Pulmonary embolism is the sudden blocking of an artery of the lung (pulmonary artery) by a collection of solid material brought through the bloodstream (embolus)—usually a blood clot (thrombus) or rarely other foreign material.
The pulmonary arteries carry blood from the heart to the lungs. The blood picks up oxygen from the lungs and travels back to the heart. From the heart, the blood is pumped to the rest of the body to provide oxygen to the tissues (see Biology of the Lungs and Airways: Exchanging Oxygen and Carbon Dioxide). When a pulmonary artery is blocked by an embolus, people may not be able to get sufficient oxygen into the blood. Large emboli may cause so much blockage that the heart has to strain to pump blood through the pulmonary arteries that remain open. If too little blood is pumped or the heart is strained excessively, the person can go into shock and die. Sometimes, the blockage of blood flow causes lung tissue to die (a condition called pulmonary infarction).
The body usually breaks up small clots quickly, keeping damage to a minimum. Large clots take much longer to disintegrate, so more damage is done.
About 1% of people admitted to the hospital have a pulmonary embolism. In about 5% of people in whom autopsy is done to find the cause of death, pulmonary embolism is unexpectedly found to be the cause.
The most common type of pulmonary embolism is a blood clot, usually one that forms in a leg or pelvic vein (see Venous Disorders: Deep Vein Thrombosis (DVT)) when blood flow slows down or stops, as may occur in the leg veins when a person stays in one position for a long time. People who have been on prolonged bed rest and those recovering from major surgery are at risk. Those sitting for long time periods without moving around (as may happen during air travel) are at slightly increased risk. Far less often, blood clots form in the veins of the arms or in the right side of the heart. Once a clot breaks free into the bloodstream, it usually travels to the lungs.
Unusual Types of Emboli:
The sudden blocking of an artery of the lung is not only caused by blood clots. Other material can also form emboli.
Symptoms depend on the extent that the pulmonary artery is blocked and on the person's overall health. For example, people who have another disease such as chronic obstructive pulmonary disease or coronary artery disease may have more disabling symptoms.
Small emboli may not cause any symptoms, but when symptoms do occur, they usually develop abruptly. Shortness of breath may be the only symptom, especially if pulmonary infarction does not develop. Often, the breathing is very rapid, and the person may feel anxious or restless and appear to have an anxiety attack. Some people have pain in the chest. In some people, the first symptoms are light-headedness, fainting, or seizures. In older people, the first symptom may be confusion or deterioration of mental function. These symptoms usually result from a sudden decrease in the heart's ability to deliver enough oxygen-rich blood to the brain and other organs.
The heartbeat may become rapid, irregular, or both. With very large emboli, blood pressure may be dangerously low (shock), the skin may have a blue color (cyanosis), or the person may suddenly die.
The symptoms of pulmonary infarction develop over hours. If pulmonary infarction occurs, the person experiences coughing that may produce blood-stained sputum, sharp chest pain when breathing in, and in some cases fever. Symptoms of infarction often last several days but usually become milder every day.
In people who have recurring episodes of small pulmonary emboli, symptoms such as chronic shortness of breath, swelling of the ankles or legs, and weakness tend to develop progressively over weeks, months, or years.
Doctors suspect pulmonary embolism based on the person's symptoms and risk factors, such as recent surgery, a prolonged period of bed rest, or an inherited tendency to form blood clots. A large pulmonary embolism may be relatively easy for doctors to diagnose, especially when there are obvious preconditions, such as signs of a blood clot in a leg. However, in many cases, symptoms are absent or not very characteristic, which is an important reason why pulmonary embolism is often difficult to diagnose. Indeed, pulmonary embolism is one of the most difficult serious disorders for doctors to recognize and diagnose.
A chest x-ray may reveal subtle changes in the blood vessel patterns after embolism and signs of pulmonary infarction. However, the x‑ray results are often normal, and even when they are abnormal, they rarely enable doctors to establish the diagnosis with certainty.
An electrocardiogram may show abnormalities, but often these abnormalities are transient and can only support the possibility of pulmonary embolism.
