Pulmonary Embolism

Pulmonary embolism is the obstruction of one or more pulmonary arteries by material, most often a thrombus that has travelled (embolised) from the deep veins of the legs or pelvis. It is the most serious manifestation of venous thromboembolism and can range from clinically silent to rapidly fatal obstruction of the pulmonary circulation.

Definition

Pulmonary embolism is the lodging of an embolus, usually a venous thrombus, in the pulmonary arterial circulation, obstructing blood flow to part of the lung and impairing pulmonary perfusion and gas exchange.

Scope

This entry describes pulmonary embolism as a vascular pathology: the source and migration of emboli, the haemodynamic and gas-exchange consequences of pulmonary arterial obstruction, and the disease's epidemiology. It is reference-educational and does not provide anticoagulation, thrombolysis, dosing, or individualised management guidance.

Core questions

• Where does the embolus originate, and how does it reach the pulmonary arteries?
• How does arterial obstruction affect right-heart function and gas exchange?
• What conditions favouring thrombosis (Virchow's triad) underlie the embolic source?
• What distinguishes a small peripheral embolus from massive central obstruction?

Key concepts

• Venous thromboembolism and deep vein thrombosis as the source
• Virchow's triad: stasis, endothelial injury, hypercoagulability
• Embolisation and arterial obstruction
• Ventilation-perfusion mismatch (dead-space effect)
• Right ventricular pressure overload
• Massive (high-risk) versus submassive and low-risk embolism
• Pulmonary infarction

Mechanisms

Most pulmonary emboli arise from thrombi forming in the deep veins of the lower limbs or pelvis, where the conditions of Virchow's triad - venous stasis, endothelial injury, and a hypercoagulable state - promote clot formation. A fragment detaches and travels through the right heart into the pulmonary arteries, lodging where the vessel calibre narrows. Obstruction creates lung regions that are ventilated but not perfused (increased dead space) and provokes vasoconstriction, raising pulmonary vascular resistance. A large or central obstruction acutely increases right ventricular afterload, which can precipitate right-heart failure and circulatory collapse; if perfusion is sufficiently compromised, pulmonary infarction may follow, as outlined in the ESC guidelines and standard pathology texts.

Clinical relevance

Pulmonary embolism is assessed through pretest probability, biomarkers, and imaging, and risk stratification by haemodynamic impact frames prognosis; the ESC/ERS guideline provides the widely used diagnostic and risk framework. Understanding the underlying vascular pathology clarifies why obstruction threatens both gas exchange and the right heart. This entry is descriptive reference material, not individualised clinical advice.

Epidemiology

Venous thromboembolism, which encompasses deep vein thrombosis and pulmonary embolism, is a common and important cardiovascular disorder worldwide, as reviewed by Wendelboe and Raskob, with incidence rising with age and with risk factors such as immobilisation, surgery, malignancy, and inherited or acquired thrombophilia.

History

Rudolf Virchow's nineteenth-century work on thrombosis and embolism established that clots forming in veins can travel to the lungs and named the triad of factors predisposing to thrombosis. Twentieth- and twenty-first-century advances in imaging, D-dimer testing, and structured risk stratification transformed how the condition is recognised and graded.

Debates

How should pulmonary embolism severity be stratified?

Classifications based on haemodynamic stability, right-ventricular dysfunction, and biomarkers aim to separate high-risk from lower-risk embolism, and the optimal combination of markers for prognosis continues to be refined.

Key figures

• Rudolf Virchow

Seminal works

• konstantinides-2019
• wendelboe-2016

Frequently asked questions

Where do most pulmonary emboli come from?

Most arise from thrombi (clots) in the deep veins of the legs or pelvis, which detach and travel through the right heart to lodge in the pulmonary arteries.

Why can a pulmonary embolism be life-threatening?

A large or central embolus abruptly obstructs pulmonary blood flow, raising right ventricular afterload and impairing gas exchange, which can lead to right-heart failure and circulatory collapse.