Determination of Situs From Bronchial Shadow on X-ray Chest

Determination of situs from bronchial shadow on X-ray chest

Morphological right bronchus is more vertical and branches earlier while left bronchus is more horizontal and branches later as it has to travel longer to the lung hilum. Right bronchus is eparterial – superior to the right pulmonary artery, while left bronchus is hyparterial – inferior to the left pulmonary artery.

In situs ambiguous, it could be either left isomerism or right isomerism. In left isomerism, both bronchi will have features of left bronchus while in right isomerism, both bronchi will have features of right bronchus [1].

Left isomerism is associated with vena caval abnormalites. Dilated inferior vena cava may enter region of right superior vena cava/right atrial junction rather than at lower right atrium. Left hemiazygous vein may also be present, draining from the abdomen and form a bulge on the left upper border of the mediastinum on chest X-Ray.

A simple well positioned and penetrated chest X-Ray can differentiate situs ambiguous into left isomerism and right isomerism. But very often the quality of the X-Ray will not be sufficient to delineate the bronchial shadows confidently.

Reference

1. J Somerville, V Grech. The Chest X-Ray in Congenital Heart Disease 2. Images Paediatr Cardiol
   . 2010 Jan;12(1):1-8.