Normal Left Pulmonary Artery Size on Chest Radiography
The left pulmonary artery arises from the main pulmonary trunk and arches superiorly over the left mainstem bronchus before descending into the left lung. Accurate measurement on chest radiography is clinically important, as enlargement may signal elevated pulmonary arterial pressure or significant underlying cardiopulmonary disease.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | <18 mm |
Clinical Significance
A left pulmonary artery diameter of 18 mm or greater on the lateral projection is considered enlarged and warrants further evaluation. Pulmonary arterial enlargement is one of the earliest radiographic signs of pulmonary hypertension, and its recognition can prompt timely echocardiographic or CT pulmonary angiographic workup.
Enlargement of the left pulmonary artery may be caused by increased flow, increased resistance, or post-stenotic dilatation. Key pitfalls include suboptimal lateral positioning, which may cause the vessel to appear falsely prominent, and the normal variation in vessel caliber with cardiac cycle phase.
- Pulmonary arterial hypertension (primary or secondary)
- Left-to-right shunts (e.g., ASD, VSD, PDA) causing flow-related enlargement
- Chronic thromboembolic disease
- Post-stenotic dilatation from pulmonic valve stenosis
- High-output states (e.g., severe anemia, thyrotoxicosis)
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 greatest visible diameter as it courses superiorly and posteriorly over the left mainstem bronchus. A true lateral projection with minimal rotation is essential; even slight obliquity can alter the apparent vessel diameter. The measurement should be performed at the level of the vessel’s widest point, perpendicular to its long axis.
Radiography provides a useful first-line screen, but CT pulmonary angiography offers superior cross-sectional delineation and allows measurement of the main, right, and left pulmonary arteries with greater precision. Radiographic assessment should always be interpreted in conjunction with clinical history and other chest radiograph findings such as cardiomegaly or pulmonary vascular redistribution.