Prophylaxis of pulmonary embolism.

The three factors which catalyse thrombus formation and intravascular coagulation are changes in the rate of blood flow, changes in the vessel wall, and changes in the nature of the blood. These factors, which were enumerated over a hundred years ago by Rudolph Virchow,47 are still our main guide lines. The various causes of changes in blood flow are well known. The antithrombotic properties of the vein wall may be altered by local trauma, and intimal damage is positively thrombogenic. The changes in the blood include changes of viscosity, fibrinogen level, antifibrinolysis, platelet stickiness, and the concentration of the coagulation factors. All current forms of prophylaxis attempt to change one or other of these three supposedly important predisposing condi-