II. PATHOGENESIS OF DEEP VEIN THROMBOSIS
A. Immobilization
Thrombi form in veins of the lower limbs and pelvis because of certain factors that increase the tendency of free-flowing blood to clot. A sudden increase in the tendency of the blood to clot commonly occurs soon after surgical operations or fractures that immobilize the lower limbs. Sudden immobilization for more than 48 h causes the blood to flow very slowly through the limb that is immo¬bilized. This slow-flowing (stasis) blood through veins in the extremities tends to form ‘‘sludge’’ and clots. This is similar to very slow-flowing or stagnant water in a stream that forms moss on rocks. Immobilization, therefore, explains the high incidence of clots in veins after surgical operations that involve the lower limbs versus surgery of the chest and upper regions of the body where the lower limbs are free to move.
B. Coagulation Factors
Soon after surgery or the birth of a baby small particles in the blood called platelets become sticky and clump together. This together with other substances called coag¬ulation factors in the blood orchestrate the clot; once a reaction is initiated there is a cascade of processes that lead to the formation of a firm, fibrin clot.
Damage to veins by traumatic injury and infections also predispose to clot formation.