- An embolus is a detached intravascular solid, liquid, or gaseous mass that is carried by the blood from its point of origin to a distant site, where it often causes tissue dysfunction or infarction.
- The vast majority of emboli are dislodged thrombi, hence the term thromboembolism.
- Other rare emboli are composed of fat droplets, nitrogen bubbles, atherosclerotic debris (cholesterol emboli), tumor fragments, bone marrow, or even foreign bodies.
- Emboli travel through the blood until they encounter vessels too small to permit further passage, causing partial or complete vascular occlusion.
Types of Emboli
Some important types of embolism
- Pulmonary Embolism:
- Pulmonary emboli originate from deep venous thromboses and are the most common form of thromboembolic disease.
- Incidence since the 1970s of roughly 2 to 4 per 1000 hospitalized patients in the United States.
- PE causes about 100,000 deaths per year in the United States.
- In more than 95% of cases, PEs originates from leg DVT.
- Course of embolus causing Pulmonary Embolism
- Rarely, a venous embolus passes through an interatrial or interventricular defect and gains access to the systemic arterial circulation (paradoxical embolism).
- Most pulmonary emboli (60% to 80%) are clinically silent because they are small. With time they become organized and are incorporated into the vascular wall; in some cases organization of the thromboembolus leaves behind a delicate, ridging fibrous web.
- Sudden death, right heart failure (cor pulmonale), or cardiovascular collapse occurs when emboli obstruct 60% or more of the pulmonary circulation.
- Embolic obstruction of medium-sized arteries with subsequent vascular rupture can result in pulmonary hemorrhage but usually does not cause pulmonary infarction. This is because the lung is supplied by both the pulmonary arteries and the bronchial arteries, and the intact bronchial circulation is usually sufficient to perfuse the affected area. Understandably, if the bronchial arterial flow is compromised (e.g., by left-sided cardiac failure), infarction may occur.
- Embolic obstruction of small end-arteriolar pulmonary branches often does produce hemorrhage or infarction.
- Multiple emboli over time may cause pulmonary hypertension and right ventricular failure.
Figure- Embolus from a lower extremity deep venous thrombosis,
lodged at a pulmonary artery branchpoint.
- Most systemic emboli (80%) arise from intracardiac mural thrombi, two thirds of which are associated with left ventricular wall infarcts and another one fourth withleft atrial dilation and fibrillation. The remainder originates from aortic aneurysms, atherosclerotic plaques, valvular vegetations, or venous thrombi (paradoxical emboli);10% to 15% are of unknown origin.
- Arterial emboli are in contrast to venous emboli, the vast majority of which lodge in the lung, arterial emboli can travel to a wide variety of sites; the point of arrest depends on the source and the relative amount of blood flow that downstream tissues receive.
- The emboli are arterial and invariably cause infarction at the sites of lodgement . These sites, in descending order of frequency are: lower extremities (75%) or the brain (10%), internal visceral organs; the intestines, kidneys, spleen, and upper extremities, may be involved on occasion.
- The consequences of systemic emboli depend on the vulnerability of the affected tissues to ischemia, the caliber of the occluded vessel, and whether a collateral blood supply exists; in general, however, the outcome is tissue infarction.
Fat and Marrow Embolism
- Microscopic fat globules—sometimes with associated hematopoietic bone marrow—can be found in the pulmonary vasculature after fractures of long bones or, rarely, in the setting of soft tissue trauma and burns. Presumably these injuries rupture vascular sinusoids in the marrow or small venules, allowing marrow or adipose tissue to herniated into the vascular space and travel to the lung.
- Fat and marrow emboli are very common incidental findings after vigorous cardiopulmonary resuscitation and are probably of no clinical consequence. Indeed, fat embolism occursin some 90% of individuals with severe skeletal injuries but less than 10% of such patients have any clinical findings.
- Fat embolism syndrome is the term applied to the minority of patients who become symptomatic. It is characterized by pulmonary insufficiency, neurologic symptoms, anemia, and thrombocytopenia, and is fatal in about 5% to 15% of cases. Typically, 1 to 3 days after injury there is a sudden onset of tachypnea, dyspnea, and tachycardia; irritability and restlessness can progress to delirium or coma.
