Embolism - arterial and venous; types, sites of origin, target organs
An embolism is a detached mass that is carried through the blood to a distant site. More often than not, this is a thrombus that has become detached and then gotten lodged somewhere in the vessels which can go on to form tissue ischemic necrosis.
Arterial Embolism
Depending on where the embolism enters the circulation, it can be arterial or venous. So, arterial thrombi that detach would go on to become arterial emboli.
Arterial embolism can cause occlusion of any part of the body and is a major form of infarction, and tissue necrosis. Embolisms in the brain or carotid artery can lead to stroke, those that end up in the cardiac arteries can cause a myocardial infarction. As well as affecting the brain, arterial embolisms commonly affect the lower extremities and small bowel.
Risk factors include atrial fibrillation, mitral valve stenosis, age, hypertension and heart failure, as well as prosthetic valves. In effect, anything that fits Virchow's triad.
Venous Embolism
A venous embolism, originating from a systemic vein, on the other hand, will always impact the lungs after passing through the right side of the heart, and cause a pulmonary embolism. The most common sites of origin are the femoral veins.
Gross: saddle pulmonary embolus (arrows) lying across the bifurcation of the pulmonary trunk into both main pulmonary arteries; it usually comes from a deep leg vein thrombus.
Image: “Saddle thromboembolus wArrows” by Bakerstmd, CC BY-SA 4.0, via Wikimedia Commons.
Paradoxical embolisms are rare, and are those emboli that cross the veins to the arterial blood system. These are only found in those where there are septal defects between the atria or ventricles.
Example: a patient with a deep vein thrombosis and a patent foramen ovale strains or coughs, so right atrial pressure rises; a fragment of the leg clot passes into the left atrium and lodges in a cerebral artery, causing a stroke instead of a pulmonary embolism.
Types
As well as classifying Emboli as ‘Arterial’ or ‘Venous’, we can also classify them based on the direction of the embolus:
- Anterograde - movement is in direction of blood flow
- Retrograde - movement is in the opposite direction of blood flow (usually venous)
We can also classify embolisms based on the thing that is doing the blocking, as it isn’t always a thrombus on the move! Other types include: