Pulmonary Embolism
Pulmonary embolism is the sudden blocking of an artery of the lung (pulmonary artery) by an embolus--usually a blood clot (thrombus).
The functions of the arteries of the lungs are to carry enough blood containing oxygen and nutrients to keep the lung tissue healthy and to carry carbon dioxide to the lungs for removal from the body (see Section 4, Chapter 38). However, when a large artery to the lung is blocked by an embolus, the amount of blood supplied may be insufficient, eventually causing lung tissue to die.
About 10% of people with pulmonary embolism suffer some lung tissue death (called pulmonary infarction). Sometimes the body breaks up small clots quickly, keeping damage to a minimum. Large clots take much longer to disintegrate, so more damage is done. Large clots may cause sudden death by blocking so much of the lung arteries that the oxygen supply to the body is inadequate to sustain life or by placing an excessive strain on the heart.
The prevalence of pulmonary embolism in people admitted to the hospital is about 1%. When an autopsy is performed, pulmonary embolism is often unexpectedly found to be the cause of death in about 5% of people.
Causes
The most common type of embolus that travels to the lungs is a blood clot, usually one that forms in a leg or pelvic vein (see Section 3, Chapter 36) 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 sitting for long time periods without moving around (as may happen during air travel) are at particular risk. When the person starts moving again, the clot can break loose. 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.
Another type of embolus may form from fat, which can escape into the blood from the bone marrow when a bone is fractured. An embolus also may form from amniotic fluid being forced into the pelvic veins during childbirth. However, both fat and amniotic fluid emboli are rare. If they form, they usually lodge in small vessels such as the arterioles and capillaries of the lung, where they generally cause less damage than blood clots. However, if many of these smaller vessels become obstructed, acute respiratory distress syndrome (see Section 4, Chapter 56) or pulmonary hypertension (see Section 4, Chapter 54) may develop; both of these conditions can lead to respiratory failure, heart failure, and shock.
Cancerous tumor fragments may break free into the circulation to form emboli, which, if they are numerous, can cause pulmonary hypertension as the cancer spreads throughout the lungs.
Air bubbles may form an emboli and cause pulmonary embolism after a vein has been exposed to large amounts of air, as may occur during intravenous infusion of drugs, nutrients, or fluid. Air emboli may also form when a vein is being operated on (such as when a blood clot is being removed) or when a person is being resuscitated (because of the force of chest compressions). An additional risk is when a person dives underwater; the risk depends on how deep the person dives and how fast he ascends to the surface of the water (see Section 24, Chapter 295).
 See the sidebar What Predisposes Someone to Blood Clots?
Symptoms
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 most emboli cause shortness of breath, which comes on very quickly. 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. Larger emboli commonly cause sharp pain in the chest, especially when the person inhales; the pain is called pleuritic chest pain.
In some people, the first symptoms of pulmonary embolism may be light-headedness, fainting, or seizures. 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. Irregular heartbeats may also occur. People with obstruction of one or more large pulmonary arteries may have a blue skin color (cyanosis) and can die suddenly.
The symptoms of pulmonary embolism usually develop abruptly, whereas the symptoms of pulmonary infarction develop over the following hours. If pulmonary infarction occurs, the person experiences coughing that may produce blood-stained sputum, sharp chest pain when the person breathes 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.
Diagnosis
A doctor suspects pulmonary embolism based on the person's symptoms and predisposing factors, such as a recent surgery or a prolonged period of bed rest. A large pulmonary embolism may be relatively easy for a doctor to diagnose, especially when there are obvious preconditions, such as signs of a blood clot in a leg. Certain procedures are often needed to confirm the diagnosis. Even with these procedures, however, many emboli can be quite subtle and difficult for doctors to diagnose conclusively.
A chest x-ray may reveal subtle changes in the blood vessel patterns after embolism and signs of pulmonary infarction. However, the results are often normal, and even when they are abnormal, they rarely enable the doctor 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.
A lung perfusion scan is one of the best tests for diagnosing pulmonary embolism. 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. Areas without normal blood supply appear dark on the scan because no radioactive particles can reach them. 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 conditions other than pulmonary embolism, such as obstructive lung disease (for example, emphysema, which can result in decreased blood flow to areas where lung tissue has been damaged).
