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DEAR DOCTOR K: What is a pulmonary embolism?

DEAR READER: A pulmonary embolism occurs when a blood clot (called an embolus) suddenly blocks a blood vessel in the lung. A small pulmonary embolus can happen without causing any symptoms, but a large pulmonary embolus can suddenly threaten your life.

To explain pulmonary embolism, let’s begin with a refresher on the circulation of blood in our bodies. Blood carries nutrients (like oxygen and sugar) to the cells of our body and removes waste material from the cells. The blood circulates because of the pumping action of the heart.

There are two circuits. One circuit pumps oxygen-poor blood to the lungs, where it picks up oxygen. The second circuit pumps oxygen-rich blood throughout the body through “pipes” called arteries. When oxygen leaves the blood to enter the cells, the blood becomes oxygen-poor. That oxygen-poor blood returns to the heart through pipes called veins.

Blood flows more slowly through veins than through arteries. Sometimes clots form in the slowly moving blood, especially if the flow is slower than usual. This is a condition called deep vein thrombosis (DVT).

Usually, DVT occurs in the veins of the legs. Part of the clot can break away and travel through veins to the heart, and then on to the lungs. There, it travels through progressively narrower blood vessels. When the size of the clot is larger than the width of the blood vessel, it gets stuck in the blood vessel. (I’ve put an illustration on my website, AskDoctorK.com.)

DVT may cause pain or swelling of the legs. It is usually the result of extended inactivity. If you are inactive for many hours — during a long airplane flight or prolonged bed rest, for example — blood flow in your legs may slow. DVT is more likely in people who are bedridden and those who are recovering from surgery.

Symptoms of pulmonary embolism can include chest pain and unexplained shortness of breath or coughing.

Doctors treat a life-threatening pulmonary embolism by using clot-busting medicines. Or a doctor may use a catheter to remove the clot or deliver medicine to dissolve it.

Non-life-threatening pulmonary embolism is treated with blood thinners to keep clots from getting larger and prevent new clots from forming. Blood thinners don’t break up blood clots that have already formed; the body dissolves most clots with time. But the blood thinner will be necessary for several months or longer.

To reduce your risk for DVT and pulmonary embolism:

— Avoid sitting motionless for long periods of time.

— If you must sit, flex your calves, ankles and thighs from time to time to keep the blood flowing.

— Drink a lot of water.

— Walk around frequently during long plane flights or car trips.

— Get out of bed and move around as soon as possible after having surgery or being ill.

If you’ve already had DVT or pulmonary embolism, ask your doctor about compression stockings. They can help keep blood in the legs from pooling and clotting.

(Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.)

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