{
  "id": "embolism",
  "title": "Embolism",
  "lang": "en",
  "source_lang": "en",
  "summary": "Embolism is obstruction of a blood vessel by material carried through the circulation from another site.",
  "category": "health",
  "subcategory": "diseases",
  "type": "concept",
  "aliases": [],
  "generated_at": "2026-10-25",
  "model": "pro/gpt-6.1-sol",
  "url": "https://aiwiki.page/en/embolism/",
  "translations": {
    "zh-Hans": {
      "url": "https://aiwiki.page/zh/embolism/",
      "json": "https://aiwiki.page/zh/embolism.json"
    }
  },
  "sections": [
    {
      "heading": "",
      "text": "Embolism is the obstruction of a blood vessel by material, called an embolus, that has travelled through the circulation from another location. Most clinically significant emboli are fragments of blood clots, producing thromboembolism, but fat, gas, cholesterol crystals, infected material, tumour fragments, and other substances can also be involved. Embolism differs from thrombosis, in which a clot forms at the site it obstructs, although the two processes are closely connected.\n\nThe effects depend on the embolus’s composition, size, destination, and impact on blood flow. Emboli entering the pulmonary circulation can impair gas exchange and strain the right side of the heart; those entering systemic arteries can interrupt the blood supply to the brain, limbs, or internal organs. Embolism is therefore an important cause of stroke, organ injury, and sudden death. Its recognition and clinical management draw on vascular anatomy, pathology, laboratory testing, and medical imaging, with substantial differences between thrombotic and nonthrombotic forms."
    },
    {
      "heading": "Definition, terminology, and historical development",
      "text": "An embolus is a detached intravascular mass—solid, liquid, or gaseous—that is carried by the bloodstream to a site away from its origin. Embolism is the lodging of such material within a blood vessel, producing partial or complete obstruction. A single embolus may obstruct one vessel, fragment into several branches, or form part of a shower of numerous microscopic emboli. The term therefore does not imply a single large blockage or a particular clinical severity. Small emboli may produce no recognized symptoms and be detected incidentally on imaging or at autopsy; others interrupt enough blood flow to cause organ dysfunction or tissue death. The consequences depend on the affected circulation and the tissue’s alternative blood supply, not simply on whether embolic material is present.\n\nEmbolism is distinguished from thrombosis, in which a blood clot forms locally within the cardiovascular system. A thrombus remains attached at its site of formation until all or part of it detaches; the detached material then becomes a thromboembolus. Thromboembolism describes this linked process of clot formation and transport, rather than treating thrombosis and embolism as unrelated events. The distinction concerns mechanism: an artery may be blocked by a clot formed at that location or by one arriving from elsewhere. Embolism also differs from fixed vascular stenosis, such as narrowing caused by atherosclerotic plaque, and from external compression by an adjacent mass. These processes can coexist: a pre-existing stenosis may provide a lodging site for an embolus, and secondary thrombosis may extend an embolic obstruction.\n\nClassification commonly uses both the embolic material and its destination. Most clinically recognized emboli consist of thrombotic material, but other forms include fat droplets, cholesterol crystals, gas bubbles, infected material, tumour fragments, and foreign objects such as catheter fragments. Pulmonary embolism identifies obstruction in the pulmonary arterial circulation; systemic arterial embolism includes emboli reaching cerebral, limb, intestinal, renal, and other arteries. These anatomical labels do not by themselves establish the material involved. Likewise, embolization describes the process of material travelling and lodging, but is not invariably pathological. In therapeutic embolization, clinicians deliberately deliver selected agents through a catheter to reduce or stop blood flow, for example to control haemorrhage or occlude vessels supplying a tumour. This controlled intervention is distinct from unintended embolic disease.\n\nThe modern distinction between thrombosis and embolism emerged from the work of Rudolf Virchow in the 1840s and 1850s. Through post-mortem investigation and animal experiments, Virchow established that material originating in peripheral venous thrombi could travel through the right heart and lodge in pulmonary arteries. This explained why an obstructing pulmonary clot need not have formed in the lung itself. His work helped replace explanations that attributed such pulmonary obstructions primarily to local inflammation or clot formation. His studies of thrombosis and embolism were included in his collected scientific papers published in 1856.\n\nSubsequent recognition expanded from gross autopsy findings to microscopic pathology, which could identify small emboli and distinguish thrombotic material from fat, tumour cells, or cholesterol clefts. During the twentieth century, pulmonary angiography and radionuclide lung scanning made investigation possible during life rather than principally after death. From the 1990s, spiral computed tomography enabled increasingly practical visualization of pulmonary arterial filling defects, with later multidetector systems depicting smaller vessels. These developments also increased recognition of incidental emboli in patients imaged for other reasons, separating the anatomical detection of embolism from the presence of an obvious clinical syndrome."
