Deep Vein Thrombosis

Topic Overview

What is deep vein thrombosis?

Deep vein thrombosis (DVT) is a blood clot (thrombus) in a deep vein, usually in the legs.

Clots can form in superficial veins and in deep veins. Blood clots with inflammation in superficial veins (called superficial thrombophlebitis or phlebitis) rarely cause serious problems. But clots in deep veins (deep vein thrombosis) require immediate medical care. See pictures of a developing blood clot and the leg veins .

These clots are dangerous because they can break loose, travel through the bloodstream to the lungs, and block blood flow in the lungs (pulmonary embolism). A pulmonary embolism is often life-threatening. DVT can also lead to long-lasting problems. DVT may damage the vein and cause the leg to ache, swell, and change color. It can also lead to leg sores after years of having a DVT.

Blood clots most often develop in the calf and thigh veins, and less often in the arm veins or pelvic veins. This topic focuses on blood clots in the deep veins of the legs, but diagnosis and treatment of DVT in other parts of the body are similar.

What causes deep vein clots to form?

Blood clots can form in veins when you are inactive. For example, clots can form if you are paralyzed or bedridden or must sit while on a long flight or car trip. Surgery or an injury can damage your blood vessels and cause a clot to form. Cancer can also cause deep vein thrombosis. Some people have blood that clots too easily, a problem that may run in families.

What are the symptoms?

Symptoms of DVT include swelling of the affected leg. Also, the leg may feel warm and look redder than the other leg. The calf or thigh may ache or feel tender when you touch or squeeze it or when you stand or move. Pain may get worse and last longer or become constant.

If a blood clot is small, it may not cause symptoms. In some cases, pulmonary embolism is the first sign that you have DVT.

How is deep vein thrombosis diagnosed?

If your doctor suspects that you have DVT, you probably will have an ultrasound test to measure the blood flow through your veins and help find any clots that might be blocking the flow. Other tests, such as a venogram, are sometimes used if ultrasound results are unclear. A venogram is an X-ray test that takes pictures of the blood flow through the veins.

How is it treated?

Treatment begins right away to reduce the chance that the blood clot will grow or that a piece of the clot might break loose and flow to your lungs.

Treatment for DVT usually involves taking blood thinners (anticoagulants) such as heparin and warfarin (Coumadin, for example). Heparin is given through a vein (intravenously, or IV) or as an injection. Warfarin is given as a pill. Treatment usually involves taking blood thinners for at least 3 months to prevent existing clots from growing.

Your doctor may need to adjust the dose of your medicine. You will have blood tests often so he or she can see how well the blood thinners are working.

Your doctor also may recommend that you prop up or elevate your leg when possible, use a heating pad, take walks, and wear tight-fitting, elastic stockings (compression stockings). These measures may help reduce the pain and swelling that can happen with DVT.

In rare cases, a vena cava filter may be used. A vena cava filter is inserted into the vena cava, the large vein that returns blood to the heart from the abdomen and legs. A vena cava filter helps prevent blood clots from traveling to the lungs. This device is usually only used if a person is at high risk for pulmonary embolism and is not able to take blood thinners. It may also be used if you have DVT that comes back again or you had a sudden blockage of blood flow in the lung (pulmonary embolism) while taking blood thinners.

How can deep vein thrombosis be prevented?

There are things you can do to prevent deep vein thrombosis. Many doctors recommend that you wear compression stockings during a journey longer than 8 hours. On long flights, walk up and down the aisle hourly, flex and point your feet every 20 minutes while sitting, drink plenty of water, and avoid alcohol and beverages with caffeine.

Frequently Asked Questions

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	Deep vein thrombosis: Taking anticoagulants safely

Cause

Three factors can increase the risk of deep vein thrombosis:

• Slowed blood flow. If you have surgery or are inactive for a period of time (for example, if you are bedridden or must sit while on a long flight or car trip), blood flow in your legs is slowed, which allows a blood clot to form.
• Damage to the blood vessels. Surgery or injury may damage your blood vessels.
• Changes in the blood itself. Cancer or certain inherited factors may cause your blood to clot more quickly than normal.

Symptoms

Deep vein thrombosis often does not cause symptoms or causes only minimal symptoms. When symptoms occur, they include:

• Swelling.
• Warmth.
• Pain or tenderness. The pain may be in the calf or thigh and may be present only when the affected area is touched or when standing or walking.
• Redness.