- Thrombocytopenia is attributed to platelet adhesion to fat globules and subsequent aggregation or splenic sequestration; anemia can result from similar red cell aggregation and/or hemolysis.
- A diffuse petechial rash (seen in 20% to 50% of cases) is related to rapid onset of thrombocytopenia and can be a useful diagnostic feature.
- Fat microemboli and associated red cell and platelet aggregates can occlude the pulmonary and cerebral microvasculature.
- Release of free fatty acids from the fat globules exacerbates the situation by causing local toxic injury to endothelium, and platelet activation and granulocyte recruitment (with free radical, protease, and eicosanoid release) complete the vascular assault. Because lipids are dissolved out of tissue preparations by the solvents routinely used in paraffin embedding, the microscopic demonstration of fat microglobules typically requires specialized techniques, including frozen sections and stains for fat.
Figure– Bone marrow embolus in the pulmonary circulation. The cellular elements on the left side of the embolus are hematopoietic cells, while the cleared vacuoles represent marrow fat. The relatively uniform red area on theright of the embolus is an early organizing thrombus.
- Gas bubbles within the circulation can coalesce to form frothy masses that obstruct vascular flow and cause distal ischemic injury.
- For example, a very small volume of air trapped in a coronary artery during bypass surgery, or introduced into the cerebral circulation by neurosurgery in the “sitting position,” can occlude flow with dire consequences.
- A particular form of gas embolism, called decompression sickness, occurs when individuals experience sudden decreases in atmospheric pressure.
- Scuba and deep sea divers, underwater construction workers, and individuals in unpressurized aircraft in rapid ascent are all at risk. When air is breathed at high pressure (e.g., during a deep sea dive), increased amounts of gas (particularly nitrogen) are dissolved in the blood and tissues. If the diver then ascends (depressurizes) too rapidly, the nitrogen comes out of solution in the tissues and the blood.
- The rapid formation of gas bubbles within skeletal muscles and supporting tissues in and about joints is responsible for the painful condition called the bends
- In the lungs, gas bubbles in the vasculature cause edema, hemorrhage, and focal atelectasis or emphysema, leading to a form of respiratory distress called the chokes. A more chronic form of decompression sickness is called caisson disease (named for the pressurized vessels used in bridge construction; workers in these vessels suffered both acute and chronic forms of decompression sickness).
- In caisson disease, persistence of gas emboli in the skeletal system leads to multiple foci of ischemic necrosis; the more common sites are the femoral heads, tibia, and humeri. Individuals affected by acute decompression sickness are treated by being placed in a chamber under sufficiently high pressure to force the gas bubbles back into solution. Subsequent slow decompression permits gradual resorption and exhalation of the gases, which prevents the
obstructive bubbles from reforming.
Amniotic Fluid Embolism
- Amniotic fluid embolism is the fifth most common cause of maternal mortality worldwide; it accounts for roughly 10% of maternal deaths in the United States and results in permanent neurologic deficit in as many as 85% of survivors.
- Amniotic fluid embolism is an ominous complication of labor and the immediate postpartum period.
- Although the incidence is only approximately 1 in 40,000 deliveries, the mortality rate is up to 80%. The onset is characterized by sudden severe dyspnea, cyanosis, and shock, followed by neurologic impairment ranging from headache to seizures and coma.
- If the patient survives the initial crisis, pulmonary edema typically develops, frequently accompanied by disseminated intravascular coagulation.
- The underlying cause is the infusion of amniotic fluid or fetal tissue into the maternal circulation via a tear in the placental membranes or rupture of uterine veins.
- Classicfindings at autopsy include the presence of squamous cells shed from fetal skin, lanugo hair, fat from vernix caseosa, and mucin derived from the fetal respiratory or gastrointestinal tract in the maternal pulmonary microvasculature
- Other findings include marked pulmonary edema, diffuse alveolar damage and the presence of fibrin thrombi in many vascular beds due to disseminated intravascular coagulation.
Figure-Amniotic fluid embolism. Two small pulmonary arterioles are packed with laminated swirls of fetal squamous cells. There is marked edema and congestion. Elsewhere the lung contained small organizing thrombi consistent with disseminated intravascular coagulation. (Courtesy Dr. Beth Schwartz, Baltimore, Md.)