Mediastinal shift may be caused by volume expansion on one side of the thorax, volume loss on one side of the thorax, mediastinal masses and vertebral or chest wall abnormalities. An emergent condition classically presenting with mediastinal shift is tension pneumothorax.

What occurs when a patient experiences a mediastinal shift?

This mediastinal shift also causes compression of the lung on the unaffected side. This is a life-threatening situation because the pressure on the great vessels causes a decrease in venous return, reduced cardiac output, decrease in blood pressure and hypoxia, which will lead to shock.

Which way does the mediastinum shift in a pneumothorax?

A large left sided pneumothorax is present with near complete collapse of the left lung. Mediastinal shift towards the right is present in keeping with tension (a medical emergency).