There are many other conditions with similar symptoms, such as a ruptured Baker's cyst or cellulitis, which can make diagnosing deep vein thrombosis difficult.

Sometimes life-threatening pulmonary embolism is the first indication that you have deep vein thrombosis. Pulmonary embolism is the sudden blockage of an artery in the lung. Blood clots in the deep veins of the leg are the most common cause of pulmonary embolism. Symptoms of pulmonary embolism include:

• Sudden shortness of breath.
• Chest pain that may get worse with a deep breath.
• A cough that may bring up blood.
• A fast heart rate.
• Fainting (syncope).

What Happens

If you have symptoms of deep vein thrombosis, testing will begin immediately to determine whether you have a blood clot in your leg.

Often people with deep vein thrombosis do not have any symptoms. In these people, this condition is usually suspected only after a blood clot is discovered in the lung (pulmonary embolism). Typically, the blood clot in the lung came from a deep vein clot in the leg that was not causing symptoms.

When you are diagnosed with deep vein thrombosis, treatment begins if it is likely that the blood clot will grow or that a piece of the clot might break loose and flow to the lungs (pulmonary embolism). If you have a blood clot in your upper (proximal) leg vein, you will likely need to take anticoagulant medicine for 3 to 6 months, and possibly longer.¹

After 3 to 6 months, your doctor may recommend that you continue warfarin (such as Coumadin) on an ongoing basis to prevent deep vein clots from recurring.²

Typically, if you have a blood clot in the lower (distal) deep leg veins, you will need to take medicine to prevent more blood clots (anticoagulant medicine) for about 3 to 6 months. The length of time will vary based on your own health. Sometimes your doctor will not start this medicine right away. He or she will wait 24 to 48 hours to see if your blood clot is growing. For symptom relief, your doctor may recommend a nonsteroidal anti-inflammatory drug (NSAID), such as ibuprofen.

The main goal of treatment is to prevent the blood clot from growing or moving to the lungs. If a blood clot in the deep veins of the leg breaks loose, it can travel to the lungs and block blood flow (pulmonary embolism). Pulmonary embolism occurs in 25% of cases of untreated, diagnosed deep leg vein thromboses.³ In people who receive treatment for deep vein thrombosis, the rate of pulmonary embolism falls drastically. For more information, see the topic Pulmonary Embolism.

Blood clots in the lung (pulmonary emboli) occur more often in people with deep vein blood clots in the upper rather than the lower leg veins. Only about 25% of blood clots in the veins of the calf will become larger and extend into the upper leg or groin veins.⁴ Blood clots that extend into the upper leg veins usually require treatment with anticoagulant medicine to prevent pulmonary embolism.

The recurrence rate for deep vein thrombosis varies depending on what caused the blood clot and how it was treated. Recurrence is most common in people who have continuing risk factors (such as cancer or inherited blood-clotting problems) and in people who have had more than one blood clot in the leg. Recurrence is lowest in people who have a short-term risk factor, such as surgery or temporary inactivity.

In about 25% of people who have had deep vein thrombosis with symptoms, a condition called postthrombotic syndrome may develop.⁴ This condition can cause pain, swelling, discoloration, and sores on the leg. Postthrombotic syndrome usually develops within 2 years of the original blood clot.⁴ One study showed that compression stockings can cut your chance of developing postthrombotic syndrome nearly in half.⁵

What Increases Your Risk

Many factors increase your risk for deep vein thrombosis. Some risk factors do not change, such as genetic blood irregularities, while other risk factors may change according to circumstances, such as pregnancy.

Major risk factors

Major risk factors for deep vein thrombosis include:

• Prolonged bed rest (more than 3 days), such as during a hospital stay.
• Abnormal blood clotting (hypercoagulable state), usually a result of inherited genes from one or both parents.
• Injury.
• Surgery, particularly major hip or knee surgery, neurosurgery, and abdominal or chest surgery associated with cancer.
• Cancer and its treatment.
• Paralysis from a spinal cord injury.
• Having a central venous catheter during a hospital stay.

Minor risk factors

Most of these risk factors are minimal by themselves but may become more significant in combination. Research continues on the importance of these risk factors and how they interrelate.

