Science Fair Project Encyclopedia
Deep venous thrombosis
- DVT can also refer to Driving Van Trailer
Deep venous thrombosis (or DVT) is the occlusion of a deep vein by a blood clot ("thrombus"). It generally affects the leg veins, such as the femoral vein or the popliteal vein, or occasionally the veins of the arm ("Paget-von Schroetter syndrome"). See also: thrombophlebitis, the more general class of pathologies of this kind.
Signs and symptoms
Classical symptoms of DVT include pain, swelling and redness of the leg, starting with the calf and progressing upwards. In many patients, the symptoms are more insidious. In up to 25% of all hospitalised patients, there may be some form of DVT, which often remains clinically inapparent (unless pulmonary embolism develops).
There are several techniques during physical examination to increase the a priori likelihood of DVT. These include measuring the circumference of the affected and the contralateral limb at a fixed point (to objectivate oedema), and palpating the venous tract, which is often tender.
A careful history has to be taken considering risk factors (see below), including the use of the oral contraceptive pill, recent long-haul flying, a history of miscarriage (which is a feature of several disorders that can also cause thrombosis). A family history can reveal a hereditary factor in the development of DVT.
In a low-probability situation, current practice is to commence investigations by testing for D-dimer levels. This fibrin degradation product is an indication that thrombosis is occurring, and that the blood clot is being dissolved by plasmin. A low D dimer level should prompt other possible diagnoses (such as a ruptured Baker's cyst, if this has not been considered as part of the history).
Other blood tests usually performed at this point are:
- full blood count
- coagulation studies: PT, APTT, thrombin time , INR
- liver enzymes
- renal function and electrolytes
In cases of higher suspicion, compression ultrasound scanning of the leg veins, combined with duplex measurements (to determine blood flow), can reveal a blood clot and the extent of it (i.e. whether it is below or above the knee).
When all the above remains inconclusive, the gold standard is intravenous venography, which involves injecting a peripheral vein of the affected limb with a contrast agent and taking X-rays, to reveal whether the venous supply has been obstructed. Impedance plethysmography can also be used as a non-invasive alternative.
Generally, patients are initiated on heparin treatment while they start on a 3- to 6-month course of warfarin (or related vitamin K inhibitors). Often, low molecular weight heparin is substituted for the regular heparin. In patients who have had recurrent DVTs (two or more), anticoagulation is generally "life-long".
Main article: thrombosis
Thrombosis is a multifactorial process, caused by the nature of blood flow, the consistency of the blood, and qualities of the vessel wall (Virchow's triad). Amongst the plethora of risk factors, immobilisation, female gender, use of oral contraceptives and air travel ("economy class syndrome") are some of the better-known causes. Thrombophilia (tendency to develop thrombosis) often expresses itself with recurrent thromboses.
It is recognised that thrombi usually develop first in the calf veins, "growing" in the direction of flow of the vein. DVTs are distinguished as being above or below the popliteal vein. Very extensive DVTs can extend into the iliac vein or the inferior vena cava. The risk of pulmonary embolism is higher in more extensive clots.
DVT's occur in about 1 per 1000 persons per year. About 1-5% will die from the complications (i.e. pulmonary embolism).
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