pass through. Once lodged, it blocks blood flow like a cork in a pipe.
The damage depends entirely on where the blockage lands. A clot that reaches the lungs can strain the heart. One that reaches the brain causes a stroke. The size of the embolus matters too. A large clot blocking a major vessel can cause life-threatening organ failure within minutes, while a small one in a minor vessel may produce mild symptoms or none at all.
Types of Emboli
Blood clots are by far the most common type, but an embolus can be made of several different materials:
- Blood clot (thromboembolus): The most frequent cause. Clots from deep veins in the legs account for the vast majority of pulmonary embolisms.
- Air or gas: Air bubbles can enter veins during certain surgeries (particularly thyroid or neck procedures), blood transfusions, or trauma. Even a small volume of air in the bloodstream can block a vessel.
- Fat: Fat globules can enter the bloodstream after fractures of long bones or during orthopedic surgery.
- Amniotic fluid: A rare but serious complication of childbirth, where amniotic fluid enters the mother’s bloodstream.
- Tumor fragments: Cancer cells can break off from a tumor, travel through the blood, and block vessels in distant organs.
- Cholesterol: Cholesterol crystals from diseased artery walls can break free and block smaller downstream vessels.
- Foreign bodies: Fragments of medical devices, catheter tips, or other materials can occasionally act as emboli.
Pulmonary Embolism: The Most Common Form
A pulmonary embolism (PE) happens when a clot travels to the lungs and blocks one or more of the pulmonary arteries. Most of these clots originate in the deep veins of the lower legs, then the thigh-area veins, and less commonly the veins near the pelvis. The condition is closely linked to deep vein thrombosis (DVT), which is a clot that forms in one of these deep veins. Together, DVT and PE are referred to as venous thromboembolism.
PE symptoms are often vague, which makes it tricky to recognize. The most common signs include sudden shortness of breath, sharp chest pain (especially when breathing in), a persistent cough, coughing up blood, feeling lightheaded, and fainting. Some people experience an unusually fast heartbeat. When a large clot blocks a major pulmonary vessel, the right side of the heart has to work much harder to push blood through. This pressure overload can lead to right-sided heart failure and, if untreated, death.
The age-adjusted mortality rate for PE in the U.S. was about 10 per 100,000 people in 2018 and 2019, then jumped nearly 24% in 2020. COVID-19 played a role in that spike, with PE reported in about 1.4% of COVID-related deaths that year.
Arterial Embolism and Its Targets
While most venous emboli end up in the lungs, clots that form or travel on the arterial side of the circulation can land in very different places. The brain and the legs are the two most common targets. Strokes and transient ischemic attacks are the single most important consequence of arterial emboli. When a clot blocks blood flow to part of the brain, brain tissue begins dying within minutes.
Limb ischemia is the second most common result. Emboli to the legs are about four times more frequent than those to the arms. When a clot blocks the main artery supplying a leg, the limb becomes painful, pale, and cold. Without restoring blood flow quickly, the tissue can die permanently. Less commonly, arterial emboli lodge in the arteries supplying the intestines (the superior mesenteric artery is the usual site), the kidneys, or rarely the arteries feeding the heart or the eyes.
Risk Factors
Anything that slows blood flow, damages blood vessels, or makes blood clot more easily raises the risk. The CDC identifies several specific factors:
- Immobility: Being confined to bed after surgery or illness, wearing a leg cast, sitting for long stretches during travel, or paralysis.
- Surgery and injury: Major operations on the abdomen, pelvis, hip, or legs carry elevated risk. Bone fractures and severe muscle injuries also contribute.
- Personal or family history: A prior DVT or PE significantly increases the chance of another. Inherited clotting disorders, such as a genetic mutation that makes blood clot too easily, raise baseline risk.
- Other medical factors: Active cancer, pregnancy, hormone therapy, and obesity all independently increase the likelihood of clot formation.
Multiple risk factors compound each other. Someone who is both obese and taking hormone therapy, for example, faces a higher combined risk than either factor alone.
How Embolisms Are Diagnosed
For suspected pulmonary embolism, doctors typically start with a blood test called a D-dimer. This test measures a protein fragment produced when a blood clot dissolves. A normal D-dimer result makes PE unlikely, which is useful for ruling it out quickly. If the D-dimer is elevated or clinical suspicion is high, imaging follows.
CT pulmonary angiography (CTPA) is the gold standard for diagnosing PE. It uses a CT scanner with contrast dye injected into a vein to produce detailed images of the lung arteries. The test has a specificity of 96%, meaning it very rarely identifies a PE that isn’t there. It also provides fast results, which matters when time is critical.
For deep vein thrombosis, ultrasound has replaced older dye-based techniques as the standard approach. A technician presses the ultrasound probe against the skin over the vein. A healthy vein collapses flat under pressure; a vein containing a clot doesn’t. This compression ultrasound detects clots in the upper leg veins with about 96% sensitivity. It’s less reliable for clots in the smaller calf veins, catching about 71 to 75% of those. For pregnant patients, ultrasound of the legs is recommended as the first imaging step, since it avoids radiation exposure.
Treatment Overview
Treatment depends on the size and location of the embolism and how much it’s affecting the body. The foundation for most blood clot embolisms is anticoagulation, commonly called blood thinners. These medications don’t dissolve existing clots but prevent them from growing and stop new ones from forming, giving the body time to break the clot down naturally.
For large, life-threatening pulmonary embolisms that cause the heart to struggle, doctors may use clot-dissolving medications (thrombolytics). These drugs actively break apart the clot but carry a higher risk of serious bleeding, so they’re reserved for the most dangerous situations. In cases where medications aren’t an option or aren’t working, a procedure to physically remove the clot may be performed, either through a catheter threaded into the blood vessel or through open surgery.
After the initial treatment, most people with a PE or DVT stay on blood thinners for at least three months, and sometimes longer depending on what caused the clot and whether the risk factors are still present.
Reducing Your Risk
For long-distance travel lasting more than six hours, the key recommendations for people with increased risk include walking around periodically, doing calf muscle exercises while seated, choosing an aisle seat for easier movement, and wearing properly fitted compression stockings that provide 15 to 30 mmHg of pressure at the ankle. For travelers without additional risk factors, compression stockings aren’t necessary.
People at substantially higher risk during travel (those with recent surgery, prior clot history, active cancer, postpartum status, or two or more smaller risk factors) may benefit from compression stockings or a preventive dose of a blood-thinning injection before a long flight. Aspirin alone is not recommended for clot prevention during travel unless other options aren’t available.
In hospital settings, prevention is a major focus after surgery. Early movement after an operation, compression devices that periodically squeeze the legs, and short-term blood thinners are standard approaches for patients recovering from major procedures. If you’re facing a planned surgery, especially on the hip, knee, or abdomen, your surgical team will typically assess your clot risk and build prevention into your recovery plan.