Embolism

Overview
In medicine, an embolism occurs when an object (the embolus, plural emboli) migrates from one part of the body (through circulation) and cause(s) a blockage (occlusion) of a blood vessel in another part of the body. The term was coined in 1848 by Rudolph Carl Virchow.

This can be contrasted with a "thrombus" which is the formation of a clot within a blood vessel, rather than being carried from somewhere else. Thrombus mainly form in the Arteries whereas Emboli form in the Veins.

Blood clots form the most common embolic material by far: other possible embolic materials include fat globules (a fat embolism), air bubbles (an air embolism), talc embolism (often following drug abuse), septic emboli (containing pus and bacteria), or amniotic fluid.

Emboli often have more serious consequences when they occur in the so-called "end-circulation": areas of the body that have no redundant blood supply, such as the brain, heart, and lungs.

Assuming a normal circulation, a thrombus or other embolus formed in a systemic vein will always impact in the lungs, after passing through the right side of the heart. This forms a pulmonary embolism that can be a complication of deep-vein thrombosis. Note that, contrary to popular belief, the most common site of origin of pulmonary emboli are the femoral veins, not the deep veins of the calf. Deep veins of the calf are the most common site of thrombi, not emboli origin.

Some congenital abnormalities of the circulation, especially septal defects (holes in the cardiac septum), allow an embolus from a systemic vein to cross into the arterial system and land anywhere in the body. The most common such abnormality is patent foramen ovale, occurring in about 25 % of the adult population, but here the defect functions as a valve which is normally closed, because pressure is slightly higher in the left side of the heart. In certain circumstances, e.g. if patient is coughing just when an embolus is passing, passage to the arterial system may occur.

Emboli starting in the heart (from a thrombus in the left atrium secondary to atrial fibrillation or septic emboli from endocarditis) can cause emboli in any part of the body.

An embolus landing in the brain from either the heart or a carotid artery will likely cause an ischemic stroke.

Emboli of cardiac origin are also frequently encountered in clinical practice. Thrombus formation within the atrium in valvular disease occurs mainly in patients with mitral valve disease, and especially in those with mitral valve stenosis with atrial fibrillation (AF). In the absence of AF, pure mitral regurgitation has low incidence of thromboembolism.

Absolute risk of emboli in idiopathic AF depends on other risk factors such as increasing age, hypertension, diabetes, recent heart failure, or previous stroke.

Thrombus formation can also take place within the ventricles, and it occurs in approximately 30% of anterior wall myocardial infarctions, compared to only 5% of inferior ones. Other risk factors include poor ejection fraction (<35%), size of infarct, as well as presence of AF. In the first three months after infarction, left ventricle aneurysms have 10% risk of embolization.

Patients with prosthetic valves also carry a significant increase in risk of thromboembolism. Risk varies on the valve type (bioprosthetic or mechanical), the positon (mitral or aortic), and presence of other factors such as AF, left ventricular dysfunction, previous emboli, etc.