Normal Right Descending Pulmonary Artery Size on Radiography
The right descending pulmonary artery (RDPA), also called the right interlobar pulmonary artery, is visible on frontal chest radiographs as a vertically oriented vessel descending lateral to the right hilum. Accurate measurement of its transverse diameter is a practical, low-cost method for screening pulmonary arterial enlargement at the point of initial imaging.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Posteroanterior | <16 mm |
Clinical Significance
A transverse RDPA diameter of 16 mm or greater on a posteroanterior (PA) chest radiograph is considered abnormal and raises concern for pulmonary arterial hypertension (PAH) or pulmonary vascular overcirculation. This threshold has long been incorporated into plain-film pattern recognition as an accessible marker when advanced imaging is unavailable or not yet performed.
Enlargement of the RDPA should prompt clinical correlation with symptoms, echocardiography, and, when indicated, CT pulmonary angiography or right heart catheterization. It is important to note that isolated RDPA enlargement is a non-specific finding; technique, patient rotation, and overlying structures can affect apparent vessel caliber.
- Pulmonary arterial hypertension (primary or secondary)
- Left-to-right shunt (e.g., ASD, VSD) causing increased pulmonary flow
- Chronic pulmonary thromboembolism
- Mitral valve disease with secondary pulmonary venous hypertension
- Hyperkinetic states (e.g., high-output cardiac failure, anemia)
Reference: Reed JC. Chest Radiology Plain Film Patterns and Differential Diagnoses, E-Book. Mosby. (2010).
Imaging Notes
On the posteroanterior chest radiograph, the RDPA is measured at its widest transverse diameter just below the right hilum, where it descends alongside the intermediate bronchus. A true PA projection is essential; patient rotation artificially magnifies or reduces apparent vessel width, potentially leading to misclassification. Standard radiographic technique with full inspiration improves hilar conspicuity and reproducibility.
Because plain radiography provides only a two-dimensional projection, CT remains the reference standard for precise pulmonary artery measurement and assessment of underlying etiology. The RDPA diameter on chest X-ray serves as a practical screening tool rather than a definitive diagnostic criterion.