Your risk for deep vein thrombosis may be increased by:

• Certain health conditions such as varicose veins, heart attack, heart failure, and stroke.
• A long airplane flight or car trip.
• Pregnancy, especially immediately after giving birth or after a cesarean section.
• Increasing age. People older than 40 have a greater risk of developing deep vein thrombosis.
• Being overweight.
• Taking birth control hormones, such as daily pills or weekly patches. Current evidence shows that a woman's risk of developing pulmonary embolism, a complication of deep vein thrombosis, increases while she is taking birth control hormones. Past use of birth control pills does not appear to increase this risk.
• Current use of hormone therapy (hormone replacement therapy or estrogen replacement therapy), raloxifene (Evista) for osteoporosis, or the breast cancer treatment tamoxifen (Nolvadex).
• Smoking.

When To Call a Doctor

Call 911 or other emergency services if you:

• Have a sudden onset of shortness of breath and/or chest pain. Chest pain from a blood clot that travels to the lungs (pulmonary embolism) often gets worse with deep breathing.
• Are coughing up blood (hemoptysis).
• Faint or lose consciousness (syncope).

Call your doctor immediately if you have:

• Swelling, warmth, or tenderness in the soft tissues of your leg.
• Pain in your leg that gets worse when you stand or walk. This is especially important if there is also swelling or redness in your leg.

Watchful Waiting

Watchful waiting is not appropriate if you think you have a blood clot in your leg (deep vein thrombosis). Call your doctor if you are not sure whether you need to be seen right away. If you have symptoms of a blood clot in your leg, you should be seen immediately.

Who To See

Health professionals who can diagnose a blood clot (thrombus) in the leg include:

• Nurse practitioners.
• Physician assistants.
• Family medicine doctors.
• Internists.
• Surgeons.
• Emergency room doctors.

To prepare for your appointment, see the topic Making the Most of Your Appointment.

Exams and Tests

Assessing your risk

Deep vein thrombosis may first be suspected after a medical history and physical exam. The information gathered from these initial tests will help your doctor determine whether your risk level for having deep vein thrombosis is low, medium, or high. Your risk level will help your doctor decide the appropriate testing for deep vein thrombosis.

Ultrasound test

Ultrasound is the main test used to help diagnose deep vein thrombosis. It creates a picture of the flow of blood through the veins. You might need more than one ultrasound, usually done a few days to a week apart. This is called serial testing. The testing sequence for deep vein thrombosis is based on your risk level and the results from your initial ultrasound.

Additional tests

Additional tests may be used when ultrasound results are unclear. These tests may help diagnose or exclude a blood clot in the leg but are not frequently needed. Additional tests may include:

• D-dimer test.
• Magnetic resonance imaging (MRI).
• Computed tomography (CT) scan.
• Venogram.

If you are treated with anticoagulant medicines, you may need periodic blood tests to monitor the effects of the anticoagulant on the blood. Blood tests include:

• Activated partial thromboplastin time (APTT) to monitor treatment with standard or unfractionated heparin.
• Prothrombin time (PT), also referred to as INR, to monitor treatment with warfarin (Coumadin).

If you are suspected of having pulmonary embolism, you may have a lung scan, a spiral CT scan, or a pulmonary angiogram. For more information, see the topic Pulmonary Embolism.

Early Detection

Special blood tests may help identify inherited blood-clotting abnormalities that can increase your risk of forming blood clots. However, screening for these factors is not routinely done and is somewhat controversial.

In general, screening is sensible if you have or have had one or more of the following:⁶

• A blood clot in a vein that has no clear cause
• A blood clot at age 45 or younger
• A first-degree family member (mother, father, brother, or sister) who has had a blood clot in a vein before age 45 or has had problems with blood clotting
• A blood clot in a vein at an unusual location, such as the gastrointestinal region, the brain, or the arms

Some doctors believe that checking for clotting problems with a blood test can help prevent deep vein thrombosis in people who have an increased risk and are in a high-risk situation (such as upcoming surgery). If a blood test finds clotting problems, then preventive measures can be taken.

Treatment Overview

The main goals of treatment for deep vein thrombosis are:

• To prevent the blood clot from becoming larger.
• To prevent the blood clot from traveling to the lungs (pulmonary embolism).
• To prevent postthrombotic syndrome, a condition that can cause pain, swelling, and sores of the affected leg.
• To prevent the recurrence of blood clots.

Initial treatment

If you have symptoms of deep vein thrombosis, testing will begin immediately to determine whether you have a blood clot in your leg. Alternately, if a blood clot is discovered in your lung (pulmonary embolism), your doctor may test you for deep vein thrombosis.

