Obstruction of the blood flow to the lungs by one or more blood clots
In pulmonary embolism, a piece of a blood clot (embolus) becomes lodged in an artery in the lungs and partially or completely blocks blood flow in the affected area. Usually, the clot has broken off from a larger clot in the veins of the legs or the pelvic region (see Deep vein thrombosis) and travelled to the lungs in the bloodstream. Pulmonary embolism may be mild if the blocked artery is small, but blockage of a major artery may suddenly cause severe symptoms and can be fatal. Rarely, small clots can block numerous small arteries in the lungs over months or years, and symptoms may not appear for some time. This condition is called recurrent pulmonary embolism.
Pulmonary embolism is most likely to occur in people who have developed deep vein thrombosis as a result of a period of immobility, such as that following childbirth or surgery (especially repair of fractures or pelvic surgery) or, rarely, during a long journey. A tendency to develop blood clots (see Hypercoagulability) increases the risk of deep vein thrombosis, as does smoking and using combined oral contraceptives or hormone replacement therapy.
The symptoms depend on the extent of blockage to the flow of blood. Massive pulmonary embolism, in which a large blood clot blocks a major pulmonary artery, may cause sudden death. Single small clots may not cause symptoms. In most cases, however, symptoms develop over a few minutes and may include:
Shortness of breath.
Sharp chest pain that may be worse when inhaling.
Coughing up blood.
In some cases of recurrent pulmonary embolism, shortness of breath that worsens over months is the only symptom. Eventually, so many vessels may become blocked that pressure in the pulmonary arteries is increased (see Pulmonary hypertension) and chronic heart failure may then develop.
If your doctor suspects that you have a pulmonary embolism, you will be admitted to hospital as an emergency. You will be given tests to measure the levels of oxygen and carbon dioxide in your blood (see Measuring blood gases). You will also have a chest X-ray to exclude other lung conditions, and the blood flow in the veins of your legs may be measured using Doppler ultrasound scanning. You may also have a special blood test (the D-dimer test) to check if there has been significant clot formation and breakdown in the body; a radionuclide lung scan; and a contrast CT scan to image the blood vessels in the lungs and pinpoint the exact location of blockages. You may also have a blood test to check if you have a clotting disorder.
Treatment depends on the severity of the blockage. You may have a continuous intravenous infusion or injections of heparin, an anticoagulant drug that acts immediately to prevent the existing clots from enlarging and new clots from developing (see Drugs that prevent blood clotting). At the same time, you may be started on an oral anticoagulant drug, such as warfarin, which also prevents further clotting by thinning the blood but takes a few days to become effective. Anticoagulant drug treatment may last for 3 months if pulmonary embolism followed a single period of immobility. If the embolism is due to a long-term condition, such as hypercoagulability, you may need drug treatment indefinitely. Treatment is usually monitored with regular blood tests to check your blood clotting, and the drug dosage is altered as necessary.
In severe cases, thrombolytic drugs may be used to dissolve the clot. If the main artery supplying the lungs is affected, life-saving emergency surgery may be necessary to remove the clot. If clots recur despite anticoagulant drug therapy, or if anticoagulants are not suitable, you may need to have a procedure in which a filter to trap blood clots is placed in the main vein that leads from the lower half of the body to the heart.
Massive pulmonary embolism is fatal in about 1 in 3 cases. However, if you survive the first few days, you are likely to make a full recovery. People who have recurrent pulmonary embolism may remain short of breath.
If you have already had pulmonary embolism, you have a higher than average risk of further episodes and should avoid periods of prolonged immobility, such as sitting down continually during a long journey. You may need to have preventive treatment, such as heparin injections, after any surgery that involves prolonged immobility.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.