An embolus is any detached, traveling intravascular mass carried by
circulation, which is capable of clogging arterial capillary beds at a
site distant from its point of origin. By contrast there are non-traveling
blockages that develop locally from vascular trauma or epithelial pathology
and vascular inflammation — like atheroma, thrombi. However, if a
thrombus breaks loose from its genesis site it becomes a thrombo-embolus and if
not broken down during transit, may cause embolism(s).
The term was coined in 1848 by Rudolf Virchow.
Classification by substance Thromboembolism – embolism of thrombus.
Cholesterol embolism – embolism of cholesterol, often from atherosclerotic
plaque inside a vessel. Fat embolism – embolism of bone fracture
or fat droplets. Air embolism – embolism of air bubbles.
Septic embolism – embolism of bacteria-containing pus.
Tissue embolism – embolism of small fragments of tissue.
Foreign body embolism – embolism of foreign materials such as talc and other
small objects. Amniotic fluid embolism – embolism of
amniotic fluid, foetal cells, hair, or other debris that enters the mother's
bloodstream via the placental bed of the uterus and triggers an allergic
reaction. In thromboembolism, the thrombus from a
blood vessel is completely or partially detached from the site of thrombosis.
The blood flow will then carry the embolus to various parts of the body
where it can block the lumen and cause vessel obstruction or occlusion. Note
that the free-moving thrombus is called an embolus. A thrombus is always
attached to the vessel wall and is never freely moving in the blood circulation.
This is also the key difference for pathologists to determine the cause of a
blood clot, either by thrombosis or by post-mortem blood clot. Vessel
obstruction will then lead to different pathological issues such as blood stasis
and ischemia. However, not only thromboembolism will cause the
obstruction of blood flow in vessels, but also any kind of embolism is capable
of causing the same problem. Fat embolism usually occurs when
endogenous fat tissue escapes into the blood circulation. The usual cause of
fat embolism is therefore the fracture of tubular bones, which will lead to the
leakage of fat tissue within the bone marrow into ruptured vessels. There are
also exogenous causes such as intravenous injection of emulsions.
An air embolism, on the other hand, is usually always caused by exogenic
factors. This can be the rupture of alveoli, and inhaled air can be leaked
into the blood vessels. Other more-common causes include the puncture
of the subclavian vein by accident or during operation where there is negative
pressure. Air is then sucked into the veins by the negative pressure caused by
thoracic expansion during the inhalation phase of respiration. Air embolism can
also happen during intravenous therapy, when air is leaked into the system.
Gas embolism is a common concern for deep-sea divers because the gases in
human blood can be easily dissolved at higher amounts during the descent into
deep sea. However, when the diver ascends to the normal atmospheric
pressure, the gases become insoluble, causing the formation of small bubbles
in the blood. This is also known as decompression sickness or the Bends.
This phenomenon is explained by Henry's Law in physical chemistry.
Embolism by other materials are rather rare. Septic embolism happens when a
purulent tissue is dislodged from its original focus. Tissue embolism is a
near-equivalent to cancer metastasis, which happens when cancer tissue
infiltrates blood vessels, and small fragments of them are released into the
blood stream. Foreign-body embolism happens when exogenous—and only
exogenous—materials such as talc enter the blood stream and cause occlusion or
obstruction of blood circulation. Bullet embolism occurs in approximately 0.3%
cases of gunshot wounds. Amniotic-fluid embolism is a rare complication of
childbirth. See also
Paradoxical embolism Pulmonary embolism
References