Lobar collapse occurs secondary to obstructing mass lesions, mucus plugging, foreign body aspiration (though this may present with air trapping due to a ball valve effect instead), ETT misplacement or extrinsic compression (e.g. acute oedema in a tumour after radiotherapy). Whole lung collapse can be differentiated from pleural effusion because there is volume loss and mediastinal shift to that side. Lobar collapse presents with specific patterns on the chest radiograph.

The left lower lobe collapses medially and posteriorly (and so this is also the direction the oblique fissure moves). This creates a triangular density behind the heart. The heart border is preserved, but the medial aspect of the left hemidiaphragm is obscured. The triangular opacity may look a bit like a sail, hence the “sail sign”.

The left upper lobe collapses anteriorly. Since it collapses in this direction, it envelops the hilum and contact the heart border, thus obscuring both. There is volume loss, but the left lower lobe remains aerated behind so the opacity is not complete and appears like a veil.

The right lower lobe collapses medially and posteriorly creating a triangular opacity adjacent to the right heart border on the PA radiograph. On the lateral radiograph it can be seen to lie in the posterior chest and is not actually adjacent to the right heart border.

The right middle lobe collapses anteriorly and medially to abut the right heart border thus causing it to lose clarity. On the lateral radiograph there is a wedge shaped collapse extending anteroinferiorly from the hilum.

The right upper lobe collapses medially and superiorly. The horizontal fissure is elevated. If there is an obstructing lesion, this may become apparent due to a bulge in the otherwise collapsed lobe – the Golden S sign.

In all these cases look for volume loss; crowding of the ribs; elevation of the hemidiaphragm, the horizontal fissure or diaphragm and hilar position. Also, an obstructing lesion may be visible.

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