ABC of Vascular Disease

Deep Vein Thrombosis

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1. What is a Deep Vein Thrombosis?
A deep vein thrombosis or DVT is a blood clot that develops inside the veins that lie deep within the muscles of the leg.

2. What causes a DVT?
The blood contains molecules that will naturally cause the blood to clot and become solid.  This is called a thrombosis.  When blood is moving through the normal veins, these molecules are unable to act and the blood stays fluid.  Damage to the inside "non stick" lining of the vein causes these molecules to become activated very quickly and a thrombosis of the vein will result within a few minutes.  This is a normal survival mechanism to prevent major loss of blood even from a small cut.  However, if the blood is allowed to stagnate in a normal, undamaged vein for long enough, a blood clot can start to form.  Blood is moved through the deep veins in the leg mainly by the action of the muscles that surround the veins.  If the person is completely immobile for several hours (e.g. during an operation) then a blood clot can start to form.  Small clots are quite common and normally the body dissolves the clot once the person becomes active again.  However, if the clot is too extensive, then the body may not be able to dissolve it and an established deep vein thrombosis develops.  The third factor that determines if a clot will form is the "stickiness" of the blood.  People with a history of recent childbirth, illness, trauma or surgery have "stickier" blood and are more prone to form clots.  Some people are born with "stickier" blood and a predisposition to develop blood clots.  One of the ways to prevent unwanted blood clots is to take drugs that reduce the ability of the blood to clot.  These drugs are called anticoagulants.

3. How would I know if I had a DVT?
The extent of a deep vein thrombosis may vary from a minor, asymptomatic clot, to complete occlusion of the major deep veins of the leg and pelvis which causes painful swelling of the leg.  Sometimes a DVT can be quite extensive but completely asymptomatic until a complication occurs.

4. What complications can a DVT cause?
Most minor deep venous thromboses are asymptomatic do not cause any complications. Occasionally a deep vein thrombosis may lead to one of a number of potentially disabling and life threatening complications; pulmonary embolus (PE), phlegmasia (pronounced fleg-may-see-ah) and phlebitic (pronounced fleh-bih-tick) syndrome.

5.  What is a pulmonary embolus?
If a large blood clot in a deep vein in the leg dislodges it can be carried in the veins (called an embolus) to the heart and then into the arteries to the lungs (the pulmonary arteries) where it lodges and reduces the blood flow to the lungs.  This event may occur without warning and cause sudden collapse and even death!  Less severe cases cause shortness of breath and chest pain, even less severe cases may be asymptomatic.

6. What does phlegmasia mean?
Phlegmasia is where the deep vein thrombosis blocks the major veins and the leg swells and becomes blue and painful.  Unless treated quickly, this condition may lead to collapse and potentially loss of the leg through gangrene.  In less severe cases the deep veins may be permanently damaged resulting in phlebitic syndrome and long term disability.

7. What is phlebitic syndrome?
If the deep veins of the leg are damaged or blocked by blood clots, then the normal function of the veins in the leg is impaired and over years this can results in swelling, pain, skin damage and leg ulcers.

8. How likely am I to get a DVT after surgery?
This depends on the the patient, the type of illness and the type of surgery.  Individuals can be roughly grouped into low, medium and high risk of DVT.  The higher the risk the more likely a DVT will occur.  It is impossible to predict which patients with a DVT will get a PE so it is safer to attempt to prevent all DVT's using prophylactic measures.

9. What is DVT prophylaxis?
This is the principle that it is better to prevent a DVT than to cure one.  Knowing how DVT's develop there are two main methods of preventing them:
 

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Maintaining blood flow in the deep veins
The blood flow can be maintained by wearing compression socks that force the blood to flow in the deep veins.  This can be further improved by inflatable socks that actually squeeze the leg at intervals to simulate the pumping action of the muscle.  Such socks are often used during the operation.

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Reducing the stickiness of the blood
Clearly, any operation that requires an incision will produce some bleeding and it is important that the blood clotting mechanism is not disabled.  All that is required is a partial reduction in the stickiness of the blood and this is achieved with small injections under the skin of a substance called heparin. Heparin is a naturally occurring anticoagulant that reduces the stickiness of the blood very quickly but only for a few hours.  The effect is proportional to the amount given.

10. How is a DVT diagnosed?
There are two main ways to prove that a DVT is present. A blood test can be done which measures if the body is actively breaking down a blood clot.  If the test is positive it is evidence that a thrombosis is present somewhere.  The other test is an ultrasound scan of the veins of the leg.  The operator will be able to "see" the clots in the veins and show that the blood flow is reduced by the thrombosis.

11. How is a DVT treated?
This depends on the extent and severity of the DVT.  Most DVT's are small and are treated as an outpatient with support socks, a course of heparin injections and regular exercise.  This treatment is designed to prevent the DVT from worsening and to assist the bodies normal mechanism that dissolves the clot that is already present.  If the DVT is more extensive then it may be necessary to be admitted to hospital for bed rest, elevation of the affected leg, and an intravenous infusion of the heparin.  Once the problem has settled then it is usual to require a course of anticoagulant tablets for 3 to 6 months to prevent a relapse.  The most commonly used is a powerful drug called warfarin. Treatment with warfarin must be monitored carefully with regular blood tests to get the dose for the patient just right. Too much warfarin can lead to bleeding, not enough risks a further DVT.  For very severe DVT where virtually all the veins are clotted up, the patient is very ill and at risk of limb loss.  In these rare cases it may be necessary to attempt to dissolve the clot with infusions through special catheters using very powerful drugs.  This treatment can only be done in specialist centres.

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S.R.Dodds 2001

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