The person's symptoms and risk factors and the results of tests help doctors estimate the likelihood of a pulmonary embolism. This estimate determines what other tests are done. Doctors try to use tests that do not involve making an incision or entering the person's body (noninvasive tests) before they use an invasive test. Noninvasive tests are usually easier to perform and carry less risk of side effects. For example, if pulmonary embolism appears unlikely, testing may be limited to a blood test that measures a substance called d‑dimer. If pulmonary embolism seems more likely or if the result of the d-dimer test is abnormal, further testing is done, which may include a CT angiogram, an ultrasound examination of the legs, or a lung perfusion scan. These are noninvasive tests. If the diagnosis is still unclear after noninvasive tests are done, an invasive test (for example, pulmonary angiography) may be done.
A CT angiogram is a type of computed tomography (CT) scan. It is fast, noninvasive, and fairly accurate, particularly for large clots. In this test, contrast material is injected into a vein. The contrast material travels to the lungs, and a CT scanner generates images of blood in the arteries to determine if a pulmonary embolism is blocking blood flow. A CT angiogram is the imaging test most often used to diagnose pulmonary embolism.
An ultrasound examination of the legs is noninvasive and can identify clots in the legs, which are the usual sources of pulmonary embolism. The absence of clots on this test does not rule out pulmonary embolism. However, if the ultrasound examination reveals blood clots, people are usually treated as they would be for pulmonary embolism without any further testing.
A lung perfusion scan is noninvasive and fairly accurate but is not very rapid. A tiny amount of radioactive substance is injected into a vein and travels to the lungs, where it outlines the blood supply (perfusion) of the lung. Completely normal scan results indicate that the person does not have a significant blood vessel obstruction. Abnormal scan results support the possibility of pulmonary embolism but may also reflect disorders other than pulmonary embolism, such as emphysema, which can result in decreased blood flow to areas where lung tissue has been damaged.
Usually, the perfusion scan is done with a lung ventilation scan. The person inhales a harmless gas containing a trace amount of radioactive material, which is distributed throughout the small air sacs of the lungs (alveoli). The areas where carbon dioxide is being released and oxygen is taken up can then be seen on a scanner. By comparing this scan to the pattern of blood supply shown on the perfusion scan, doctors can usually determine whether a person has had a pulmonary embolism.
Pulmonary angiography (see Diagnosis of Lung Disorders: Chest Imaging) is the most accurate means of diagnosing pulmonary embolism, but it is invasive and poses some risk and is more uncomfortable than the other tests. It is usually performed only if the results of other tests are not conclusive.
The likelihood of dying from pulmonary embolism is very low, but massive pulmonary embolism can cause sudden death. Most deaths occur before the diagnosis is made, often within 1 to 2 hours of the embolism occurring. If a person is alive when diagnosed, the chance of survival is about 95%. Important factors include the size of the embolus, the size and number of pulmonary arteries blocked, and the person's overall health status. Anyone with a serious heart or lung problem is at greater risk of dying from pulmonary embolism. A person with normal heart and lung function usually survives unless the embolus blocks half or more of the pulmonary arteries.
Given the danger of pulmonary embolism and the limitations of treatment, doctors try to prevent blood clots from forming in the veins of people at risk. In general, people, particularly those who are prone to clotting, should try to be active and move around as much as possible. For example, when traveling on an airplane for a long period, people should try to get up and move around every 2 hours.
For certain people, an anticoagulant drug is given, most often heparin. Heparin comes in two forms: traditional and low molecular weight. They appear equally effective. Heparin is the most widely used drug for reducing the likelihood of clots forming in calf veins after any type of major surgery, especially surgery on the legs. Small doses are injected just under the skin shortly before surgery, and ideally additional doses are given until the person is up and walking again. Hospitalized people at high risk of developing pulmonary embolism (such as those with heart failure, immobility, obesity, or who have had clots in the past) benefit from small doses of heparin even if they are not undergoing surgery. Low-dose heparin does not increase the frequency of major bleeding complications, but heparin can increase minor oozing of blood from wounds.
Warfarin, an anticoagulant given by mouth, may be given to people with one or more risk factors. It is also given to those who have undergone certain kinds of surgery that are particularly likely to result in clots, such as surgery for a hip fracture or a joint replacement. Warfarin therapy may need to be continued for several weeks or months. Low-molecular-weight heparin is also effective for people in this situation.