    },
    {
      "heading": "Circulatory pathways and mechanisms of injury",
      "text": "Emboli usually travel in the direction of blood flow until they reach a vessel too narrow to permit passage. Material entering the systemic veins normally passes through the venae cavae, right atrium, right ventricle, and pulmonary arteries. The pulmonary vascular bed therefore acts as a filter for many venous emboli: larger fragments lodge in major pulmonary arteries, whereas smaller particles reach distal arteries, arterioles, or capillaries. By contrast, emboli originating in the left atrium, left ventricle, or aorta enter the systemic arterial circulation and may reach the cerebral, intestinal, renal, splenic, or limb arteries. Their destination depends partly on the source’s position and the distribution of blood flow. Material released from an aortic lesion ordinarily travels downstream rather than into arteries arising upstream of that lesion.\n\nParadoxical embolism occurs when material from the venous circulation bypasses the pulmonary filter through a right-to-left shunt and enters systemic arteries. An intracardiac route is a patent foramen ovale, a persistent flap-like communication between the atria found in approximately one-quarter of adults. Passage requires a right-to-left pressure gradient, which may occur transiently during straining or with increased right-sided cardiac pressure. Pulmonary arteriovenous malformations provide another route by connecting pulmonary arteries to pulmonary veins without an intervening capillary bed. A patent foramen ovale is a potential conduit, not proof that an arterial event arose from a venous embolus: it is common in people without embolic disease, and competing arterial or cardiac sources may exist.\n\nThe site of impaction reflects the relationship between embolus dimensions, shape, deformability, and vessel calibre. A branching point can trap an embolus that straddles the daughter vessels; fragmentation may instead distribute material among multiple smaller branches. Consequently, one proximal obstruction and numerous distal obstructions can produce different patterns of injury even when the total amount of embolic material is similar. Collateral circulation can maintain some perfusion beyond an occlusion, particularly where pre-existing arterial narrowing has allowed alternative pathways to enlarge. Abrupt obstruction in a territory with little collateral supply is generally less well tolerated. Pulmonary tissue has both pulmonary and bronchial arterial supplies, helping explain why pulmonary arterial obstruction does not invariably cause lung infarction.\n\nMechanical blockage reduces downstream perfusion, but embolic injury is not solely a plugging phenomenon. Activated platelets and vascular cells can release mediators, including thromboxane and serotonin, that promote vasoconstriction and increase resistance beyond that attributable to the obstruction alone. Endothelial injury, inflammatory signalling, and activation of coagulation may further impair microvascular flow. Additional thrombus formation around impacted material can extend the obstructed segment. The contribution of these processes varies with the embolic material: infected fragments, fat droplets, gas bubbles, and blood clots produce overlapping but distinct cellular responses.\n\nIschemia is an inadequate blood supply relative to tissue requirements; sustained severe ischemia can cause irreversible cell death and infarction. The outcome depends on residual flow, oxygen demand, tissue susceptibility, and the duration of obstruction, rather than embolus size alone. Restoration of circulation may rescue viable tissue, but can also produce reperfusion injury. Reoxygenation promotes reactive oxygen species formation, intracellular calcium disturbances, and inflammatory-cell activation; damaged microvascular endothelium can become leaky, producing edema or hemorrhage. Persistent capillary obstruction and swelling may prevent effective tissue perfusion despite reopening of the larger supplying artery, a phenomenon termed no-reflow."
    },
    {
      "heading": "Venous thromboembolism and pulmonary embolism",
      "text": "Venous thromboembolism comprises deep-vein thrombosis (DVT) and pulmonary embolism (PE), which are linked manifestations of clot formation and migration. Most pulmonary thromboemboli originate in deep veins of the legs or pelvis, particularly the popliteal, femoral, and iliac veins. A fragment detaches, passes through the inferior vena cava and right heart, and lodges in the pulmonary arterial circulation. Calf-vein thrombi can also embolize, but their clinical importance includes the possibility of extension into proximal veins, where embolization risk is greater. Less common sources include upper-extremity deep veins, often in association with central venous catheters, and thrombi within the right heart. DVT may produce no recognized leg symptoms, and failure to identify a residual venous clot does not exclude PE.\n\nThe anatomical spectrum ranges from small emboli confined to subsegmental branches to multiple bilateral emboli or obstruction of the main pulmonary arteries. Some are discovered incidentally during computed tomography performed for another reason, such as assessment of cancer. A saddle embolus straddles the bifurcation of the main pulmonary artery and extends into its right and left branches. This term describes location, not the degree of circulatory failure: a saddle embolus may allow substantial residual flow, whereas emboli distributed across smaller vessels can collectively cause severe obstruction. Physiological severity depends on the total affected vascular bed, the speed of obstruction, and pre-existing cardiac and pulmonary reserve, rather than the appearance of a single clot alone.\n\nObstruction disrupts the relationship between ventilation and blood flow. Alveoli supplied by blocked arteries may remain ventilated but receive little or no perfusion, increasing physiological dead space. Redistribution of blood to unobstructed regions can create areas with relatively low ventilation in relation to perfusion. Together with atelectasis and, in severe cases, reduced oxygen content in blood returning to the lungs, these changes can lower arterial oxygen levels. Hypoxemia and stimulation of respiratory receptors commonly increase ventilation, so arterial carbon dioxide is often reduced rather than elevated. Normal oxygenation nevertheless does not exclude PE.\n\nMechanical obstruction and accompanying vasoconstriction increase pulmonary vascular resistance. The right ventricle, normally adapted to a low-pressure circulation, must generate greater pressure to maintain pulmonary blood flow. Acute overload can cause right-ventricular dilatation, impaired contraction, and increased wall stress; reduced coronary perfusion can further compromise right-ventricular function. Leftward displacement of the interventricular septum and reduced delivery of blood through the lungs decrease left-ventricular filling and cardiac output. At the severe end of this sequence, systemic hypotension, obstructive shock, or cardiac arrest can develop. Substantial right-ventricular strain can also occur while systemic blood pressure remains normal.\n\nManifestations are variable and nonspecific. Shortness of breath may begin abruptly or evolve over several days; rapid breathing and tachycardia are common but not universal. Pleuritic chest pain, worsened by inspiration, often accompanies peripheral emboli that irritate the pleura. Pulmonary hemorrhage or infarction can cause hemoptysis, although infarction is not an inevitable consequence because lung tissue also receives bronchial arterial blood. Syncope may accompany an abrupt reduction in cardiac output. Hypotension, altered consciousness, cool extremities, and other signs of poor perfusion indicate hemodynamic compromise. Conversely, some emboli cause few or no symptoms, and neither the presence nor absence of any single manifestation establishes or excludes the condition."
