Normal Right Interlobar Pulmonary Artery Size on Radiography
The right interlobar pulmonary artery (RIPA) is the descending branch of the right pulmonary artery, visible on frontal chest radiographs as it courses lateral to the bronchus intermedius. Its diameter serves as a reliable, reproducible marker of pulmonary arterial caliber on plain radiography. Accurate knowledge of age-appropriate dimensions is essential for detecting pulmonary hypertension, increased pulmonary blood flow, or vascular underperfusion in pediatric patients.
Normal Reference Values
| Orientation | Age | Measurement |
|---|---|---|
| Anteroposterior | 1 y/o | 8 mm |
| Anteroposterior | 3 y/o | 9 mm |
| Anteroposterior | 5 y/o | 10 mm |
| Anteroposterior | 6 y/o | 11 mm |
| Anteroposterior | 8 y/o | 12 mm |
| Anteroposterior | 11 y/o | 14 mm |
| Anteroposterior | 12 y/o | 15 mm |
| Anteroposterior | 14 y/o | 16 mm |
Clinical Significance
In children, the RIPA diameter increases steadily with age, ranging from approximately 8 mm at 1 year to 16 mm at 14 years. Enlargement beyond the expected age-matched value raises concern for elevated pulmonary arterial pressure or volume overload. Conversely, a smaller-than-expected diameter may indicate reduced pulmonary blood flow.
Careful comparison to age-specific norms is critical because an RIPA diameter that is normal in a teenager would be frankly abnormal in a toddler. Overpenetrated or rotated radiographs can artificially alter apparent vessel width, so technique must be optimized before drawing clinical conclusions.
- Pulmonary arterial hypertension — primary or secondary (e.g., congenital heart disease, chronic lung disease)
- Left-to-right shunt lesions — atrial septal defect, ventricular septal defect, patent ductus arteriosus
- Pulmonary stenosis or hypoplasia — reduced RIPA diameter
- Chronic thromboembolic disease — asymmetric enlargement or pruning
- High-altitude or hypoxic pulmonary vasoconstriction — diffuse arterial prominence
Reference: Leinbach LB. Roentgenologic evaluation of normal pulmonary arteries in children. Am J Roentgenol Radium Ther Nucl Med. 1998;89: 995-8.
Imaging Notes
On a standard posteroanterior (or anteroposterior in younger children) chest radiograph, the RIPA is measured at its widest visible diameter as it descends in the right hilum, lateral to the bronchus intermedius and medial to the intermediate bronchus air column. The measurement is made perpendicular to the long axis of the vessel on a well-inspired, non-rotated projection. Rotation of the patient introduces significant measurement error and should be excluded by verifying symmetry of the clavicular heads relative to the spinous processes.
Because portable AP projections magnify mediastinal and vascular structures compared with standard PA technique, measurements obtained on AP films should be interpreted with this magnification factor in mind. Whenever possible, dedicated PA erect radiographs with a standard 72-inch focus-to-film distance should be used for serial comparison.