Expansion
Pushed away by extra volume, pulled towards by lost volume
Expansion
Pushed AWAY from the lesion - something is adding volume:
- Tension pneumothorax
- Large pleural effusion or haemothorax
- Large mass or diaphragmatic hernia
Pulled TOWARDS the lesion - something is losing volume:
- Lobar or whole-lung collapse
- Pneumonectomy
- Pulmonary fibrosis and scarring
Check the shift at two levels: the trachea for the upper mediastinum and the cardiac apex for the lower.
A simple consolidation does not shift the mediastinum, because the lung is filled but not expanded or shrunken. That fact alone separates pneumonia from collapse when both produce a white hemithorax.