A 52-year-old man with acute dyspnea and chest pain undergoes CTPA. A filling defect is identified in the right main pulmonary artery extending into the lower lobe branches. What is the most common source of this pulmonary embolism?

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B. Deep vein thrombosis of the lower limbs. ## Most Common Source of Pulmonary Embolism **Key Point:** Deep vein thrombosis (DVT) of the lower limbs is the source of >90% of clinically significant pulmonary emboli, making it the most common origin. ### Pathophysiology of PE Origin ```mermaid flowchart TD A[Venous Thromboembolism]:::outcome --> B{Source of thrombus?}:::decision B -->|Lower limb DVT| C[90% of PE]:::action B -->|Upper limb DVT| D[5-10% of PE]:::action B -->|Right heart/atrial| E[Rare: <2%]:::action B -->|Paradoxical/other| F[Rare: <1%]:::action C --> G[Embolus travels via IVC to right heart]:::outcome G --> H[Lodges in pulmonary arteries]:::outcome ``` ### Why Lower Limb DVT is the Most Common Source 1. **Virchow's triad** — Lower limbs are prone to venous stasis (especially in immobility, surgery, malignancy), endothelial injury, and hypercoagulability. 2. **Anatomical factors** — The deep veins of the legs (femoral, popliteal, tibial) are large-caliber vessels that can harbor large thrombi. 3. **Clinical prevalence** — DVT of the lower limbs occurs in 20–30% of hospitalized patients without thromboprophylaxis. 4. **Mechanical pathway** — Thrombi from the legs travel via the inferior vena cava (IVC) directly to the right heart and pulmonary circulation. ### Distribution of PE Sources | Source | Frequency | Notes | | --- | --- | --- | | Lower limb DVT | 90–95% | Femoral, popliteal, calf veins | | Upper limb DVT | 5–10% | Central lines, malignancy, thoracic outlet syndrome | | Right atrial/cardiac | <2% | Atrial fibrillation, right ventricular infarction | | Paradoxical embolism | <1% | PFO, ASD, venous thrombus crossing to systemic circulation | | Tumor embolism | <1% | Lung cancer, renal cell carcinoma | **High-Yield:** When PE is diagnosed on CTPA, always perform bilateral lower limb venous ultrasound to identify the source DVT and guide anticoagulation duration. **Clinical Pearl:** Approximately 30–50% of patients with PE have no imaging evidence of DVT on ultrasound ("isolated PE"), but DVT may still be present in distal calf veins or have already embolized. ### Risk Factors for Lower Limb DVT Leading to PE - **Immobility** — Prolonged bed rest, long flights, paralysis - **Surgery** — Orthopedic procedures (hip/knee replacement) carry highest risk - **Malignancy** — Hypercoagulable state - **Hypercoagulable states** — Factor V Leiden, prothrombin G20210A mutation, antithrombin deficiency - **Trauma** — Long bone fractures - **Central venous catheters** — Iatrogenic endothelial injury [cite:Harrison 21e Ch 297]

Answer

A. Tumor embolism from lung malignancy

Answer

C. Right atrial thrombus secondary to atrial fibrillation

Answer

D. Paradoxical embolism from a patent foramen ovale

A 52-year-old man with acute dyspnea and chest pain undergoes CTPA. A filling defect is identified in the right main pulmonary artery extending into the lower lobe branches. What is the most common source of this pulmonary embolism?

Explanation

Most Common Source of Pulmonary Embolism

Pathophysiology of PE Origin

Why Lower Limb DVT is the Most Common Source

Distribution of PE Sources

Risk Factors for Lower Limb DVT Leading to PE

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Source	Frequency	Notes
Lower limb DVT	90–95%	Femoral, popliteal, calf veins
Upper limb DVT	5–10%	Central lines, malignancy, thoracic outlet syndrome
Right atrial/cardiac	<2%	Atrial fibrillation, right ventricular infarction
Paradoxical embolism	<1%	PFO, ASD, venous thrombus crossing to systemic circulation
Tumor embolism	<1%	Lung cancer, renal cell carcinoma