    },
    {
      "heading": "Systemic arterial thromboembolism",
      "text": "Thromboemboli entering the systemic arterial circulation commonly arise in the left heart or a diseased proximal artery. In atrial fibrillation, impaired atrial contraction and local blood stasis favour thrombus formation, particularly within the left atrial appendage. Left-ventricular thrombi can develop after myocardial infarction, especially when an injured ventricular apex is akinetic or forms an aneurysm; severe ventricular dysfunction provides another setting. Diseased or prosthetic cardiac valves may carry thrombotic material, although emboli from infected vegetations belong to the distinct category of septic embolism. Mural thrombus within an aortic aneurysm or overlying diseased aortic tissue can also detach. A single source may produce successive emboli or distribute fragments to several organs.\n\nIn cerebral arteries, embolic occlusion can cause ischemic stroke, with deficits determined by the affected vascular territory: middle cerebral artery occlusion, for example, may produce contralateral weakness and language disturbance when the dominant hemisphere is involved. Transient focal neurological dysfunction caused by ischemia without acute infarction constitutes a transient ischemic attack; symptom resolution alone does not establish that infarction has been avoided. Cardioembolism denotes embolization from the heart, whereas artery-to-artery embolism commonly involves thrombotic material released from an atherosclerotic carotid plaque into downstream cerebral or ophthalmic vessels. The two mechanisms can produce similar clinical manifestations. Retinal arterial occlusion typically causes sudden, painless loss of vision in one eye; central retinal artery occlusion affects a larger retinal territory than branch occlusion. Retinal emboli are not invariably thrombotic: cholesterol crystals and calcific material can also obstruct these vessels.\n\nEmbolization to a limb artery produces acute limb ischemia, conventionally describing a sudden reduction in perfusion with symptoms lasting less than two weeks. Emboli frequently lodge at arterial branch points, including the common femoral bifurcation. The traditional “six Ps” are pain, pallor, pulselessness, paresthesia, paralysis, and poikilothermia, meaning an abnormally cool limb. These findings need not all be present, particularly early in the event. Sensory loss and motor impairment indicate compromised nerve and muscle function rather than merely reduced skin perfusion. Abrupt occlusion of a previously patent artery may cause particularly severe ischemia because collateral channels are poorly developed. By contrast, local thrombosis on longstanding peripheral arterial disease may occur in a limb with pre-existing collaterals and a history of exertional symptoms, although clinical overlap is substantial.\n\nMesenteric emboli most often affect the superior mesenteric artery, sometimes lodging beyond its origin and producing segmental intestinal ischemia. Early acute mesenteric ischemia may cause severe abdominal pain despite relatively limited abdominal examination findings; progression to bowel infarction can produce peritoneal inflammation and systemic deterioration. This differs anatomically from local thrombosis, which commonly develops near the origin of a mesenteric artery over an atherosclerotic lesion. Renal arterial emboli can cause flank or abdominal pain, hematuria, and renal infarction. The functional effect depends on the volume of tissue affected and whether the opposite kidney functions normally. Splenic emboli may produce left upper abdominal pain and wedge-shaped infarcts, but small lesions can be clinically silent.\n\nCoronary thromboembolism is a less common cause of myocardial infarction than local thrombus formation following coronary plaque disruption. Potential sources include left-sided intracardiac thrombi and prosthetic valves; embolic material can obstruct a coronary artery even without substantial underlying coronary atherosclerosis. Across these territories, an arterial occlusion or downstream infarct demonstrates the site of injury but does not by itself establish whether the obstructing thrombus arrived from elsewhere or formed locally."
    },
    {
      "heading": "Fat and cholesterol embolism",
      "text": "Fat embolism occurs when fat droplets enter the circulation, most commonly after fractures of long bones, particularly the femur, or fractures of the pelvis. Disruption of marrow-containing bone and adjacent venous channels allows bone-marrow fat to enter venous blood, especially when pressure within the marrow cavity rises. Similar embolization can occur during orthopedic surgery, including intramedullary instrumentation and joint replacement. Microscopic fat embolization is considerably more frequent than clinically apparent disease: detecting circulating fat or fat within pulmonary vessels does not, by itself, establish fat embolism syndrome. The syndrome denotes organ dysfunction associated with fat embolization, rather than the mere presence of embolic droplets.\n\nBoth mechanical obstruction and biochemical injury contribute to fat embolism syndrome. Fat droplets lodge in pulmonary microvessels, while lipase-mediated breakdown of fat releases free fatty acids that can damage the vascular endothelium and provoke inflammation. Increased capillary permeability produces pulmonary edema and impaired oxygen exchange; severe cases can develop acute respiratory distress syndrome. Fat may also reach the systemic circulation through right-to-left shunts or passage of small droplets through the pulmonary microcirculation, although the relative contribution of these pathways varies. Clinical manifestations commonly develop 24–72 hours after injury, rather than immediately. The characteristic combination is respiratory impairment, neurological disturbance, and a petechial rash, but all three need not be present. Respiratory findings include rapid breathing and reduced blood oxygen; neurological abnormalities range from confusion and agitation to seizures or coma. Petechiae, when present, often affect the upper chest, axillae, conjunctivae, and oral mucosa.\n\nRecognition is complicated by accompanying trauma: lung contusion, blood loss, head injury, and infection can produce overlapping abnormalities. Gurd’s clinical criteria, first published in 1970, organized manifestations into major and minor features, but neither these nor later scoring systems constitute a universally accepted definitive test. Fat droplets in blood, urine, or respiratory samples are not sufficiently specific to establish the syndrome. In cerebral involvement, diffusion-weighted magnetic resonance imaging may reveal numerous punctate lesions, often called a “starfield” pattern; this supports the clinical interpretation but is not independently diagnostic.\n\nCholesterol crystal embolization instead originates from atherosclerotic plaques, usually in the aorta or other large arteries. Disruption of a plaque’s surface releases cholesterol crystals and associated debris into downstream arteries. Release may occur spontaneously or after arterial instrumentation, such as catheter angiography, or vascular surgery involving a diseased aorta. The crystals lodge predominantly in small arteries and arterioles, causing both luminal obstruction and an inflammatory reaction that can include foreign-body giant cells and subsequent fibrosis. Tissue processing commonly dissolves the crystals, leaving characteristic needle-shaped or elongated empty clefts in affected vessels.\n\nThe distribution depends on the plaque source and may involve several organs. Renal embolization can cause progressive kidney dysfunction, sometimes emerging over days to weeks after a vascular procedure. Cutaneous manifestations include livedo reticularis—a mottled, netlike discoloration—and blue-toe syndrome, in which affected toes become painful and blue or purple. Peripheral pulses may remain palpable because the obstruction lies in vessels smaller than those assessed by pulse examination. Gastrointestinal involvement can cause abdominal pain or bleeding. Eosinophilia and reduced complement concentrations may accompany the inflammatory response, but neither is specific. Unlike a large thromboembolus that suddenly blocks a major artery, cholesterol embolization often produces repeated showers of microscopic material, patchy tissue ischemia, and a delayed or progressive multisystem illness."