Usually, the perfusion scan is coupled 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 taken up can then be seen on a scanner. By comparing this scan to the pattern of blood supply shown on the perfusion scan, a doctor can usually determine whether a person has had a pulmonary embolism by a mismatch between ventilation and blood perfusion.
Pulmonary angiography (see Section 4, Chapter 39) is an accurate means of diagnosing pulmonary embolism, but it poses some risk and is more uncomfortable than the other tests. It is usually performed only if the other tests fail to demonstrate a conclusive diagnosis of a pulmonary embolism. In an x-ray procedure, a radiopaque dye is injected into the pulmonary arteries. A pulmonary embolism shows up as blockage in an artery. A certain type of computed tomography (CT) called CT angiography is another accurate test. CT angiography can be used if pulmonary angiography is not available or if the person should not undergo this test for some reason.
Additional tests, such as an ultrasound to examine the legs for blood clots in the veins, may be performed to find out where the embolus originally developed. A blood test (D-dimer test) can provide additional support of the diagnosis. A normal test result can help to exclude pulmonary embolism as the cause of a person's symptoms.
Prevention
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 of pulmonary embolism. In general, a person who is 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, the person should try to get up and move around every two hours.
For people who have undergone surgery--especially older people--the risk of clot formation can be reduced by the following measures: 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 and thighs. However, these devices alone are inadequate to prevent clot formation in people who have undergone hip or knee surgery.
Anticoagulant drugs are given. Heparin is the most widely used therapy for reducing the likelihood of clots forming in calf veins after any type of major surgery, especially surgery for the legs (see Section 14, Chapter 173). Hospitalized people at high risk of developing pulmonary embolism (such as those with heart failure, an acute myocardial infarction, chronic lung disease, obesity, a stroke or other neurologic problem or who have had clots in the past) benefit from small doses of heparin even if they are not undergoing surgery. Small doses are injected just under the skin shortly before the operation and ideally until the person is up and walking again. Low-dose heparin does not increase the frequency of major bleeding complications, but heparin can increase minor oozing of blood from wounds. Low-dose heparin can also be used for operations involving the spine or brain.
A different form of heparin, called low-molecular-weight heparin, is equally or even more effective in preventing clots than the use of traditional heparin. Low-molecular-weight heparin is also injected just under the skin and is usually continued until the risk of developing clots has passed.
Warfarin, an anticoagulant given by mouth, may be given when a person has 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.
Treatment
Treatment of pulmonary embolism begins with the administration of oxygen and, if necessary, analgesics to relieve pain. 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 effect within the first 24 hours of treatment. Otherwise, the person is at high risk of more pulmonary emboli, and new clots or enlargement of existing clots in leg and pelvic veins. Low-molecular-weight heparin is probably equally effective to traditional heparin and does not require the blood test monitoring that conventional heparin requires. 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 predisposing 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 due to a hereditary predisposition to clotting, usually take anticoagulants indefinitely. While taking warfarin, the person periodically has to have a blood test to determine if the dose needs to be adjusted. Changes in diet and many other drugs may affect the magnitude of anticoagulation by this drug. If excessive anticoagulation occurs, severe bleeding in a number of body organs can develop.
Thrombolytic therapy is used for people who appear to be in danger of dying of pulmonary embolism. Thrombolytic drugs such as streptokinase or tissue plasminogen activator (TPA) break up and dissolve the clot. However, 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. 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 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 Section 3, Chapter 36). 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.
For emboli that form from fat or amniotic fluid, oxygen therapy and use of a ventilator may be needed. In addition, because emboli that develop from amniotic fluid may stimulate the formation of blood clots (coagulation), agents such as cryoprecipitate are sometimes needed to block certain key steps in the formation of these clots (such as the development of fibrin deposits in the circulation).
Prognosis
About half of the people with untreated pulmonary embolism will have another embolism. As many as half of these recurrences may be fatal. Anticoagulant treatment can reduce the rate of recurrence to about 1 in 20 people; only about 1 in 5 of these people will die of pulmonary embolism. The likelihood of dying depends on 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 vessels. If death occurs from pulmonary embolism, it usually occurs rapidly, often within 1 to 2 hours.
An air embolism can cause death, but only if the amount of air that reaches the heart and pulmonary arteries is large. Death occurs with a large air embolus not only because blood flow to much of the lungs is blocked but also because the heart cannot effectively pump blood.
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