When you are diagnosed with deep vein thrombosis, treatment begins immediately to reduce the risk that the blood clot will grow or that a piece of the clot might break loose and flow to the lungs (pulmonary embolism). Early treatment also reduces the risk of postthrombotic syndrome.

Deep vein thrombosis is usually treated with anticoagulant medicines: heparin and warfarin (such as Coumadin). Heparin is given through a vein (intravenously, or IV) or as an injection, and it acts immediately. Warfarin is given by mouth, and it takes several days to become effective. Often both medicines are started at the same time, then heparin is discontinued after warfarin becomes effective. Some people may take low-molecular-weight heparin (LMWH) long term instead of warfarin.

If you have a blood clot in your upper (proximal) leg vein, you will likely need to take warfarin for 3 to 6 months, and possibly longer.¹ After 3 to 6 months and depending upon your risk factors, your doctor may recommend that you continue on lower doses of warfarin (such as Coumadin) on an ongoing basis to prevent deep vein clots from recurring.²

Two types of heparin are available for treatment of deep vein thrombosis. Unfractionated heparin (UH) is given in the hospital, whereas low-molecular-weight heparin (LMWH) can be self-injected at home, which usually is more convenient. Low-molecular-weight heparin usually does not require periodic blood tests to monitor its effects, although both of these anticoagulants are equally effective.^{7, 1}

Typically, if you have a blood clot in the lower (distal) deep leg veins (in your calf), you will need to take medicine to prevent more blood clots (anticoagulant medicine) for about 3 to 6 months. The length of time will vary based on your own health. Sometimes your doctor won't start this medicine right away. He or she may wait 24 to 48 hours to see if your blood clot is growing. For symptom relief, your doctor may recommend a nonsteroidal anti-inflammatory drug (NSAID), such as ibuprofen.

Your doctor may also recommend that you elevate your leg when possible, use a heating pad, take walks, and wear compression stockings. These measures may help reduce the pain and swelling that can occur with deep vein thrombosis.

If you are not able to take anticoagulants, you may need a vena cava filter or different medicines.

Ongoing treatment

For deep vein thrombosis in the upper leg, you will probably need to take warfarin (such as Coumadin) for 3 to 6 months and possibly longer after initial treatment.¹ Some people may take low-molecular-weight heparin (LMWH) long-term instead of warfarin.

After 3 to 6 months, your doctor may recommend that you continue anticoagulants to prevent deep vein clots from recurring.²

When you are taking an anticoagulant, you will have blood drawn regularly so that your doctor can monitor how the anticoagulant medicine is working. The test that measures how long it takes your blood to clot is called prothrombin time, or pro-time.

Medications (especially antibiotics), diet, and daily habits can affect how anticoagulant medicines work. Your doctor will check your blood regularly and may need to adjust the dose of your medicine. For more information, see:

Deep vein thrombosis: Taking anticoagulants safely.

Anticoagulants: Vitamin K and your diet.

Treatment if the condition gets worse

If your clot continues to grow or if you develop pulmonary embolism while on anticoagulation medicines, a vena cava filter may be inserted into a vein. This rarely occurs.

What To Think About

Although medical experts do not agree on the usefulness of compression stockings, they are sometimes recommended to help relieve swelling and pain. One study showed that these stockings can cut your chance of developing postthrombotic syndrome nearly in half.⁵

Pregnant women are generally not given oral anticoagulants-warfarin (such as Coumadin)-because they can cause birth defects. However, anticoagulants given through an IV (unfractionated heparin) or that are injected (low-molecular-weight heparin) usually can be given throughout the pregnancy. Oral anticoagulants can be started immediately after the baby is born.

Prevention

Preventive measures are used before and after any procedure or event that increases your risk of deep vein thrombosis. These measures include:

• Taking anticoagulants to prevent a blood clot that can develop after some types of surgery. They are also used in people who have a high risk of developing blood clots, such as those who have had previous clots.
• Exercising your lower leg muscles to improve circulation in your legs. Point your toes up toward your head so that the calves of your legs are stretched, then relax. Repeat. This exercise is especially important to do when you are sitting for long periods of time.
• Getting up out of bed as soon as possible after an illness or surgery. It is very important to get moving as soon as you are able. If you cannot get out of bed, do the leg exercise described above every hour to keep the blood moving through your legs.
• Using special stockings called compression stockings to help prevent deep vein thrombosis if you are at an increased risk.