    },
    {
      "heading": "Air and other gas embolism",
      "text": "Venous gas embolism occurs when gas enters systemic veins and travels to the right heart and pulmonary arteries. Small quantities may be trapped in the pulmonary microcirculation and gradually absorbed, whereas a rapid, larger influx can raise pulmonary vascular resistance, impair right-ventricular ejection, and reduce blood flow to the left heart. A large collection of gas can produce a “gas lock” that obstructs right-ventricular outflow. Arterial gas embolism involves gas in the systemic arterial circulation, where even a relatively small volume can interrupt cerebral or coronary perfusion, causing focal neurological deficits, loss of consciousness, myocardial ischemia, or cardiac arrest. The consequences therefore depend on location and delivery rate as well as total volume; there is no single volume threshold that reliably separates harmless from dangerous exposure.\n\nGas entry requires communication with a blood vessel and a pressure gradient favoring entry. Central venous catheter insertion, disconnection, manipulation, or removal can expose veins to atmospheric air, particularly when venous pressure is below atmospheric pressure during inspiration or when the insertion site is elevated above the heart. Open veins during surgery, especially cranial procedures performed with the operative field above the heart, provide another established route. Trauma can open veins or create communications between airways and pulmonary vessels. Pressurized infusion systems and extracorporeal circuits can introduce gas if air reaches the blood pathway. Carbon dioxide used to inflate body cavities during laparoscopic surgery can also enter injured vessels; although more soluble in blood than the principal components of air, it can still cause severe embolism when entry is sufficiently rapid.\n\nArterial gas can enter directly during arterial or cardiac procedures, or reach pulmonary veins after pulmonary barotrauma. Excessive expansion of gas within the lungs can rupture alveoli and disrupt adjacent vessels, allowing gas to pass through pulmonary veins into the left heart. This mechanism can occur during diving ascent, especially when expanding lung gas cannot escape, and during positive-pressure ventilation. Venous bubbles can also cross into systemic arteries through a right-to-left shunt, including a patent foramen ovale when right-atrial pressure exceeds left-atrial pressure. Pulmonary arteriovenous communications provide another route. The presence of a potential shunt alone does not establish that an embolus crossed it.\n\nDecompression sickness results from bubbles forming from dissolved inert gas, usually nitrogen, as ambient pressure falls after exposure to increased pressure. During a dive, increased pressure promotes inert-gas uptake into tissues; during ascent, tissue gas may become supersaturated and form bubbles within tissues or venous blood. This differs from arterial gas embolism caused by expanding lung gas entering pulmonary veins after barotrauma: the latter does not require substantial prior tissue uptake of inert gas and can follow ascent from relatively shallow water. Both conditions can occur in the same diver and are grouped clinically under decompression illness.\n\nPressure changes also alter existing bubble size: approximately, at constant temperature, gas volume varies inversely with absolute pressure. Gas solubility, diffusion, bubble fragmentation, and pulmonary filtration further modify its persistence and distribution. Injury is not exclusively mechanical. Bubbles can damage vascular endothelium, promote platelet and leukocyte adhesion, activate coagulation and complement, and increase capillary permeability. These responses contribute to edema and microvascular dysfunction even after the initial obstruction diminishes. In the lungs, obstruction and vasoconstriction can produce ventilation–perfusion mismatch and hypoxemia; in systemic arteries, interruption of flow can cause ischemia and infarction."
