An embolism is a blockage in a blood vessel caused by something that traveled there from another part of your body. Most often, that “something” is a blood clot, but it can also be a bubble of air, a globule of fat, or other debris. The object breaks free, rides the bloodstream, and gets wedged in a vessel too narrow for it to pass through, cutting off blood flow to the tissue beyond. Up to 900,000 people in the United States are affected by blood clot-related embolisms each year, and an estimated 60,000 to 100,000 of those cases are fatal.
Embolism vs. Blood Clot
The terms “blood clot” and “embolism” are often used interchangeably, but they describe two different problems. A blood clot (thrombus) forms and stays in one place, narrowing the vessel where it developed. An embolism starts somewhere else. A clot might form in a deep leg vein, break loose, and travel to the lungs. The clot itself is the embolus; the blockage it creates at its destination is the embolism.
Three conditions make clots more likely to form in the first place: damage to a blood vessel wall, sluggish blood flow (from sitting still too long, for instance), and a tendency for blood to clot more easily than normal. These three factors, described in the 1800s and still central to modern medicine, explain why long surgeries, immobility, and certain genetic conditions all raise embolism risk.
Types of Embolism
Pulmonary Embolism
A pulmonary embolism (PE) happens when a clot lodges in a blood vessel in the lungs. It is the most common life-threatening type of embolism. Most PEs start as clots in the deep veins of the legs or pelvis. When part of that clot breaks off and reaches the lungs, it blocks blood from picking up oxygen, straining both the lungs and the heart.
Symptoms typically come on suddenly: shortness of breath (even at rest), chest pain that worsens with deep breaths, rapid breathing, lightheadedness, or fainting. In about 25% of PE cases, sudden death is the first and only symptom, which is why any combination of unexplained breathlessness and chest pain warrants immediate emergency care. Without quick treatment, a PE can cause permanent heart or lung damage.
Embolic Stroke
When an embolus gets stuck in a blood vessel inside the brain, it causes an embolic stroke. The brain tissue downstream of the blockage loses its blood supply within minutes. Atrial fibrillation, a common heart rhythm disorder, is one of the leading sources of these clots. The heart’s upper chambers quiver instead of contracting fully, allowing blood to pool and clot. Those clots can then travel up to the brain.
Stroke symptoms appear fast: sudden numbness or weakness on one side, confusion, trouble speaking, vision changes, or a severe headache with no clear cause. Speed matters enormously here. Brain cells begin dying within minutes of losing blood flow.
Air Embolism
An air embolism occurs when gas bubbles enter the bloodstream and block a vessel. This is rare in everyday life but can happen during certain medical procedures, diving accidents, or trauma to the chest. In adults, it generally takes more than 5 milliliters of air per kilogram of body weight to cause symptoms through a vein. For a 70-kilogram (154-pound) person, that means roughly 300 milliliters of air in the veins is typically fatal. However, even tiny amounts of air, as little as 0.5 milliliters, can be dangerous if they reach the coronary arteries supplying the heart.
Fat Embolism
Fat embolism syndrome is a rare but serious complication that can follow fractures of large bones, particularly the thighbone (femur), or orthopedic surgeries that involve those bones. When a long bone breaks, fat from the bone marrow can enter the bloodstream and travel to the lungs, brain, or skin. The classic warning signs are a triad: breathing difficulty, confusion or other neurological changes, and a distinctive reddish rash (often described as salmon-colored) that typically appears on the chest, neck, or underarms. Symptoms usually develop within 24 to 72 hours of the injury or surgery.
How Embolisms Are Diagnosed
Because embolism symptoms overlap with many other conditions, objective testing is essential. For a suspected PE, doctors commonly use a lung perfusion scan, which tracks blood flow through the lungs and reveals areas where circulation is blocked. CT scans with contrast dye are now the most widely used tool, providing detailed images of clots in the pulmonary arteries. Blood tests that measure clotting markers can help rule out an embolism when the result is negative, though a positive result alone isn’t enough to confirm one.
For embolic strokes, brain imaging with CT or MRI identifies the blocked area and guides treatment decisions. Doctors may also examine the heart with ultrasound to look for a clot source, especially in patients with atrial fibrillation or other heart conditions.
Treatment Options
Blood-thinning medication (anticoagulant therapy) is the foundation of embolism treatment. These drugs don’t dissolve an 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 most pulmonary embolisms, this is the primary treatment, and patients may continue on blood thinners for several months afterward.
When a PE is massive and the heart is failing under the strain, more aggressive treatment is needed urgently. Clot-dissolving drugs (thrombolytics) can be delivered through an IV or directed straight to the clot via a catheter. In the most severe cases, surgical removal of the clot may be necessary. These interventions carry a higher bleeding risk, so they’re reserved for situations where the embolism is immediately life-threatening.
Embolic strokes follow a similar logic: restoring blood flow as quickly as possible, often with clot-dissolving medication or a catheter-based procedure to physically pull the clot out.
Prevention in Hospitals and Beyond
Most embolism prevention focuses on stopping clots from forming in the first place, especially during high-risk periods like hospitalization, surgery, or prolonged bed rest. Hospitals routinely assess patients for clot risk and assign preventive measures based on that score.
For patients at elevated risk who aren’t likely to bleed, low-dose blood thinners given by injection are the standard approach. For patients who can’t safely take blood thinners because of bleeding risk, mechanical devices take over. Intermittent pneumatic compression sleeves wrap around the calves and inflate rhythmically, mimicking the pumping action of walking to keep blood moving. Graduated compression stockings are another option, though evidence for their effectiveness in preventing symptomatic clots in hospitalized patients is weaker.
Outside the hospital, the same principles apply on a simpler scale. Moving regularly during long flights or car rides, staying hydrated, and following post-surgical activity guidelines all reduce the risk of clots that could become embolisms. People with atrial fibrillation are often prescribed long-term blood thinners specifically to prevent the clots that cause embolic strokes.
Warning Signs That Need Immediate Attention
Certain symptoms signal a potentially fatal embolism and require emergency care, not a wait-and-see approach. For pulmonary embolism, the red flags are sudden shortness of breath, chest pain that sharpens when you breathe deeply, rapid breathing, feeling faint, or actually passing out. These can appear whether you’ve been physically active or sitting still.
For embolic stroke, the classic signs are sudden weakness or numbness on one side of the body, difficulty speaking or understanding speech, sudden severe headache, or loss of vision. In both cases, every minute without treatment increases the risk of permanent damage or death. A PE is a medical emergency that can cause heart and lung failure without prompt intervention.