The AP chest x-ray shows a widened mediastinum and depressed left main bronchus, highly suspicious for a traumatic aortic injury. This was confirmed on subsequent CT scan of the chest and the patient underwent a successful repair of their thoracic aorta.

A plain radiograph can have following findings in a patient with thoracic aortic injury:

• Widened mediastinum.
• Obliteration of descending thoracic aorta.
• A left apical cap.
• Downward displacement of the left main bronchus.
• Tracheal deviation to the right.
• Obscurantism of the aortic arch.
• A right paratracheal stripe thickening.
• Deviation of the NG tube to the right.
• Left hemithorax.
• Displaced left or right para spinal line.
• Fractured first rib.

Reference: https://emedicine.medscape.com/article/416939-overview#a2