Formation of blood clot inside an artery or vein is called in medical terms as Thrombosis. In a healthy individual, blood is pumped by the heart and the oxygenated blood flows through the arteries to various organs. The deoxygenated blood containing other metabolites flows through the veins back to heart and then to the lungs and the cycle continues. The blood flow in the veins is slower compared to the arteries. The blood from the veins in the legs particularly has to flow against gravity. The action of the calf muscles helps to pump the blood through the veins and the presence of valves prevent any reflex so that the blood flows only in one direction.

There are two systems of veins in the legs — one superficial and the other deep. These are interconnected by communicating veins. The blood flows from the superficial system to deep systems and then to the larger veins in the pelvis.

Blood has a fluid component and cellular component. The cells are RBCs (Red Blood Cells), WBCs (White Blood Cells), and platelets. Under normal circumstances the cells are in the middle of the stream. The blood is kept in fluid form by a mechanism which prevents clotting, but whenever there is an injury, the clotting mechanism gets activated and the bleeding is stopped. There is a fine balance between these two systems which is actually quite complicated involving several proteins.

Blood clots can develop inside the artery or vein due to three main reasons:

*Injury to the vessel wall
*Stagnation of blood flow
*Abnormalities of the coagulation system

Veins are more prone to develop clots, particularly the deep veins of the leg. Whenever the clot forms it adheres to the vessel wall and may partially or totally block the vein. It may extend proximally or distally. As the clot extends proximally a portion of it may get detached. The loose clot (known as Embolus) may travel to the lungs to produce what is known as Pulmonary Embolism. Depending on the size of the clot and the vessel that is blocked the patient experience breathlessness or may even collapse and die.

If not treated adequately and promptly the clot in the vein gets organised and in the process, damages the valves. The blocked veins and damaged valves result in a condition known as post-thrombotic syndrome. Swelling, pigmentation and ulceration are features of this disease which may take months or years to develop.
There are several risk factors which predispose to the development of DVT and it is important for the clinician to identify these in any patient who is hospitalised. Age, prolonged immobilisation, advanced cancer, co-morbid conditions like diabetes, heart failure, trauma, etc, are some of the risk factors. Patients undergoing surgery are also at risk of developing DVT depending on the age of the patient, type of surgery, duration, etc. For example an elderly diabetic patient undergoing hip replacement has 60 per cent risk of DVT while for a young man having hernia repair the risk is less than 10 per cent. Long distance air travel is another risk factor. There have been several reports of patients developing deep venous thrombosis after a long journey by flight.

Certain prophylactic measures if taken after identifying those at moderate to high risk certainly reduces the incidence of DVT. Early mobilisation, elastic stockings and drugs like heparin are some of the measures recommended.

It is not often easy to diagnose deep vein thrombosis. A high degree of suspicion is required in any patient who complains of pain and swelling of the leg, particularly during the post operative period. Unlike in the past there are now sophisticated equipment like the duplex scan which has a high degree of accuracy in diagnosing venous thrombosis.

The author is a consultant vascular surgeon, Manipal Hospital, Bangalore