An embolic event or "embolism" describes a process by which any substance that does not readily dissolve in blood enters the circulation. Embolic substances can be solid, liquid, or gas. Examples include blood clots, fat, air, bone marrow, amniotic fluid, and cement used in joint prostheses. Once they enter the circulation, usually on the venous side, embolisms (or "emboli") have two possible pathways. Most often, they follow the venous blood flow through the right side of the heart, and then travel through the pulmonary arteries to the lungs. Here they can do serious or even fatal harm by blocking the blood flow to portions of the lungs. Many people (30% in one study) have undiagnosed pathways from the pulmonary (right-heart) to the arterial (left-heart) circulation, and in these patients a venous embolism can bypass the lungs to become an arterial embolism. These "paradoxical emboli" can block the circulation to vital organs, particularly the brain or heart. Small emboli that would do only minor damage in the lungs can cause fatal harm to the brain or heart by this mechanism.
The most common source of embolism is blood clot (thrombus) from the periphery, usually from the leg veins. These venous clots can break loose from the walls of the vein in which they were formed (e.g., the saphenous vein) and thus enter the circulation. This mechanism of "Deep Venous Thrombosis" or "DVT" is the subject of APSS 12A. Another common embolic mechanism is the admission or air into the venous circulation, which can occur in trauma, during surgery, or by inadvertent injection of air into any type of venous access catheter. "Air Embolism" is the topic of APSS 12B. In both of these subAPSS, we discuss the prevention, early detection or diagnosis, and emergency treatment of the embolic event. Other subAPSS for future development may include fat, marrow, and amniotic fluid embolism.