Long airplane flights pose an increased risk for deep vein thrombosis, even for those who may not normally be at risk. Many doctors recommend that you wear compression stockings during a journey longer than 8 hours. Also, when on long flights walk up and down the aisle hourly, flex and point your feet every 20 minutes while sitting, drink plenty of water-a large glass every 2 hours-and avoid alcohol and beverages with caffeine, which can dehydrate you and increase your risk for clots.

If you are already at high risk for deep vein thrombosis, talk to your doctor before taking a long flight.

Intermittent pneumatic compression (IPC) devices are also used to prevent deep vein thrombosis. These devices alternately inflate and deflate knee-high boots, which results in decreased pooling of blood in the legs. IPC pumps are often used when people stay in a hospital.

Home Treatment

Home treatment for deep vein thrombosis focuses on safety while taking anticoagulants because of the increased risk for bleeding.

Anticoagulant treatment for a first episode of deep vein thrombosis usually lasts for 3 to 6 months. While you are being treated with anticoagulants, you need to:

• Take your anticoagulants as directed.
• Monitor your response to the medicines as your doctor recommends. This may require lab work once or twice each week at first and every 2 to 4 weeks later on.
• Avoid activities that pose a risk of serious injury, including activities at work.
• Manage your diet properly. Don't suddenly change your intake of foods that are rich in vitamin K. Vitamin K can interfere with the action of anticoagulants, making it more likely that your blood will clot. For more information, see:

  Anticoagulants: Vitamin K and your diet.

• Consult your doctor before taking any other medications, including aspirin or nonsteroidal anti-inflammatory drugs (NSAIDs), or discontinuing or changing the dose of any medication you are currently taking.
• If you drink alcohol, use it only in moderation.

For more information, see:

Deep vein thrombosis: Taking anticoagulants safely.

Elevating your leg or wearing compression stockings may also help reduce the pain and swelling that can occur with deep vein thrombosis.

Use a heating pad on the affected leg for 20 to 30 minutes 3 to 4 times daily. Your doctor may recommend that you take walks 5 or 6 times a day, if possible. These measures may help reduce the pain and swelling that can occur with deep vein thrombosis.

Medications

Anticoagulant medicines are the main form of treatment for deep vein thrombosis. Anticoagulants affect the way blood clots in the body.

Medication Choices

Anticoagulants

Anticoagulants can prevent new clots from forming and prevent existing clots from getting larger; however, they do not break up or dissolve existing blood clots.

Anticoagulants are used to:

• Treat existing deep vein thrombosis.
• Prevent a blood clot that can develop after some types of surgery.
• Prevent blood clots in people who are at high risk (such as those who have had previous clots).

Anticoagulants that are used to prevent and treat deep vein thrombosis include:

• Heparin. The two types of heparin are:
  • Low-molecular-weight heparin (LMWH). This type can be self-injected at home, which is more convenient.
  • Unfractionated heparin (UH). This type is given through a vein (IV) or through an injection under the skin. UH typically requires regular monitoring and is usually given in the hospital.
• Warfarin (such as Coumadin), an oral anticoagulant.

Heparin acts immediately, while warfarin takes several days to become effective. Heparin will be discontinued when warfarin is at a therapeutic level.

Low-molecular-weight heparin (LMWH) and unfractionated heparin (UH) are both effective at treating deep vein thrombosis. LMWH is typically preferred over UH, because LMWH can be given at home and typically does not require monitoring with blood tests.

The ideal length of time to continue treatment with an oral anticoagulant varies and is still being researched. In general, treatment of a blood clot with oral anticoagulant medicines will continue for about 3 to 6 months. The length of time will vary based on your own health.

• If you have a short-term risk of deep vein thrombosis because of surgery, treatment with oral anticoagulant medicines usually lasts a shorter period of time.
• If you have recurrent blood clots or continuing risk factors (such as cancer), anticoagulant treatment usually continues as long as those risk factors are present, which could be for the rest of your life.
• If you have inherited blood-clotting disorders, you may need oral anticoagulants indefinitely.⁶

Studies show that proper anticoagulant therapy reduces the rate of recurrent blood clots from 25% to less then 5% in the first 3 months.⁸ When used to initially treat deep leg vein thrombosis, heparin reduces the risk of developing deep vein blood clots and fatal blood clots in the lungs (pulmonary embolism) by 60% to 70%.⁹ Some people may take low-molecular-weight heparin (LMWH) long-term instead of warfarin.

