The narrow segment just distal to the left subclavian artery, tethered by the ligamentum arteriosum
Expansion
The isthmus is the short segment of aorta between the origin of the left subclavian artery and the attachment of the ligamentum arteriosum.
It matters because it is a transition point. The arch is relatively mobile within the mediastinum, while the descending aorta is fixed to the vertebral column by the intercostal arteries and pleural reflections. The ligamentum arteriosum tethers the junction between them.
In a rapid deceleration, a road traffic collision or a fall from height, the mobile arch continues to move while the fixed descending aorta stops, and shearing forces concentrate at the isthmus. Around 90 per cent of survivable traumatic aortic injuries occur here.
Radiographic clues include a widened mediastinum, loss of the aortic knuckle contour, depression of the left main bronchus and deviation of a nasogastric tube to the right.