Normal Carina Angle Size on Chest Radiography
The carina angle is the subcarinal angle formed at the bifurcation of the trachea into the left and right main bronchi, visible on the posteroanterior (PA) chest radiograph. Accurate assessment of this angle is clinically important because enlargement of adjacent mediastinal structures — most notably the left atrium — directly splays the carina and alters this measurement.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Posteroanterior | 40-80¡ |
Clinical Significance
On a standard PA chest radiograph, the normal carina angle ranges from 40° to 80°. An angle exceeding 80° is considered widened and warrants further evaluation for an underlying cause of subcarinal or left atrial enlargement. Historically, an angle greater than 70° was used as a threshold for left atrial enlargement; however, Haskin and Goodman’s reassessment demonstrated that the normal range extends to 80°, cautioning against over-diagnosis based on earlier, narrower criteria.
Pitfalls include patient rotation, cardiac projection variation with respiratory phase, and body habitus, all of which can artifactually alter the perceived angle. A widened carina angle alone is not pathognomonic and must be interpreted alongside other radiographic and clinical findings.
- Left atrial enlargement (mitral valve disease, heart failure)
- Subcarinal lymphadenopathy (lymphoma, sarcoidosis, metastatic disease)
- Subcarinal mass (bronchogenic cyst, esophageal duplication cyst)
- Pericardial effusion
- Aortic aneurysm with mediastinal widening
Reference: Haskin PH, Goodman LR. Normal tracheal bifurcation angle: a reassessment. AJR Am J Roentgenol. 1982;139(5):879-82.
Imaging Notes
The carina angle is measured on the posteroanterior (PA) chest radiograph by drawing lines along the inferior walls of the right and left main bronchi from their origin at the carina. A true PA projection with adequate inspiration is essential; expiratory or rotated films will artificially alter the angle. The carina is best identified at the level of the aortic knuckle, typically at the T4–T5 vertebral level.
When the carina angle appears borderline or the radiograph is technically suboptimal, cross-sectional imaging with CT provides a definitive assessment of subcarinal anatomy and any contributing mediastinal pathology.