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?

Question

Accepted Answer

C. Right lower lobe pulmonary artery. ## 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).

Answer

A. Right upper lobe pulmonary artery

Answer

B. Segmental arteries of the left lower lobe

Answer

D. Left main pulmonary artery

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

Anatomical Basis for Right Lower Lobe Predilection

Mechanism of Right Lower Lobe Predilection

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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)