    },
    {
      "heading": "Septic, tumour, amniotic-fluid, and foreign-material embolism",
      "text": "Septic emboli contain infected material, usually fragments of platelet–fibrin vegetations from infective endocarditis, although infected venous thrombi and intravascular devices can also provide a source. Left-sided cardiac vegetations can embolize to the cerebral, splenic, renal, or limb arteries; right-sided vegetations, particularly on the tricuspid valve, typically produce pulmonary emboli. Unlike a sterile thromboembolus, a septic embolus can both obstruct blood flow and inoculate the affected tissue with microorganisms. Consequences include infarction, local suppuration, abscess formation, and infection of the arterial wall, sometimes producing an infected aneurysm. Septic pulmonary emboli may produce multiple peripheral nodules, wedge-shaped lesions, and cavitation as infected tissue breaks down. Staphylococcus aureus is an important associated pathogen, including in infections involving vascular catheters and injection drug use.\n\nTumour emboli consist of malignant cells or tumour fragments carried within blood vessels, sometimes accompanied by thrombus. They may arise when a tumour invades a vein or cardiac chamber: renal-cell carcinoma, for example, can extend through the renal vein into the inferior vena cava and occasionally the right atrium, creating a potential source of pulmonary tumour emboli. Microscopic pulmonary tumour emboli lodge in small arteries and arterioles and may cause progressive dyspnoea, hypoxaemia, and pulmonary hypertension without a large central arterial obstruction. In pulmonary tumour thrombotic microangiopathy, tumour microemboli are associated with coagulation activation and fibrocellular thickening of the vascular intima, further narrowing the pulmonary circulation; gastric adenocarcinoma is a well-established association. Tumour embolization is not synonymous with metastasis: the former describes intravascular transport and lodging, whereas successful metastasis also requires survival and growth at a secondary site. Conventional cancer-associated thromboembolism can occur without tumour cells in the embolus.\n\nAmniotic fluid embolism is a rare obstetric syndrome, with population-based estimates commonly around 2–6 cases per 100,000 deliveries, depending on case definition and ascertainment. It usually develops during labour, delivery, or shortly afterwards and may present with abrupt hypoxaemia, hypotension, cardiac arrest, and disseminated intravascular coagulation, sometimes with severe haemorrhage. Entry of amniotic components into the maternal circulation is implicated, but simple plugging of vessels by fetal debris does not adequately explain the syndrome. Proposed interacting mechanisms include pulmonary vasoconstriction, acute right-ventricular dysfunction, inflammatory mediator release, endothelial injury, and tissue-factor-associated coagulation activation. Consumptive coagulopathy and enhanced fibrinolysis can contribute to bleeding. Fetal squamous cells in maternal pulmonary vessels are neither uniquely diagnostic nor sufficient to explain the physiological collapse; the relative contributions of the proposed mechanisms remain incompletely resolved.\n\nForeign-material emboli include broken catheter segments, guidewire fragments, injected particles, and material displaced during medical procedures. A catheter fragment entering a systemic vein may travel into the right heart or pulmonary arteries, where it can lodge, provoke thrombosis, or injure adjacent structures. Intravenous injection of crushed tablets can introduce insoluble excipients, including talc or microcrystalline cellulose, into pulmonary arterioles and capillaries. Persistent particles may provoke foreign-body granulomas, fibrosis, and pulmonary hypertension rather than only an acute obstruction. Documented procedural examples include pulmonary cement embolism after vertebroplasty and migration of coils or other vascular devices. These unintended events differ from therapeutic embolization, in which coils, particles, liquid agents, or other materials are deliberately delivered to selected vessels to control haemorrhage, exclude an abnormal vascular connection, or reduce a tumour’s blood supply."
    },
    {
      "heading": "Epidemiology and predisposing conditions",
      "text": "Venous thromboembolism (VTE), comprising deep-vein thrombosis and pulmonary embolism, occurs at approximately 1–2 events per 1,000 people annually in many European and North American populations. Estimates vary with age distribution, case ascertainment, and whether recurrent or incidentally detected events are included. The two manifestations are commonly reported together because they frequently represent different stages of the same disease: a venous thrombus can detach and obstruct pulmonary arteries, and both conditions may coexist when either is diagnosed. Counting them separately can therefore double-count patients. There is no comparable, meaningful incidence figure for embolism as a whole: arterial thromboembolism, gas embolism, cholesterol embolization, and other forms have different populations at risk, definitions, and detection methods.\n\nThe framework traditionally associated with Rudolf Virchow, Virchow’s triad, groups thrombotic predisposition into venous stasis, endothelial injury, and hypercoagulability. Stasis reduces the clearance of activated coagulation factors and promotes local conditions favouring clot formation. Endothelial injury or activation alters the normally antithrombotic vessel surface. Hypercoagulability increases the tendency of blood to clot through changes in procoagulant, anticoagulant, or fibrinolytic pathways. These mechanisms often overlap: major orthopedic surgery, for example, combines tissue and vascular injury with reduced mobility and a postoperative procoagulant response. The triad explains predisposition to thrombosis, rather than to every form of embolism.\n\nHospitalization is a major VTE risk context because acute illness, operations, immobility, and invasive devices often occur together. Hip or knee replacement, major trauma, spinal cord injury, and prolonged bed rest are established examples. Paralysis and limb immobilization reduce the muscular pumping that assists venous return; central venous catheters provide an additional local stimulus for upper-extremity thrombosis. Long-distance travel is associated with a smaller increase in risk, particularly when other predisposing conditions coexist. Cancer creates both persistent and treatment-related risks through tumour-associated coagulation activation, venous compression, surgery, systemic therapy, and vascular access devices. Risk differs substantially by tumour site and stage; pancreatic and gastric cancers are among the higher-risk settings.\n\nPregnancy and the postpartum period together are associated with approximately 1–2 VTE events per 1,000 deliveries. Increased coagulation-factor activity, reduced activity of some natural anticoagulant pathways, venous compression by the enlarging uterus, and delivery-related vascular injury contribute. Risk is greatest shortly after delivery, especially during the first six weeks, although an elevation remains detectable through about 12 weeks. Estrogen-containing contraceptives and oral menopausal hormone therapy also increase venous thrombotic risk; the magnitude varies by formulation, route, and accompanying patient characteristics. These associations concern thromboembolism and are distinct from the rare obstetric syndrome of amniotic fluid embolism.\n\nInherited thrombophilias include factor V Leiden, the prothrombin G20210A variant, and deficiencies of antithrombin, protein C, or protein S. Their frequency and associated risk differ across populations and genetic states; carrying a variant does not mean that thrombosis will inevitably occur. Acquired predispositions include antiphospholipid syndrome and certain myeloproliferative neoplasms. Advancing age, obesity, previous VTE, and chronic inflammatory disease also influence risk. VTE is uncommon in children outside specific clinical settings, whereas annual incidence in older populations can reach several events per 1,000 people.\n\nPredisposing conditions also differ between venous and arterial events. Atrial fibrillation, recent myocardial infarction with ventricular thrombus, and diseased cardiac valves are established contexts for systemic arterial embolism; atherosclerotic disease can provide arterial thrombotic or cholesterol-crystal sources. In VTE classification, surgery, trauma, and temporary immobilization are examples of transient provoking factors, whereas active cancer and some chronic prothrombotic disorders are persistent conditions. Events termed unprovoked have no identified provoking factor after assessment; the term does not establish the absence of an underlying biological predisposition."
