A blood clot in the lung, called a pulmonary embolism, almost always starts somewhere else in the body. In the vast majority of cases, a clot forms in the deep veins of the legs or pelvis, breaks loose, and travels through the bloodstream until it gets stuck in one of the arteries feeding the lungs. This blocks blood flow, strains the heart, and can be fatal: pulmonary embolism kills roughly 100,000 people per year in the United States.
How a Clot Travels to the Lungs
The process begins with a condition called deep vein thrombosis, or DVT, where a blood clot forms in a large vein, usually in the lower leg, thigh, or pelvis. As long as the clot stays attached to the vein wall, it causes local symptoms like leg swelling and pain but doesn’t threaten the lungs. The danger comes when part or all of the clot detaches.
Once free, the clot fragment rides the flow of blood back toward the heart. It passes through the right side of the heart and enters the pulmonary arteries, which branch into smaller and smaller vessels as they spread through the lungs. The clot eventually reaches a vessel too narrow to pass through and lodges there, partially or completely blocking blood flow. A small clot may block a minor branch and cause mild symptoms. A large clot can obstruct a major pulmonary artery, causing the right side of the heart to fail under the sudden pressure. The risk of a clot traveling to the lungs is highest when the DVT extends to the veins at or above the back of the knee.
The Three Underlying Causes
Clots don’t form randomly. Three conditions, often working together, create the environment for abnormal clotting. These are sluggish blood flow, damage to the blood vessel lining, and blood that clots too easily. Most risk factors for pulmonary embolism fit into one of these categories.
Sluggish blood flow. When blood moves slowly or pools in the veins, it’s more likely to clot. This is why immobility is one of the biggest risk factors. Sitting still on a long flight, recovering in bed after surgery, or being hospitalized for any reason slows the return of blood from the legs to the heart. Even a leg cast or prolonged bed rest at home can be enough.
Vessel damage. Injury to the inner lining of a blood vessel triggers the body’s clotting response. Surgery, trauma, or an intravenous catheter can all damage veins directly and set the stage for a clot to form at the injury site.
Blood that clots too easily. Some people’s blood has a heightened tendency to clot. This can be inherited (genetic clotting disorders like Factor V Leiden) or acquired through conditions like cancer, pregnancy, or certain medications. Active cancer and cancer treatment within the past six months are recognized risk factors.
Surgery and Hospitalization
Major surgery, particularly on the hips and knees, carries one of the highest risks of pulmonary embolism. It combines all three clotting triggers at once: the surgery damages blood vessels, anesthesia and recovery keep you immobile, and the body’s stress response makes the blood more prone to clotting.
The numbers are striking. After total hip replacement, pulmonary embolism occurs in anywhere from 0.7% to 30% of patients depending on the study and whether preventive blood thinners are used. After hip fracture repair, the rate runs from 4.3% to 24%, with fatal pulmonary embolism occurring in 3.6% to 12.9% of cases. Total knee replacement carries a 1.8% to 7% risk. These wide ranges reflect differences in how aggressively hospitals use prevention measures like blood-thinning medications and compression devices. Modern protocols have pushed the rates toward the lower end, but the risk remains real.
Hormonal Birth Control
Combined oral contraceptives (the pill containing both estrogen and progestin) increase the tendency of blood to clot. A national study in New Zealand found that women currently using combined oral contraceptives had roughly 9.6 times the risk of fatal pulmonary embolism compared to non-users. The absolute risk was still small, about 10.5 deaths per million women per year of use, but third-generation oral contraceptives (newer formulations) carried roughly double the risk of older versions.
Hormone replacement therapy and pregnancy itself also shift the clotting balance. Pregnancy increases clotting factors to protect against hemorrhage during childbirth, but that same protective mechanism raises the risk of dangerous clots.
Travel and Prolonged Sitting
Any trip lasting more than four hours, whether by plane, car, bus, or train, raises the risk of blood clots according to the CDC. The combination of cramped seating, dehydration, and low cabin pressure (on flights) slows blood flow in the legs. The risk climbs further if you already have other factors like obesity, recent surgery, or a clotting disorder. The clot doesn’t always appear during the trip itself. It can form during travel and grow or break loose in the days or weeks afterward.
Other Risk Factors
A previous blood clot is one of the strongest predictors of a future one. People with a history of DVT or pulmonary embolism are significantly more likely to develop another. Beyond that, several other conditions raise the risk:
- Cancer. Many cancers release substances that activate the clotting system. Chemotherapy adds to the risk.
- Obesity. Excess weight puts pressure on the veins in the pelvis and legs, slowing blood flow.
- Smoking. Tobacco damages blood vessel linings and makes the blood stickier.
- Age. Risk increases with age, particularly after 60.
- Heart failure. A weakened heart pumps blood less effectively, allowing it to pool.
Warning Signs to Recognize
Pulmonary embolism can range from silent to immediately life-threatening. The most common sign is a rapid breathing rate; about 96% of people with a diagnosed pulmonary embolism breathe faster than 20 breaths per minute. A fast heart rate (over 100 beats per minute) shows up in about 44% of cases. Sudden shortness of breath, sharp chest pain that worsens with deep breathing, and coughing up blood are classic symptoms, though not everyone experiences all of them.
Interestingly, low oxygen levels and bluish skin are not prominent features in most cases. Some patients, even with large clots, maintain near-normal oxygen readings. This means a normal pulse oximeter reading does not rule out a pulmonary embolism. Doctors instead look at the full picture: symptoms, risk factors, leg swelling, heart rate, recent surgery or immobilization, and history of prior clots.
How It’s Diagnosed
When doctors suspect a pulmonary embolism, they typically start with a blood test that measures a substance called D-dimer, a protein fragment released when a blood clot dissolves. The test is extremely sensitive, catching about 97% of pulmonary embolisms. However, it’s not very specific: only about 41% of people with an elevated D-dimer actually have a clot. Many other conditions, including infection, inflammation, pregnancy, and recent surgery, can also raise D-dimer levels. Because of this, a normal D-dimer is useful for ruling out a clot in lower-risk patients, but an elevated result usually leads to imaging, most commonly a CT scan of the chest with contrast dye to visualize the pulmonary arteries directly.
Why Prompt Treatment Matters
Untreated pulmonary embolism has a high mortality rate, but with prompt anticoagulation (blood-thinning treatment), the picture improves dramatically. Clinical trials have shown a mortality rate below 2% in patients with intermediate-to-high-risk pulmonary embolisms who receive timely treatment and close monitoring. The goal of treatment is to prevent the existing clot from growing and to stop new clots from forming, giving the body time to gradually dissolve the blockage on its own. In severe cases where the clot is massive and the heart is failing, more aggressive interventions to break up or remove the clot may be needed.
After a pulmonary embolism, most people take blood thinners for at least three months, and sometimes indefinitely if the underlying cause can’t be removed. People whose clot was triggered by a temporary factor, like surgery or a long flight, generally have a lower risk of recurrence than those with ongoing risk factors like cancer or a genetic clotting disorder.