A 58-year-old man with metastatic lung cancer is admitted with acute dyspnea and hemoptysis. Imaging confirms a massive pulmonary embolism. Which is the most common site of thrombus formation in the pulmonary arterial tree?

Explanation

## Site of Pulmonary Arterial Thromboembolism **Key Point:** The right main pulmonary artery is the most common site of lodgement of emboli in the pulmonary circulation, accounting for approximately 60–70% of massive PE cases. ### Anatomical Distribution of PE Lodgement | Pulmonary Arterial Site | Frequency | Clinical Consequence | |-------------------------|-----------|---------------------| | Right main pulmonary artery | 60–70% | Massive PE; high mortality if untreated | | Left main pulmonary artery | 20–30% | Massive PE; less common due to narrower angle | | Lobar arteries (right lower lobe) | Common | Segmental infarction; lower mortality | | Segmental and subsegmental arteries | Most frequent by count | Small emboli; often clinically silent | ### Why the Right Main Pulmonary Artery? 1. **Straighter course** — the right main pulmonary artery is more in line with the main pulmonary artery trunk; emboli preferentially lodge here due to straighter trajectory 2. **Larger diameter** — the right pulmonary artery is wider and receives more blood flow, making it a natural "catch basin" for emboli 3. **Angle of branching** — the left main pulmonary artery branches at a sharper angle, causing emboli to preferentially enter the right side 4. **Hemodynamic factors** — blood flow distribution favors the right pulmonary artery (~55% of cardiac output) **High-Yield:** In massive PE, a thrombus lodged in the right main pulmonary artery causes acute right ventricular failure and cardiogenic shock. This is a medical emergency requiring thrombolysis or embolectomy. **Clinical Pearl:** Smaller emboli lodge distally (segmental and subsegmental arteries), causing wedge-shaped pulmonary infarcts in the periphery. These are more common in absolute numbers but less immediately life-threatening. **Mnemonic:** **RIGHT = MASSIVE** — Right main pulmonary artery = Massive PE; Left = Less common; Lobar/Segmental = Lower mortality. [cite:Robbins 10e Ch 4]