Clotting and fibrinolysis.

Vascular disease accounts for more than 50% of deaths and in most cases the cause lies in obstruction to blood flow by the twin perils of atheroma formation and thrombosis. The major constituents of thrombi are platelets and fibrin, together with trapped red cells and other elements. Our main antithrombotic drugs are the antiplatelet agents to prevent platelets sticking together; anticoagulants to prevent fibrin formation; and fibrinolytic agents which, hopefully, will lyse and remove formed fibrin deposits. Antithrombotic therapy may also conveniently be classified as (1) prophylactic, to cover an event, such as a surgical operation, known to be associated with an increased thrombotic risk, and (2) therapeutic, to treat a thrombus which has already formed. It must also be remembered that the problems of arterial and venous thrombosis are different. Arterial thrombi are composed largely of aggregated platelets, together with fibrin; it is thought that platelet adhesion to the vessel wall is an important event which may initiate arterial thrombosis. Conversely, thrombi formed in veins consist mainly of fibrin and it is in this situation where the conventional anticoagulants are likely to be useful.