## Pathological Features of Acute Pulmonary Embolism **Key Point:** Acute PE classically causes **hemorrhagic (red) infarction** in the peripheral, subpleural zones of the lungs. This occurs when the embolus occludes a distal pulmonary artery branch, compromising both the pulmonary and bronchial circulations. **High-Yield:** Not all PE causes infarction. Infarction occurs in only 10% of PE cases because: - The lungs have dual blood supply (pulmonary and bronchial arteries). - Infarction requires occlusion of a distal vessel AND compromised bronchial circulation (e.g., in patients with heart failure or low cardiac output). **Clinical Pearl:** When infarction does occur, it is typically: - **Hemorrhagic (red),** not pale, because blood from the bronchial circulation seeps into the infarcted zone. - **Wedge-shaped,** with the base at the pleura. - **Peripheral,** in the lower lobes (due to preferential blood flow). **Mnemonic: HIP** — Hemorrhagic Infarction in Peripheral zones. ## Why Hemorrhagic Infarction? When a distal pulmonary artery is occluded by an embolus AND the patient has compromised systemic circulation (heart failure, shock), the bronchial circulation cannot adequately perfuse the infarcted zone. Blood from the bronchial system leaks into the necrotic alveolar spaces, creating the characteristic hemorrhagic appearance.
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