    },
    {
      "heading": "Diagnostic assessment and identification of the source",
      "text": "Assessment combines the onset and progression of symptoms, physical examination, predisposing conditions, and plausible alternative diagnoses. Abrupt focal neurological deficits, an acutely painful pulseless limb, and unexplained breathlessness suggest different vascular territories but do not by themselves establish embolism. Examination assesses physiological consequences, including hypoxaemia, circulatory instability, and impaired tissue perfusion. Evidence of obstruction must be distinguished from evidence of its origin: pulmonary arterial filling defects establish pulmonary embolism, whereas a leg-vein thrombus identifies a possible source. Conversely, detecting atrial fibrillation or an aortic plaque does not prove that it caused a particular arterial occlusion. Imaging may also leave uncertainty between an embolus and locally formed thrombus.\n\nFor suspected venous thromboembolism, validated probability assessments, including the Wells and revised Geneva scores, combine clinical findings and history to structure investigation. In appropriately selected patients with a very low clinical probability of pulmonary embolism, the pulmonary embolism rule-out criteria can identify a group not requiring D-dimer testing. D-dimer is a product of cross-linked fibrin degradation: a negative result with a sufficiently sensitive assay can exclude venous thromboembolism within validated low- or intermediate-probability pathways, but an elevated result cannot confirm it. Levels also rise with increasing age, infection, malignancy, pregnancy, and recent surgery. For patients older than 50 years, validated age-adjusted pathways commonly use a threshold of age multiplied by 10 ng/mL in fibrinogen-equivalent units, rather than a fixed 500 ng/mL threshold; interpretation depends on the assay and reporting units. High clinical probability generally leads directly to imaging rather than reliance on a negative D-dimer result.\n\nCT pulmonary angiography uses computed tomography and intravenous iodinated contrast to show intraluminal filling defects in pulmonary arteries. It also depicts alternative thoracic abnormalities and may demonstrate right-ventricular enlargement, but image quality can be limited by motion or inadequate contrast opacification. Ventilation–perfusion scanning evaluates regional air distribution and pulmonary blood flow; segmental perfusion defects with preserved ventilation support pulmonary embolism. It is particularly useful when iodinated contrast is unsuitable, although results require interpretation alongside clinical probability and underlying lung disease. Compression ultrasonography examines accessible deep veins, especially in the legs: failure of a vein to collapse under probe pressure is a principal sign of thrombosis. A demonstrated proximal deep-vein thrombus supports venous thromboembolic disease, whereas a negative leg examination does not exclude pulmonary embolism.\n\nArterial investigations are tailored to the affected organ. In suspected embolic stroke, noncontrast brain CT assesses haemorrhage, CT angiography identifies major arterial occlusions, and diffusion-weighted magnetic resonance imaging detects acute ischaemic injury. CT angiography also maps many limb and mesenteric occlusions; Doppler ultrasonography provides information about flow and accessible arterial anatomy. Catheter angiography is used selectively, particularly when detailed vascular mapping or an endovascular procedure is contemplated. Source investigation includes electrocardiography and, in selected cases, prolonged rhythm monitoring for intermittent atrial fibrillation. Transthoracic echocardiography assesses ventricular function, intracardiac masses, and some thrombi; transoesophageal examination offers more detailed assessment of the left atrial appendage, valves, and thoracic aorta. Contrast-bubble studies can demonstrate right-to-left shunting, but a patent foramen ovale alone does not establish paradoxical embolism.\n\nNonthrombotic embolism often requires integration of procedural or exposure history with laboratory and imaging findings. Blood cultures and cardiac imaging support investigation of septic emboli and infective endocarditis. Eosinophilia and impaired kidney function may accompany cholesterol embolization but are nonspecific; selected skin or kidney biopsies can demonstrate characteristic cholesterol clefts. Tissue examination occasionally identifies tumour cells or foreign material within vessels. Fat embolism syndrome and amniotic fluid embolism remain clinical diagnoses without a single definitive laboratory test; characteristic circumstances, organ dysfunction, and exclusion of competing explanations are therefore central to their recognition."
