## Most Common Source of Pulmonary Embolism **Key Point:** Approximately 90–95% of pulmonary emboli originate from deep vein thrombosis (DVT) of the lower extremities, particularly the iliofemoral, popliteal, and calf veins. ### Anatomical Distribution of Thrombus Origin | Site | Frequency | Clinical Context | |------|-----------|------------------| | Deep veins of lower extremities (femoral, popliteal, calf) | 90–95% | Most common; post-operative, immobilization, malignancy | | Pelvic veins | 5–10% | Malignancy, pregnancy, pelvic surgery | | Right heart chambers | <1% | Acute MI, cardiomyopathy, arrhythmia | | Upper extremity veins | <1% | Central lines, thoracic outlet syndrome, malignancy | ### Why Lower Extremity DVT Predominates 1. **Virchow's Triad factors** — prolonged immobilization (surgery, bed rest) causes venous stasis in dependent limbs. 2. **Vein caliber** — larger veins of the lower extremity hold larger thrombi capable of embolizing to the pulmonary circulation. 3. **Post-operative risk** — hip and knee surgery (as in this case) is a major risk factor; DVT occurs in 40–60% of hip fracture patients without prophylaxis. **Clinical Pearl:** In a post-operative patient with PE, always perform bilateral lower extremity venous ultrasound to identify the source, even if CTPA is positive. **High-Yield:** The iliofemoral and popliteal veins are the most common specific sites within the lower extremity; calf vein thrombi are less likely to embolize but can propagate proximally.
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