A 58-year-old woman is admitted with acute dyspnea and syncope. Chest X-ray shows a wedge-shaped opacity in the right lower lobe. CT pulmonary angiography confirms pulmonary embolism with infarction. Which is the most common site of pulmonary infarction in PE?

Explanation

## Anatomical Distribution of Pulmonary Infarction in PE **Key Point:** The right lower lobe is the most common site of pulmonary infarction in PE, accounting for approximately 40–50% of all infarcts. **High-Yield:** Pulmonary infarction occurs in only 10% of PE cases (most PE do not cause infarction due to dual blood supply from pulmonary and bronchial circulations). When infarction does occur, the right lower lobe is the predominant site. ### Distribution of Pulmonary Infarction | Lobe/Site | Frequency | Reason | |-----------|-----------|--------| | Right lower lobe | 40–50% | Largest lobe; most dependent; highest perfusion | | Right upper lobe | 15–20% | Second most common | | Left lower lobe | 15–20% | Smaller than right; less frequent | | Left upper lobe | 5–10% | Smallest lobe; least frequent | | Right middle lobe | 5–10% | Smallest right lobe segment | **Clinical Pearl:** Infarction is more common in patients with pre-existing cardiopulmonary disease (heart failure, chronic lung disease) because compromised collateral circulation from bronchial arteries increases tissue ischemia risk. **Mnemonic — Why Right Lower Lobe:** **BIG** — **B**iggest lobe (right lower is the largest pulmonary segment), **I**s most dependent (gravity-related perfusion), **G**reatest risk of infarction (highest blood flow → largest embolus impact). ### Pathophysiology of Infarction 1. **Dual blood supply:** Pulmonary arteries + bronchial arteries normally prevent infarction in most PE cases. 2. **Infarction occurs when:** Bronchial circulation is inadequate (heart failure, shock, pre-existing lung disease). 3. **Right lower lobe vulnerability:** Largest lobe with highest perfusion; occlusion of right lower lobe artery causes maximal tissue ischemia. **Warning:** Do not confuse "most common site of PE" (which is the right lower lobe artery) with "most common site of infarction" (also right lower lobe, but infarction is rare). Only ~10% of PE result in infarction. [cite:Harrison 21e Ch 297]