After your initial treatment with warfarin, your doctor may recommend that you take warfarin on an ongoing basis to prevent deep vein clots from recurring.²

If you take warfarin, don't suddenly change your intake of foods that are rich in vitamin K. Vitamin K can interfere with the action of anticoagulants, making it more likely that your blood will clot. For more information, see:

Anticoagulants: Vitamin K and your diet.

What To Think About

Aspirin may help prevent blood clots and reduce the risk of pulmonary embolism. But experts do not agree on how well aspirin works for preventing pulmonary embolism.

Thrombolytics are sometimes used in certain situations to treat deep vein thrombosis. But thrombolytics have a high risk of causing bleeding. They may be used if you have problems when you take heparin.

People with other illnesses such as liver or kidney problems, a recent stroke, recent surgery, inherited bleeding disorders, a bleeding ulcer, or other internal bleeding may not be able to take anticoagulants or thrombolytic medications.

Pregnant women with deep vein thrombosis should not use warfarin. Only unfractionated heparin or low-molecular-weight heparin should be given.

Surgery

Surgical removal of a blood clot resulting from deep vein thrombosis is usually considered only in rare cases where the clot is very large and blocking a major blood vessel, causing severe symptoms. Surgery increases the risk of forming new blood clots.

Other Treatment

Vena cava filters are used for some people with deep vein thrombosis who have bleeding disorders or other illnesses (including some forms of cancer or a recent bleeding ulcer) and cannot take anticoagulant medicines. This filter can prevent blood clots from traveling to the lungs (pulmonary embolism). But the filter does not stop a clot from forming.

Vena cava filters may also be used if you:

• Continue to develop blood clots even though you have been taking anticoagulant medicines.
• Have recurrent blood clots and pulmonary hypertension.
• Cannot take anticoagulant medicine (such as when you have a bleeding disorder, some forms of cancer, or a recent bleeding ulcer).

Compression stockings can also help relieve symptoms of deep vein thrombosis. A recent study showed that these stockings can cut your chances of developing postthrombotic syndrome nearly in half.⁵

References

Citations

1. Turpie AG, et al. (2002). Venous thromboembolism: Treatment strategies. BMJ, 325(7370): 948-951.

2. Ridker PM, et al. (2003). Long-term, low-intensity warfarin therapy for the prevention of recurrent venous thromboembolism. New England Journal of Medicine, 348(15): 1425-1434.

3. Hyers TM (2003). Management of venous thromboembolism: Past, present, and future. Archives of Internal Medicine, 163(7): 759-768.

4. Kearon C, Hirsh J (2007). Venous thromboembolism. In DC Dale, DD Federman, eds., ACP Medicine, section 1, chap. 18. New York: WebMD.

5. Prandoni P, et al. (2004). Below-knee elastic compression stockings to prevent the post-thrombotic syndrome. Annals of Internal Medicine, 141(4): 249-256.

6. Seligsohn U, Lubetsky A (2001). Genetic susceptibility to venous thrombosis. New England Journal of Medicine, 344(16): 1222-1231.

7. Van Dongen CJJ, et al. (2007). Fixed-dose subcutaneous low-molecular-weight heparins versus adjusted-dose unfractionated heparin for venous thromboembolism. Cochrane Database of Systematic Reviews (2).

8. Raskob GE, et al. (2006). Venous thrombosis. In MA Lichtman et al., eds., Williams Hematology, 7th ed., pp. 2055-2065. New York: McGraw-Hill.

9. Hirsh J, et al. (2001). Guide to anticoagulation therapy: Heparin. A statement for healthcare professionals from the American Heart Association. Circulation, 103(24): 2994-3018.

Other Works Consulted

• Hirsch J, et al. (2008). Executive summary: American College of Chest Physicians evidence-based clinical practice guidelines (8th ed.). Chest, 133(6): 71-109.

• Snow V, et al. (2007). Management of venous thromboembolism: A clinical practice guideline from the American College of Physicians and the American Academy of Family Physicians. Annals of Internal Medicine, 146(3): 204-210.

• Tapson VF, Becker RC (2007). Venous thromboembolism. In EJ Topol, ed., Textbook of Cardiovascular Medicine, 3rd ed., pp. 1569-1584. Philadelphia: Lippincott Williams and Wilkins.

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