A 68-year-old man with a history of hip fracture surgery 2 weeks ago presents with acute dyspnea and chest pain. Imaging confirms pulmonary embolism. Which is the most common site of lodgement of the embolus within the pulmonary arterial tree?

Explanation

## Site of PE Lodgement in the Pulmonary Circulation **Key Point:** The **right lower lobe pulmonary artery** is the most common site of PE lodgement, accounting for approximately 50–60% of emboli. This is followed by the left lower lobe artery (~25%), and upper lobes are involved in <15% of cases. ### Anatomical Basis for Right Lower Lobe Predilection | Feature | Implication | | --- | --- | | **Right pulmonary artery is more vertical** | Thrombus from IVC travels straight into right PA; gravity favors right-sided lodgement | | **Right PA is larger in diameter** | Accommodates larger thrombi; lower resistance pathway | | **Right lower lobe artery is the most dependent** | Receives blood flow preferentially due to gravity and hemodynamics | | **Bilateral lower lobes > upper lobes** | Lower lobes have higher blood flow (~60% of total pulmonary blood flow) | **High-Yield:** The **right lower lobe is involved in >50% of PE cases**; the **left lower lobe in ~25%**. Upper lobe involvement is uncommon and should raise suspicion for alternative diagnoses (e.g., in situ thrombosis, malignancy, or septic emboli). **Clinical Pearl:** On chest X-ray, a wedge-shaped opacity in the right lower lobe (Hampton's hump) is a classic but insensitive sign of pulmonary infarction secondary to PE. Infarction occurs in only ~10% of PE (when there is also bronchial artery occlusion or pre-existing cardiopulmonary disease). ### Mechanism of Right Lower Lobe Predilection 1. **Hemodynamic trajectory:** The thrombus enters the right atrium from the IVC and follows the path of least resistance into the right main PA, which is more vertical than the left. 2. **Gravity:** The right lower lobe is the most dependent portion of the pulmonary circulation. 3. **Blood flow distribution:** The right lung receives ~55% of cardiac output; the right lower lobe receives ~30% of total pulmonary blood flow. **Mnemonic:** **RLL-PE** — Right Lower Lobe is the Preferred site for Embolism lodgement (~50% of cases).