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 **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]