    },
    {
      "heading": "Clinical management by embolism type",
      "text": "Management separates several objectives: maintaining oxygenation and circulation, limiting further tissue injury, restoring blood flow when beneficial, and controlling the embolic source. These objectives are not interchangeable: improving blood pressure does not remove an arterial obstruction, while reopening a vessel cannot reverse tissue that has already infarcted. Support may include supplemental oxygen, ventilatory assistance, and treatment of circulatory shock. In severe pulmonary embolism, acute right-ventricular failure complicates both fluid administration and positive-pressure ventilation, which can further impair cardiac output. Treatment therefore depends on the affected organ and physiological disturbance, not simply the presence or anatomical size of an embolus.\n\nAnticoagulation is central to most venous thromboembolism treatment. Heparins, direct oral anticoagulants, and vitamin K antagonists such as warfarin inhibit components of coagulation, limiting clot extension and additional embolization. They do not directly dissolve an established clot; clearance depends largely on endogenous fibrinolysis and subsequent organization of thrombus. Treatment for pulmonary thromboembolism generally lasts at least three months, with longer courses determined by recurrence risk, persistent provoking conditions, and bleeding risk. Anticoagulation also addresses selected systemic embolic sources, including atrial fibrillation and some intracardiac thrombi, but is not a universal immediate treatment for every arterial embolic event. In particular, urgent full-dose anticoagulation is not routine treatment for acute ischemic stroke.\n\nThrombolysis accelerates fibrin breakdown through drugs that promote plasmin generation. In pulmonary embolism, systemic thrombolysis is used principally when hemodynamic instability indicates a high immediate risk of death. Routine systemic thrombolysis in stable patients is limited by major bleeding, including intracranial hemorrhage. Catheter-directed thrombolysis delivers a fibrinolytic drug into the pulmonary arteries, whereas catheter thrombectomy removes or disrupts clot mechanically. Surgical embolectomy provides another reperfusion route in selected severe cases, including some in which thrombolysis is contraindicated or unsuccessful. These procedures differ in their bleeding, vascular-injury, anesthesia, and technical risks; their availability also depends on specialist expertise.\n\nReperfusion selection differs in systemic arterial territories. In acute ischemic stroke, intravenous thrombolysis is used in eligible patients within defined time windows, conventionally up to 4.5 hours after symptom onset, with imaging-based selection permitting treatment in some other circumstances. Mechanical thrombectomy can remove an accessible large-vessel occlusion; selected patients benefit up to 24 hours after they were last known to be well. These windows do not apply indiscriminately to all strokes. Acute limb ischemia may involve immediate anticoagulation followed by catheter-directed thrombolysis, mechanical thrombectomy, or open embolectomy, according to limb viability, symptom duration, and arterial anatomy. Irreversible ischemic injury may preclude useful revascularization, while reperfusion itself can cause compartment syndrome and systemic metabolic disturbances.\n\nNonthrombotic emboli require cause-specific treatment. Septic embolism involves antimicrobial therapy directed at the organism and control of the infected source, potentially including removal of an infected device, abscess drainage, or valve surgery for defined indications in infective endocarditis. Anticoagulation does not sterilize infected emboli and is not initiated solely to prevent embolization from endocarditis. Fat embolism syndrome is treated chiefly with respiratory and circulatory support alongside management of the associated injury. Amniotic fluid embolism similarly requires resuscitation and organ support, often with blood-component replacement for severe coagulopathy and hemorrhage; no treatment reliably reverses the syndrome through simple removal of obstructing material.\n\nGas embolism management includes stopping further gas entry and administering high-concentration oxygen, which supports ischemic tissue and promotes nitrogen elimination from bubbles. Hyperbaric oxygen therapy further reduces bubble volume through increased ambient pressure and raises dissolved oxygen delivery; it is used particularly in arterial gas embolism and serious decompression illness. Mechanical retrieval may be possible for embolized catheter fragments or other accessible foreign bodies. Neither anticoagulants nor fibrinolytic drugs dissolve air, fat, cholesterol crystals, or device fragments, and unnecessary exposure can add bleeding without treating the obstructing material."
    },
    {
      "heading": "Prevention and health-care safety",
      "text": "Hospital prevention programmes assess the risk of venous thromboembolism alongside the risk of bleeding, because an intervention that reduces clotting can also cause haemorrhage. Assessments incorporate previous venous thrombosis, active cancer, reduced mobility, the type of surgery, and patient-specific factors such as renal impairment or thrombocytopenia. The Padua Prediction Score is used in some medical inpatient populations, while Caprini-based models are used in surgical settings; neither replaces assessment of bleeding risk or reassessment when the clinical situation changes. Institutional safeguards include standardized admission documentation, prescribing protocols, and review after surgery or transfer between units. Early mobilization reduces immobility but does not substitute for indicated pharmacological or mechanical prophylaxis.\n\nPharmacological prophylaxis commonly uses low-molecular-weight heparin or low-dose unfractionated heparin; selected surgical populations also receive other agents, including direct oral anticoagulants after hip or knee replacement. Prophylactic doses are generally lower than doses used to treat established thromboembolism. Selection and timing depend on bleeding risk, renal function, and procedures such as neuraxial anesthesia, in which anticoagulant exposure can increase the risk of spinal or epidural haematoma. Mechanical methods, particularly intermittent pneumatic compression, periodically compress the legs to increase venous flow. They are used when pharmacological prophylaxis is unsuitable and, in selected high-risk surgical patients, alongside it. Their effectiveness depends on correct sizing and consistent use; compression devices and stockings can also cause skin injury or be unsuitable in significant peripheral arterial disease. Extended prophylaxis after discharge is established in some settings, notably major joint replacement and selected abdominal or pelvic cancer operations, rather than being routine for every hospital admission.\n\nPrevention of cardiac-source embolism depends on the underlying disorder. In atrial fibrillation, oral anticoagulation reduces embolic stroke risk in patients selected through validated thromboembolic risk assessment. Direct oral anticoagulants are generally preferred over warfarin in eligible patients, whereas mechanical prosthetic valves require vitamin K antagonist therapy; warfarin also remains the established option in atrial fibrillation with moderate-to-severe rheumatic mitral stenosis. Aspirin, alone or with clopidogrel, is not an equivalent substitute for anticoagulation in atrial fibrillation. By contrast, antiplatelet therapy is central to prevention after many non-cardioembolic ischaemic strokes associated with arterial disease. Combining an anticoagulant with an antiplatelet drug increases bleeding and is reserved for defined additional indications, such as particular circumstances following coronary intervention. Left atrial appendage occlusion provides an alternative for selected patients, including some with a contraindication to long-term anticoagulation.\n\nProcedural safety addresses both thrombotic and nonthrombotic emboli. Air-entry precautions include priming infusion tubing, removing air from syringes and catheter systems, securing connections, and promptly occluding open vascular access points. Central venous catheter insertion and removal protocols use positioning and respiratory manoeuvres appropriate to the procedure to limit air entrainment. Catheter-associated thrombosis is addressed through appropriate device selection, positioning, maintenance, and removal when access is no longer required; routine systemic anticoagulation solely because a central catheter is present is not standard for all patients. Inspection of devices, avoidance of force during difficult withdrawal, and confirmation that removed components are intact help prevent retained fragments. During arterial interventions, careful guidewire and catheter manipulation limits vessel injury and plaque disruption; embolic-protection devices are used in selected procedures rather than universally.\n\nInferior vena cava filters intercept emboli travelling from the lower-body veins toward the lungs but neither treat existing thrombosis nor prevent new deep-vein thrombosis. Their principal accepted role is in selected patients with acute venous thromboembolism and a contraindication to anticoagulation, such as active major bleeding. They are not routinely added when effective anticoagulation can be given. The PREPIC randomized trial, reported initially in 1998 with eight-year follow-up published in 2005, found that permanent filters reduced pulmonary embolism but increased deep-vein thrombosis without improving survival. Device complications include caval thrombosis, migration, penetration, fracture, and difficult retrieval. For retrievable filters, a documented follow-up plan identifies responsibility for reassessment and removal once temporary protection is no longer necessary."
