What causes Flat waist sign in the lungs on chest radiograph?
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Flat waist sign on an chest radiograph is caused by:
Flattening of the left heart border, specifically the contours of the aortic knob and adjacent main pulmonary artery, occurs in left lower lobe collapse (atelectasis), which may result from endobronchial obstruction, mucus plugging, or extrinsic compression.
Why is it called so?
The flat waist sign is named for the loss of the normal concavity or “waist-like” inward curve along the left heart border between the aortic knob and main pulmonary artery, creating a flattened appearance resembling a waist without its characteristic taper.
Pathophysiology
Left lower lobe collapse causes volume loss, leading to inferior and medial displacement of the left hilum and slight right anterior oblique rotation of the heart; this rotation eliminates the normal concavity between the aortic arch and pulmonary artery, resulting in flattening of the left heart border on frontal chest radiograph.
Alternative names: Retrocardiac sail sign
Other associated named signs: Silhouette sign
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