Deep vein thrombosis

by | Heart Attacks and Strokes

Deep vein thrombosis

Deep vein thrombosis (DVT) happens when blood clots in a vein deep within the body — usually in the calf, thigh or pelvis. This is dangerous because a clot (known medically as a thrombus) can break off and travel in the bloodstream, through the heart, before lodging in an artery in the lung. This type of ‘wandering’ clot is called an embolus; when it lodges in the lung it is known as a pulmonary embolus. If the clot is large, pulmonary embolism can be fatal.

What causes DVT?

DVT happens when the normal clotting process that stops bleeding from an injured blood vessel occurs when it shouldn’t.

There are several risk factors for DVT.


The risk of DVT increases as you get older.


Some operations have a high risk of DVT, especially major abdominal surgery and operations to replace large joints such as the hip and the knee.


People who are confined to bed for long periods (for example, after major surgery or injuries) are at an increased risk of DVT. Recently there has been a lot of attention on how long plane flights and car trips can increase the risk of DVT.

Injuries that disrupt blood vessels

Including injuries such as a broken leg.

Medical conditions that increase the risk of clotting

Examples include some types of cancer, heart failure, some autoimmune diseases and inherited conditions that make the blood clot easily.


Carrying excess weight increases the risk of blood clots.

Pregnancy and childbirth

Your blood is more likely to clot when you are pregnant than when you are not. This is because levels of clotting factors (proteins in the blood that help your blood to clot when necessary) are increased and because the growing uterus (womb) can disrupt blood flow in the veins. During childbirth, blood vessels may be injured; another factor that increases the risk of clotting. In the period immediately after childbirth the risk of DVT is even higher than it is in pregnancy; this is especially true after Caesarean section.

Some medicines

The combined (oestrogen and progesterone) oral contraceptive pill and combined hormone replacement therapy (also called hormone therapy) increase the risk of DVT. However, a DVT is more likely when a woman is pregnant than when she is on the contraceptive pill. The progesterone-only pill seems much less likely to cause DVT than the combined pill.


Smoking increases the risk of DVT.

Previous DVT

People who have had a DVT or pulmonary embolism have a higher risk of having another DVT than people with no history of DVT.

DVT diagnosis

DVT doesn’t always cause symptoms. Symptoms and signs are more likely if the clot extends further up the leg than the calf — this can cause swelling and pain of the leg. The area may feel tender or warm and look red, and the person may have a mild fever.

If a doctor suspects a DVT, he or she will ask you questions and examine you. To investigate further, you will usually have a special kind of ultrasound to examine the blood flow in your veins, and possibly a blood test to look for a substance produced when the body tries to break down clots. Occasionally these tests don’t provide enough information and you will need a procedure called a venogram, in which the doctor injects a dye into your veins to visualise their flow.

Treatment of DVT

If you have a DVT, the main aim of treatment is to prevent a pulmonary embolus. You will usually be given an anti-clotting medicine (anticoagulant), initially as injections — such as enoxaparin (Clexane), dalteparin (Fragmin), fondaparinux (Arixtra) or heparin — then later as tablets — such as warfarin (Coumadin, Marevan). You’ll probably have quite a few blood tests to make sure your blood is adequately ‘thinned’. If you have pain, you’ll also be given painkillers.

In some severe cases, another medicine is given through a vein to dissolve the clot. This process is called thrombolysis and the medicines used are popularly known as ‘clot busters’ — such as streptokinase (Streptase). Although it can break down clots, thrombolysis can also cause bleeding and is not suitable in some situations. Also in severe cases, removing the clot surgically may be recommended.

Another strategy is the insertion of a filter into the main vein carrying blood back from the lower body to the heart (the inferior vena cava) — this is used to stop any clots that dislodge from reaching the lungs. Generally this type of filter is inserted only if you are unable to take anticoagulants or if the anticoagulants aren’t preventing further DVTs.

You may also be given special compression stockings to wear — these can relieve pain and swelling in the leg and reduce the risk of problems with your leg after a DVT. Compression stockings should be worn for up to 18 months after having a DVT.

How long you will need to be treated with anticoagulant medicines will depend on your particular case, including any risk factors you have for developing another DVT. The typical time is 3 to 6 months, but life-long therapy may be advised in some cases. Your doctor can advise you on the risks and benefits of long-term anticoagulant treatment, so that together you can make the best decision for your particular circumstances.

Prevention is better than cure

To reduce the risk of DVT happening in the first place, doctors generally give anticoagulants to patients in hospital who are thought to be at risk of DVT, such as those having major operations. You may also be advised to wear compression stockings.

If you are taking a long haul flight, the following measures have been recommended:

  • walk around the cabin frequently;
  • regularly move your ankles and massage and stretch your calves;
  • wear loose, non-restrictive clothing;
  • sit in an aisle seat, if possible;
  • drink plenty of non-alcoholic fluids during the flight; and
  • look out for pain in the calves during and up to a month after a long flight — if you are worried, seek medical advice as soon as possible.

The Cochrane collaboration (an international organisation aimed at helping people make well-informed decisions about healthcare by undertaking scientific reviews of evidence for certain treatments) recently looked at whether wearing compression stockings during long flights helped reduce the risk of DVT. It found that wearing these stockings can help reduce the risk of DVT.

If you are planning a long trip and have any other risk factors for DVT, it is a good idea to seek medical advice — some doctors recommend a single dose of anticoagulant or compression stockings for people with other risk factors for DVT who are flying for 4 hours or longer.

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