Normal Left Pulmonary Artery Size on Chest X-Ray
The left pulmonary artery arises from the main pulmonary trunk and arches posteriorly over the left mainstem bronchus before dividing into lobar branches supplying the left lung. Accurate measurement of its diameter on chest radiography is clinically important, as enlargement may indicate elevated pulmonary arterial pressure or increased flow states. Routine assessment on plain film remains a practical, radiation-efficient screening tool for pulmonary vascular pathology.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | 18 mm |
Clinical Significance
On the lateral chest radiograph, a left pulmonary artery diameter exceeding 18 mm is considered enlarged and warrants further evaluation. Enlargement may reflect pulmonary arterial hypertension (PAH), elevated pulmonary blood flow, or post-stenotic dilatation. Because the left pulmonary artery is visualized in profile on the lateral view, this projection offers a reliable plane for measurement compared with the frontal radiograph, where vessel overlap limits accuracy.
Clinicians should correlate radiographic enlargement with clinical findings such as dyspnea, hypoxemia, or known cardiac shunts. Subtle enlargement may be overlooked on routine frontal views, making the lateral projection an essential complementary study. It is also important to account for technical factors such as patient rotation and projection geometry, which can artifactually alter apparent vessel caliber.
- Pulmonary arterial hypertension (primary or secondary)
- Left-to-right cardiac shunt (e.g., ASD, VSD, PDA) with increased pulmonary flow
- Chronic thromboembolic pulmonary hypertension (CTEPH)
- Post-stenotic dilatation distal to pulmonary valve stenosis
- Congestive heart failure with pulmonary venous hypertension
Reference: Reed JC. Chest Radiology Plain Film Patterns and Differential Diagnoses, E-Book. Mosby. (2010)
Imaging Notes
On the lateral chest radiograph, the left pulmonary artery is measured at its widest point as it courses superiorly and posteriorly over the left mainstem bronchus. A true lateral projection with minimal rotation is essential; patient rotation introduces significant measurement error. The vessel should be traced from its origin at the main pulmonary trunk and measured perpendicular to its long axis at the point of maximum diameter.
Standard PA and lateral chest radiographs should be obtained at full inspiration to ensure consistent vascular caliber. When radiographic enlargement is identified, cross-sectional imaging with CT pulmonary angiography or echocardiography is recommended for definitive characterization of pulmonary arterial pressure and morphology.