Normal Descending Right Pulmonary Artery Size on Radiography
The descending right pulmonary artery (DRPA) is the continuation of the right pulmonary artery as it courses inferiorly within the right hilum, visible as a distinct vascular opacity on the posteroanterior (PA) chest radiograph. Accurate measurement of this structure is clinically important because enlargement is one of the earliest and most reliable plain-film indicators of pulmonary arterial hypertension.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Posteroanterior | 16 mm |
Clinical Significance
On a standard PA chest radiograph, the transverse diameter of the descending right pulmonary artery should not exceed 16 mm in adults. Measurements beyond this threshold raise concern for pulmonary arterial enlargement and warrant further evaluation. This single linear measurement, taken at the widest point of the visible DRPA shadow just below the right hilum, has been used for decades as a practical screening marker.
Enlargement of the DRPA can reflect increased pressure, increased flow, or vessel wall pathology. Common causes of an enlarged DRPA include:
- Pulmonary arterial hypertension (primary or secondary)
- Left-to-right cardiac shunts (e.g., ASD, VSD) causing flow-related enlargement
- Chronic thromboembolic disease with right heart strain
- Mitral stenosis with resultant pulmonary venous hypertension
- Cor pulmonale secondary to chronic lung disease
A key pitfall is patient rotation or suboptimal inspiration, both of which can artifactually widen the hilar shadow and mimic DRPA enlargement. Correlation with the left hilum, cardiac size, and clinical context is essential before concluding true vascular dilation.
Reference: Reed JC. Chest Radiology Plain Film Patterns and Differential Diagnoses, E-Book. Mosby. (2010)
Imaging Notes
The DRPA is best assessed on an upright, full-inspiratory PA chest radiograph. The measurement is taken as the maximal transverse diameter of the descending right pulmonary artery at its widest visible point, typically just lateral to the right cardiac border below the right hilum. The vessel margin should be clearly defined against the adjacent lung parenchyma to ensure accuracy.
Technique pearls: ensure true PA projection (medial clavicular heads symmetric), adequate inspiration (at least 9 posterior ribs visible), and appropriate exposure to resolve the hilar vascular margins. Lateral views may help distinguish hilar vascular from lymph node enlargement, but the standard DRPA measurement is performed exclusively on the PA projection.