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 (SVT) and in deep veins. Blood clots with inflammation
in superficial veins (called superficial thrombophlebitis or phlebitis) rarely
cause serious problems. Blood clots within superficial veins are called superficial thrombosis. But clots in deep veins (deep vein thrombosis) require
immediate medical care. 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 an acute condition and is often
life-threatening - just as threatening as a major heart attack! DVT can also lead to chronic or long-lasting problems in the lower legs called the "post-phlebetic syndrome". A DVT may damage
the vein and cause the leg to ache, swell, and change color (hyperpigmentation - which is irreversible). It can also lead
to leg sores or ulceration (venous stasis ulceration) many after years of having a DVT. This is why seeing a specialist such as Dr Balder becomes so important - in both the short and long term.
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. Blood clots which occur in the upper extremities are almost always a function of a foriegn body in a vein - such as an IV or hemodyalysis access device - "line".
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 and pain 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 with shortness of breath and chest pain or air hunger may be 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 ultrasoundtest
to measure the blood flow through your veins and help find any clots that might
be blocking the flow. This is a simple non invasive test which can be done in the Advanced Surgical Associates / Advanced Vein office by Dr Balder or his RVT - Scott almost any day - even on a walk in basis. An MRI (MRV) can often be helpful as well. 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, this is rarely done today.
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 or more commonly today - low molecular weight heparin (lovenox) and warfarin (Coumadin, for example). Heparin is given through
a vein (intravenously, or IV) or as an injection. Low molecular weight heparin (lovenox) is given as a single or twice a day dose subcutaneoulsy (just like insulin thru a small needle just under the skin). Warfarin is given as a pill, typically each evening.
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, and may decrease the long term leg problems by 50%.
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
Deep Vein Thrombosis -
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.
Deep Vein Thrombosis -
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).
Deep Vein Thrombosis -
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
Deep Vein Thrombosis -
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, andstroke.
·
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.
Deep Vein Thrombosis -
When To Call a Doctor
Call911or 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.
Deep Vein Thrombosis -
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. (non specific)
·
Magnetic
resonance imaging (MRI). / MRV
·
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,
aspiral CT scan,
or a pulmonary
angiogram. For more information, see the topicPulmonary
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.
Deep Vein Thrombosis -
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 andwarfarin (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) was previously the treatment of choice in the hospital. Low-molecular-weight heparin (LMWH, ie. lovenox) can be self-injected in the hospital and 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.71
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.1Some 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. Generally your primary medical doctor will check your blood
regularly and may need to adjust the dose of your medicine. If you do not have a PCP, Dr Balder can help you with coordination of your "blood levels" (PT, INR)
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 medical compression stockings, they are sometimes
recommended to help relieve swelling and pain. We have these readily avaliable in the office at Advanced Surgical with a multitude of colors, types and sizes for your immediate needs. 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.
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.
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:
o
Low-molecular-weight
heparin (LMWH). This
type can be self-injected at home, which is more convenient.
o
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%.9Some 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:
Deep Vein Thrombosis -
Surgery
Surgical removal of a blood clot resulting from deep vein thrombosisis 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.
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 developingpostthrombotic
syndrome nearly in
half.5
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.