Pathophysiology

Specifically, a thrombus is the inappropriate activation of the hemostatic process in an uninjured or slightly injured vessel. A thrombus in a large blood vessel (mural thrombus) will decrease blood flow through that vessel. In a small blood vessel (occlusive thrombus), blood flow may be completely cut-off resulting in death of tissue supplied by that vessel. If a thrombus dislodges and becomes free-floating, it is termed as an embolus.

Some of the conditions which elevate risk of blood clots developing include atrial fibrillation (a form of cardiac arrhythmia), heart valve replacement, a recent heart attack, extended periods of inactivity (see deep venous thrombosis), and genetic or disease-related deficiencies in the blood's clotting abilities.

Treatment

Blood clot prevention and treatment reduces the risk of stroke, heart attack and pulmonary embolism. Heparin and warfarin are often used to inhibit the formation and growth of existing thrombi; they are able to decrease blood coagulation by inhibiting vitamin K epoxide reductase, an enzyme needed to form mature clotting factors.

Presentation

Virchow's triad describes the pathogenesis of thrombus formation:

1. Endothelial injury (e.g. trauma, atheroma)
2. Abnormal blood flow (loss of laminar flow resulting from stasis in veins or turbulence in arteries) (e.g. valvulitis, aneurysm)
3. Hypercoagulability (e.g. leukaemia, Factor V mutation (Leiden))

Disseminated intravascular coagulation (DIC) involves widespread microthrombi formation throughout the majority of the blood vessels. This is due to excessive consumption of coagulation factors and subsequent activation of fibrinolysis using all of the body's available platelets and clotting factors. The end result is hemorrhaging and ischaemic necrosis of tissue/organs. Causes are septicaemia, acute leukaemia, shock, snake bites, fat emboli from broken bones, or other severe traumas. Treatment involves the use of fresh frozen plasma to restore the level of clotting factors in the blood, platelets and heparin to prevent further thrombi formation.

Prognosis

Thrombus formation can have one of four outcomes: propagation, embolisaztion, dissolution or organization and recannalization.

1. Propagation of a thrombus occurs towards the direction of the heart. This means that it is anterograde in veins or retrograde in ateries.
2. Embolization occurs when the thrombus breaks free from the vascular wall and becomes mobile. A venous emboli (most likely from deep venous thrombisis in the lower extremities) will travel through the systemic circulation, reach the right heart, and travel through the pulmonary artery resulting in a pulmonary embolism. On the other hand, arterial thrombosis resulting from hypertension or atherosclerosis can become mobile and the resulting emboli can occlude any artery or arteriole downstream of the thrombus formation. This means that cerebral stroke, myocardial infarction, or any other organ can be affected.
3. Dissolution occurs when fibrinolytic mechanisms break up the thrombus and blood flow is restored to the vessel. This may be aided by drugs (for example after occlusion of a coronary artery). The best response to fibrinolytic drugs is within a couple of hours before the fibrin meshwork of the thrombus has been fully developed.
4. Organization and recanalization involves the ingrowth of smooth muscle cells, fibroblast and endothelium into the fibrin-rich thrombus. Recanalization provides capillary sized channels through the thrombus for continuity of blood flow through the entire thrombus but may not restore sufficient blood flow for the metabolic needs of the downstream tissue.