Newer anticoagulants include those such as hirudin, which inhibits the production of thrombin (a substance that promotes the formation of blood clots), and danaparoid and fondaparinux, which inhibit the formation of other substances that enhance the body's production of clots. These drugs are effective in prevention but are still being studied to determine whether they have advantages compared with heparin.
For people who have undergone surgery—especially older people—the risk of clot formation can be reduced by wearing compression elastic stockings, doing leg exercises, and getting out of bed and becoming active as soon as possible. For people who cannot move their legs, intermittent air compression devices can provide rhythmic external pressure to keep blood moving in the legs. However, these devices alone are inadequate to prevent clot formation in people who have undergone hip or knee surgery.
Treatment of pulmonary embolism begins with treating the symptoms. Oxygen is given if blood oxygen levels are low. Analgesics are given to relieve pain. If blood pressure is low, intravenous fluids are given and sometimes drugs that increase blood pressure are given. Mechanical ventilation (a breathing tube) may be needed if respiratory failure develops.
Anticoagulant drugs such as heparin are given to prevent existing blood clots from enlarging and additional clots from forming. Heparin is given intravenously to achieve a rapid effect, and doctors carefully regulate the dose. Doctors strive to achieve a full anticoagulant effect within the first 24 hours of treatment. Low-molecular-weight heparin is probably as effective as traditional heparin and does not require the blood test monitoring that is commonly recommended for conventional heparin. Warfarin, which also inhibits clotting but takes longer to start working, is given next. Because warfarin is taken by mouth, it can be used long-term. Heparin and warfarin are given together for 5 to 7 days, until blood tests show that the warfarin is effectively preventing clotting. Then, the heparin is discontinued.
How long anticoagulants are given depends on the person's situation. If pulmonary embolism is caused by a temporary risk factor, such as surgery, treatment is given for 2 to 3 months. If the cause is some longer-term problem, such as prolonged bed rest, treatment usually is given for 3 to 6 months, but sometimes it must continue indefinitely. For example, people who have recurrent pulmonary embolism, often because of a hereditary clotting disorder, usually take anticoagulants indefinitely. While taking warfarin, people periodically have to have a blood test to determine if the dose needs to be adjusted.
Changes in diet and use of other drugs may affect warfarin's anticoagulant effects. If excessive anticoagulation occurs, severe bleeding in a number of body organs can develop. Because many drugs can interact with warfarin, people who take anticoagulants should be sure to check with their doctor before taking any other drugs, including drugs that can be obtained without a prescription (over-the-counter drugs) such as acetaminophen or aspirin, herbal preparations, and dietary supplements. Foods that are high in vitamin K (which affects blood clotting), such as broccoli, spinach, kale, and other leafy green vegetables, liver, grapefruit and grapefruit juice, and green tea, may also need to be avoided.
Thrombolytic drugs such as streptokinase or tissue plasminogen activator (TPA) break up and dissolve blood clots. They can be used for people who appear to be in danger of dying of pulmonary embolism. However, except in the most dire situations, these drugs cannot be given to people who have had surgery in the preceding 2 weeks, are pregnant, have had a recent stroke, or tend to bleed excessively.
In some centers, if a person appears to be in danger of dying from a massive pulmonary embolism, doctors may try to shatter the embolus using a catheter inserted into the pulmonary artery. Surgery may be needed to save someone with severe embolism. Removal of the embolus from the pulmonary artery may be lifesaving. Surgery is also used to remove long-standing pulmonary artery clots that cause persistent shortness of breath and high pressures in the pulmonary artery (pulmonary hypertension).
A filter can be surgically placed in the main vein in the abdomen that drains blood from the legs and pelvis to the right side of the heart (see Venous Disorders: Umbrellas: One Way to Prevent Pulmonary Embolism). Such a filter can be used if emboli recur despite anticoagulant treatment or if anticoagulants cannot be used or cause significant bleeding. Because clots generally originate in the legs or pelvis, this filter usually prevents them from being carried into the pulmonary artery. Newer filters are removable. Removal helps prevent some complications that can occur when filters are left in place permanently.
Last full review/revision August 2007 by John H. Newman, MD