What causes deep vein thrombosis (DVT)?
- Changes in the blood flow
- Changes in the blood constituents
- Changes in the wall of the blood vessel
When any of these occur by themselves or in combination, there is an increased risk of blood clotting within the vessel.
If blood clots within the vessel and sticks the wall, it is called a thrombus. If it dislodges and moves through the blood vessels with the blood flow, it is called an embolus.
From Virchow’s triad, the common causes of deep vein thrombosis (DVT) can be ascertained.
Q1 Changes in the blood flow increasing the risk of deep vein thrombosis (DVT)
Anything that reduces the speed of the blood flow in veins, or changes the flow from being smooth to turbulent, increases the risk of thrombosis.Lying in bed for long periods, such as when when ill in hospital, or sitting still for long periods of time, such as on a long aircraft journey (the so-called “economy class syndrome”) or on a long coach trip, reduces the venous flow in the deep veins to minimal and therefore increases the risk of deep vein thrombosis (DVT).
Venous reflux or venous incompetence, meaning that the valves have failed and therefore blood is falling the wrong way down the vein, has changed the direction of blood flow and also increases the risk of thrombus within the veins. If this is in the deep venous system, then a deep vein thrombosis (DVT) is more likely.
Q2 Changes in the blood constituents increasing the risk of deep vein thrombosis (DVT)
When the blood chemistry itself changes, there is a risk that the blood will become more “sticky” which also increases the risk of deep vein thrombosis (DVT).The two most common factors that change blood chemistry in this way are smoking and the oral contraceptive pill.
- Smoking increases a protein called fibrinogen which makes the blood thicker and more syrupy, and is also a precursor to fibrin which is the main constituent protein of a thrombus.
- The oral contraceptive pill changes the blood chemistry which similarly increases the risk of thrombosis.
Interestingly, women who are both smokers and who are on the oral contraceptive pill do not have an addition of these risk factors, but research has shown that the risks multiply. As such, the combination of both smoking and being on the pill increases the risks of the deep vein thrombosis very significantly.Another major factor that changes blood constituents is dehydration. This is a major factor in patients who are starved for 4-6 hours before a general anaesthetic or who have a very high temperature from illness or who are in very hot climates such as when on holiday.
In some patients, deep vein thrombosis (DVTs) can run in families. In these patients the blood constituents are abnormal due to a genetic problem called “thrombophilia” – literally a “love of thrombus”. Thrombophilia occurs when certain proteins or other substances are produced genetically, which increase the likelihood of venous thrombosis. Although many of these can be measured by blood tests, there are even more that are currently unknown and thus there is no test for them.
Other patients can develop deep vein thrombosis’ (DVT) if they have a disease that affects the whole of their body (called a systemic disease). Deep vein thrombosis’ (DVTs) are more common in people with certain cancers including pancreatic cancer. The sudden onset of multiple deep vein thrombosis’ (DVTs) in someone who is otherwise healthy, has no known cause for deep vein thrombosis (DVT) and no family history of deep vein thrombosis (DVT) should probably be investigated for a hidden cancer (known as an “occult malignancy”).
Q3 Changes in the blood vessel wall, increasing the risk of deep vein thrombosis (DVT)
The walls of the deep veins are specifically adapted to allow blood to flow through them without clotting. The inner lining of the vein wall is covered by a layer of cells called endothelial cells.Endothelial cells have special properties that allow them to form a very smooth surface and also to secrete certain chemicals that prevent blood clotting and thrombus formation.
When endothelial cells are damaged, the protein of the vein wall underlying the endothelial cells is exposed to blood, which starts a clot forming on this bare protein.
Therefore anything that damages the endothelium can cause deep vein thrombosis (DVT).
One of the commonest things to damage endothelium is smoking. Cigarette smoke contains hydrogen cyanide and carbon monoxide, both of which damage endothelial cells. In addition, tiny particles of smoke can pass across the lungs and into the blood vessels. There has been research suggesting that mechanical damage can be caused by these particles impacting on the endothelial cells themselves.
Any trauma to the vein wall, whether from an accident or from a medical intervention such as surgery, can also damage the vein wall increasing the risk of deep vein thrombosis.