## Source of Pulmonary Embolism **Key Point:** Deep vein thrombosis (DVT) of the lower extremities accounts for approximately 90–95% of clinically significant pulmonary emboli. The thrombus dislodges and travels via the inferior vena cava to the right heart and lodges in the pulmonary arteries. ### Pathophysiology of PE Origin The vast majority of PE arise from venous thromboembolism (VTE) in the deep veins of the legs: | Source | Frequency | Clinical Significance | | --- | --- | --- | | **Lower limb DVT** | 90–95% | Most common; often asymptomatic until PE occurs | | Right atrial thrombus | 1–2% | Rare; associated with central lines, arrhythmias | | Paradoxical embolism | <1% | Requires PFO or ASD; venous thrombus crosses to systemic circulation | | Tumor embolism | <1% | Seen in advanced malignancy; usually with other emboli | **High-Yield:** The **popliteal vein and above** (iliofemoral and popliteal veins) are the primary sites of DVT that lead to PE. Calf vein thrombi rarely cause clinically significant PE unless they propagate proximally. **Clinical Pearl:** Approximately 50% of patients with proximal DVT have asymptomatic PE on imaging, and conversely, 30–40% of patients presenting with PE have no clinical evidence of DVT on examination—emphasizing that DVT may be "silent" before PE manifests. ### Why Lower Limb DVT Dominates 1. **Anatomical factors:** The lower limbs are dependent, with slow venous return and high hydrostatic pressure. 2. **Virchow's triad:** Venous stasis (immobility, surgery), endothelial injury (trauma, lines), and hypercoagulability (malignancy, thrombophilia) are common in the legs. 3. **Thrombus size:** Lower limb thrombi are large enough to cause hemodynamically significant PE. **Mnemonic:** **DVT-PE axis** — Deep Vein Thrombosis (legs) → Pulmonary Embolism (lungs). This is the cardinal pathway in >90% of cases.
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