An embolus is any object or substance that travels through the bloodstream until it gets stuck in a vessel too narrow to pass through, blocking blood flow. Most often, it’s a piece of a blood clot that has broken free from where it originally formed. But fat, air bubbles, amniotic fluid, tumor fragments, and even surgical cement can all act as emboli (the plural of embolus). Whatever the material, the damage comes from the same basic problem: the embolus lodges in an artery, cuts off blood supply to the tissue beyond it, and starves that tissue of oxygen.
How an Embolus Forms and Travels
The most common type of embolus starts as a thrombus, which is a blood clot that forms inside a vessel. Clots tend to develop in areas where blood flow is sluggish, giving clotting factors time to accumulate and platelets more opportunity to stick together. The deep veins of the legs are a frequent site, particularly the calf veins and the larger veins behind the knee and in the pelvis. At some point, a piece of that clot breaks loose. Once free, it floats along with the flowing blood until it hits a narrowing it can’t squeeze through.
Because most emboli originate from blood clots, the term “thromboembolus” is used frequently in medicine. But the concept is broader than clots alone. Bits of fatty plaque that break off from an artery wall, globules of bone marrow fat released after a fracture, or air introduced into a vein during surgery or trauma can all become emboli. The destination depends on where the embolus enters the bloodstream. A clot from a leg vein travels toward the heart and then into the lungs. A clot that forms inside the heart itself can be pumped out into arteries supplying the brain, limbs, or organs.
Pulmonary Embolism: The Most Common Type
When a blood clot breaks free from a deep vein in the leg, it rides the venous bloodstream back to the heart, passes through the right side of the heart, and gets pumped into the pulmonary arteries supplying the lungs. There, it lodges and blocks blood flow. This is a pulmonary embolism, or PE, and it’s the most clinically significant form of embolism.
The United States has one of the highest incidence rates globally, at roughly 115 cases per 100,000 people per year. Mortality from PE has been declining in high-income countries over the past two decades, dropping from about 3.7 deaths per 100,000 in 2001 to 2.2 per 100,000 in 2023. That improvement reflects better diagnosis and treatment. However, the trend has reversed in some groups: mortality has been rising among young and middle-aged adults in the U.S., and lower-to-middle-income countries have seen rates climb from under 1 per 100,000 to nearly 5 per 100,000 over the same period.
Symptoms of a pulmonary embolism typically include sudden shortness of breath, chest pain that worsens with deep breaths, rapid heart rate, and sometimes coughing up blood. A large PE can cause the right side of the heart to fail under the strain, leading to a drop in blood pressure and collapse. High-risk PE carries a mortality rate of roughly 21 to 42 percent even with modern treatment.
Embolic Stroke: When Clots Reach the Brain
The second major destination for an embolus is the brain, where it causes an ischemic stroke. This happens most commonly when a clot forms inside the heart rather than in a vein. Atrial fibrillation, a condition where the upper chambers of the heart quiver instead of contracting in a coordinated way, is the classic setup. The uncoordinated motion lets blood pool and stagnate in the atrium, and that stagnant blood clots. A piece then breaks off, gets pumped into the aorta, and travels up into the arteries feeding the brain. When it lodges, it cuts off blood supply to brain tissue downstream.
There’s also a less common route called a paradoxical embolism. Normally, clots from veins travel to the lungs and never reach the arterial side of the circulation. But about one in four people have a patent foramen ovale, a small opening between the left and right sides of the heart that never fully closed after birth. In these individuals, a venous clot can cross through that gap directly into the arterial system and travel to the brain, causing a stroke from what was originally a leg clot.
Emboli in the Limbs
When an embolus lodges in an artery supplying an arm or leg, the result is acute limb ischemia. The classic signs, known as the “five Ps,” are progressive pain in the affected limb, pulselessness (you can’t feel a pulse below the blockage), pallor (the skin turns pale or mottled), paresthesia (numbness or tingling), and paralysis. Foot drop, where you lose the ability to lift the front of your foot, is a particularly urgent sign because it indicates nerve damage is already underway. Acute limb ischemia is a time-sensitive emergency because the tissue can die within hours without restored blood flow.
Types Beyond Blood Clots
While blood clots account for the vast majority of emboli, several other materials can cause the same kind of blockage.
Fat emboli occur when fat or bone marrow particles enter the bloodstream, typically after a long bone fracture or orthopedic surgery. Fat embolism syndrome is a more severe reaction that develops 24 to 72 hours after injury. Its hallmarks include a characteristic rash of tiny red or purple spots (usually across the chest, neck, and armpits), breathing difficulty, and confusion or altered mental status in someone who hasn’t had a head injury. Fever above 38.5°C and a rapid heart rate above 110 beats per minute are also common.
Air emboli result from air being introduced into veins, which can happen during certain surgeries, central line placement, or trauma. Symptomatic air embolism generally requires more than 5 milliliters of air per kilogram of body weight entering the venous system. In practical terms, 50 to 100 milliliters of air can cause cardiovascular instability, and around 300 milliliters is typically fatal. Air introduced closer to the right side of the heart carries the greatest risk. By contrast, even 1 to 2 milliliters of air reaching the central nervous system can be lethal, which is why air in arterial lines is treated with extreme caution.
Amniotic fluid emboli are rare but serious events that occur during or shortly after childbirth, when amniotic fluid enters the mother’s bloodstream and triggers a severe inflammatory reaction in the lungs.
How an Embolism Is Diagnosed
For suspected pulmonary embolism, the primary imaging tool is a CT scan of the chest with contrast dye injected into a vein, which directly visualizes clots in the pulmonary arteries. An alternative is a ventilation-perfusion scan (often called a V/Q scan), which compares airflow and blood flow in the lungs to identify areas where blood supply is blocked. Newer versions of V/Q scanning that combine with CT technology have shown 100% sensitivity and over 94% specificity when compared against standard CT imaging, and they deliver lower radiation doses, making them a good option for patients with kidney problems or contrast dye allergies.
For emboli in other locations, the imaging depends on where the blockage is suspected. Ultrasound is commonly used to find clots in leg veins. CT or MRI scans of the brain identify strokes caused by emboli. Angiography, where dye is injected directly into arteries and tracked with X-rays, can pinpoint blockages in limb arteries.
Treatment Options
Blood-thinning medication is the foundation of treatment for most emboli caused by blood clots. These drugs don’t dissolve the existing clot, but they stop it from growing and prevent new clots from forming, giving the body time to break down the blockage naturally.
For more serious cases, particularly large pulmonary emboli that cause dangerously low blood pressure or strain on the heart, clot-dissolving medications can be given to break up the embolus directly. This approach carries a meaningful risk of bleeding, so it’s reserved for situations where the embolism is life-threatening. Lower doses and catheter-based delivery (where a thin tube is threaded directly to the clot site) have been developed to reduce bleeding complications while still clearing the blockage.
When clot-dissolving drugs can’t be used safely, mechanical options are available. Aspiration thrombectomy uses a catheter to physically suction the clot out of the artery. Surgical removal of the clot is another option in extreme cases. For patients at high risk of recurrent pulmonary embolism who can’t take blood thinners, a small filter can be placed in the large vein leading to the heart to catch clots before they reach the lungs.
Treatment for non-clot emboli depends on the material involved. Fat embolism syndrome is primarily managed with supportive care, including oxygen and sometimes mechanical ventilation. Air embolism may be treated by repositioning the patient and, in severe cases, using a hyperbaric oxygen chamber to help the body reabsorb the trapped gas.