There are 3 factors that are known to affect clot formation; these are referred to as Virchow’s Triad
· Rate of flow
· The consistency (thickness) of the blood.
· Qualities of the vessel wall.
Blood normally flows quickly through veins, and does not usually clot. Blood flow in leg veins is helped along by leg movements, because muscle action squeezes the veins.
Occasionally a DVT occurs for no apparent reason. However, the following can increase the risk of having a DVT.
· Immobility, which causes blood flow in the veins to be slow. Slow flowing blood is more likely to clot than normal flowing blood.
· A surgical operation that lasts more than 30 minutes is the most common cause of a DVT. The legs become still when you are under anaesthetic. Blood flow in the leg veins can become very slow
· Any illness or injury that causes immobility increases the risk of a DVT.
· Long journeys by plane, train or car may cause a slightly increased risk of DVT.
· A Recent French study has shown that journeys more than five hours long by car, plane or train increase the risk of DVT fourfold.
· There is particular risk with air travel because of the combination of inactivity and dehydration, which makes the blood more sticky.
· A recent study in The Lancet showed that as many as one in ten passengers could suffer DVT unless they wore the special compression stockings.
· Damage to the inside lining of the vein increases the risk of a blood clot forming. For example, a DVT may damage the lining of the vein. So, if you have a DVT, then you have a higher than average risk of having another one sometime in the future.
· Some conditions such as vasculitis (inflammation of the vein wall) and some drugs (for example, some chemotherapy drugs) can damage the vein and increase the risk of having a DVT.
· Conditions that cause the blood to clot more easily than normal (thrombophilia) can increase the risk of having a DVT.
· Some medical conditions can cause the blood to clot more easily than usual. For example, nephrotic syndrome and antiphospholipid syndrome.
· We are currently aware of five genetic hereditary conditions, which increase the likelihood of getting a DVT, this however does not mean that if you do not test positive for one of these conditions you do have not inherited a family history of DVT. The five conditions are:-
· Factor V Leiden
· Prothrombin Mutation
· Protein C deficiency
· Protein S deficiency
· Antithrombin deficiency
· The combined contraceptive pill that contains oestrogen can cause the blood to clot slightly more easily.
· Women taking the combined contraceptive pill are three or four times more likely to suffer a DVT than those who don't.
· Hormone replacement therapy (HRT) that contains oestrogen can cause the blood to clot slightly more easily, but for many women, the other benefits outweigh the increase in risk of DVT.
· People with cancer or heart failure have an increased risk of having a DVT.
· Older people are more likely to have a DVT, particularly if you have poor mobility or have a serious illness such as cancer.
· DVTs are rare in young people, usually occurring in people aged over 40.
· Pregnancy increases the risk. About 1 in 1000 pregnant women have a DVT.
· Obesity also increases the risk of having a DVT.
- Sitting for Long Periods in the same position, for instance at a desk, can also increase your risk of DVT.