Embolism is the intra-vascular impaction of an undissolved material (solid, liquid or gaseous) carried by the blood stream to a site distant from its point of origin.

Material impacted is called embolus.

Most (99%) emboli arise from thrombi, hence the term thromboembolism.

Other forms include droplets of fat, gas bubbles, atherosclerotic debris ( atheroemboli ), tumour fragments, bone marrow, or foreign bodies such as bullets.

Emboli lodge in vessels too small to permit further passage, resulting in partial or complete vascular occlusion and ischemic necrosis of the distal tissue ( infarction ).

Pulmonary Thromboembolism:

In more than 95 %, pulmonary emboli originate from deep leg vein thrombi. Depending on the size, a pulmonary embolus may occlude the main pulmonary artery, impact across the bifurcation (saddle embolus), pass into smaller arterioles.

Multiple emboli may occur, either sequentially or as a shower of small emboli from a single large mass. In general, one pulmonary embolism puts a patient at risk for more.

Rarely, emboli pass through atrial or ventricular defects into the systemic circulation (paradoxical embolism) - Example: cerebral embolism with hemiplegia due to puerperal thrombosis of pelvic veins.

Fat Embolism:

Air Embolism:

Decompression Sickness:

Amniotic Fluid Embolism:

Source of Emboli:

Left Cardiac thrombi (80 %) are mural thrombi in myocardial infarct and thrombi in left atrium in rheumatic mitral valve disease.

Systemic artery thrombi, from aortic aneurysm, and ulcerated atherosclerotic plaques in aorta and other arteries are impacted in organs like liver, brain, viscera and in extremities.

Venous and right cardiovascular thrombi produce pulmonary embolism e.g. detached venous thrombi, tumour cells invading veins, fat embolism, air embolism amniotic fluid embolism.

Portal and mesenteric vein thrombi due to inflammation in gastrointestinal tract are impacted in liver.

Effects of Embolism: Depends on the type and site of embolism.

i) Septic emboli produce abscess.

ii) Tumor emboli cause metastatic tumor

iii) Organ with end-arteries/poor collateral circulation produces infarct.

iv) Condition of heart and collateral circulation e.g., femoral artery with sound heart causes temporary paresis with re-establishment of circulation by collaterals.

Femoral artery embolism with feeble heart in old age causes dry gangrene

Retrograde Embolism:

Normally emboli move in the direction of the flow.

If it moves to opposite direction and gets impacted, it is called retrograde embolism.

Examples:

i) Virchow’s gland-(deep supra-clavicular lymph node) in gastric carcinoma.

ii) Inguinal lymph nodes in testicular tumour.

iii) Krukenburg’s tumor in mucoid carcinoma of gastrointestinal tract.