    },
    {
      "heading": "Outcomes, long-term complications, and evidence limitations",
      "text": "Prognosis depends on the vascular territory affected, the resulting physiological disturbance, the amount of irreversible tissue injury, accompanying illness, and the interval before effective treatment—not simply on embolus dimensions. In pulmonary embolism, acute right-ventricular failure and circulatory shock are major determinants of early death; a saddle embolus describes location rather than, by itself, physiological severity. Conversely, a small embolus in a cerebral or coronary artery can cause profound injury. Collateral blood supply and tissue tolerance of interrupted circulation also influence outcomes. Even after blood flow is restored, established infarction may leave permanent impairment, while reperfusion can produce additional inflammatory and metabolic injury.\n\nRecurrent thromboembolism remains an important long-term outcome. Risk varies with the underlying cause, particularly whether a transient provoking factor has resolved or a persistent condition, such as active cancer, remains. A 2019 systematic review of patients who stopped anticoagulation after a first unprovoked venous thromboembolism estimated cumulative recurrence at approximately 10% after one year and 36% after ten years. These figures do not apply uniformly to provoked events or to patients continuing anticoagulation. Recurrence may present as deep-vein thrombosis or pulmonary embolism, and treatment-related bleeding contributes separately to long-term morbidity.\n\nPersistent breathlessness, reduced exercise tolerance, fatigue, and impaired quality of life can follow pulmonary embolism even without recurrent obstruction or pulmonary hypertension. Potential contributors include residual vascular obstruction, impaired cardiopulmonary function, deconditioning, and pre-existing disease; persistent symptoms do not necessarily indicate recurrent embolism. Post-thrombotic syndrome, by contrast, is a consequence of associated deep-vein thrombosis rather than pulmonary embolism itself. Residual venous obstruction and damaged venous valves produce chronic venous hypertension, causing limb swelling, heaviness, pain, skin changes, and occasionally ulceration. It is commonly reported in approximately 20–50% of patients after deep-vein thrombosis, although estimates depend on diagnostic criteria and follow-up duration.\n\nUnresolved pulmonary thrombi may become organized fibrous lesions that narrow or occlude pulmonary arteries. Chronic thromboembolic pulmonary disease describes symptomatic chronic thromboembolic obstruction, with mismatched perfusion defects and characteristic imaging abnormalities after at least three months of therapeutic anticoagulation; it can occur with or without resting pulmonary hypertension. Chronic thromboembolic pulmonary hypertension additionally involves elevated pulmonary vascular resistance, reflecting both organized obstruction and remodeling of smaller pulmonary vessels. Under the 2022 European pulmonary hypertension guidelines, its haemodynamic criteria include mean pulmonary arterial pressure above 20 mmHg, pulmonary arterial wedge pressure no greater than 15 mmHg, and pulmonary vascular resistance above 2 Wood units. It develops in roughly 2–3% of pulmonary embolism survivors, although estimates vary with case selection and diagnostic methods, and can progress to right-heart failure.\n\nSystemic arterial embolism may leave lasting consequences despite successful reopening of the affected artery. Cerebral infarction can cause persistent weakness, language impairment, cognitive changes, or swallowing difficulties, with rehabilitation needs determined by the location and extent of stroke. Retinal infarction may cause permanent visual loss. Extensive intestinal ischemia can require bowel resection and, when substantial bowel length is lost, cause short-bowel syndrome. Renal infarction may reduce kidney function, particularly when bilateral or affecting a solitary functioning kidney. Irreversible limb ischemia can necessitate amputation; survivors may require prosthetic rehabilitation and long-term assistance with mobility.\n\nEvidence is less complete for rare nonthrombotic embolic syndromes. Studies of fat and amniotic-fluid embolism often use differing clinical definitions, limiting comparisons of incidence and outcomes. Microscopic embolization may remain clinically silent, while catastrophic cases are disproportionately represented in case reports and autopsy series. Observational studies also have difficulty separating embolic injury from accompanying trauma, infection, malignancy, or obstetric complications. Mechanical obstruction is established in many forms, but its relative contribution alongside inflammation, endothelial injury, and coagulation activation remains incompletely resolved in some syndromes."
    }
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  "license": "https://creativecommons.org/licenses/by/4.0/"
}