Chapter 37 Thrombosis and embolism
Insufficiency of blood flow causing tissue ischaemia is one of the most common disease mechanisms encountered in pathology (see Chs 9 and 10). This chapter concentrates on two of the most important causes: thrombosis and embolism.
A blood clot produced in a blood vessel is called a thrombus. It depends on interactions between platelets, the clotting factors of the plasma and the vascular endothelium. Thrombi that form in flowing blood are characterized by the build-up of alternating layers of platelets and fibrin; the result is alternating pale and dark red bands, called lines of Zahn.
There are three main circumstances that predispose to thrombosis. They are known as Virchow’s triad:
Alterations in the flow of blood include turbulence, which can damage the endothelium, or stagnation, which promotes activation of the clotting cascade. Changes in the vessel wall include anything altering endothelial function (e.g. inflammation) and anatomical abnormalities that could induce turbulent flow. Changes in the constituents of the blood include alterations in viscosity, which tend to promote sludging and stagnation, and abnormalities of the proteins involved in thrombosis and thrombolysis. The last produces a ‘hypercoagulable state’; causes include increased oestrogen (pregnancy or contraceptive pill), release of procoagulant factors following surgery or other trauma, the presence of cancer, and inherited conditions such as deficiency in antithrombin III, protein C or protein S.
Thrombi can occur in the chambers of the heart, in arteries or in veins. In the heart, thrombosis can occur on the wall underlying an infarct, in fibrillating atria, within ventricular aneurysms and as a constituent of vegetations. Atherosclerosis is the commonest cause in arteries. In veins, most thrombi probably start in the vicinity of the valves, where turbulent flow occurs. When a vein becomes thrombosed it usually becomes inflamed (thrombophlebitis). This should be distinguished from thrombosis in a vein that is already inflamed for some other reason.
Deep venous thrombosis (DVT) is the formation of thrombi in the deep veins of the lower limb and pelvis. It is a common clinical problem, and its importance lies in the complication of pulmonary embolism (see below) that can follow. This is a feared complication of major surgery, since postoperative patients are at increased risk of DVT. Therefore, postoperative patients receive prophylaxis to reduce the risk of thrombosis. The factors that contribute to DVT in the postoperative patient can be understood by applying the principles of Virchow’s triad: