Correct answer:

Crush injuries of the chest, in which the whole sternum is loosened by fractured ribs on either side or several ribs are fractured in two places, result in the condition of flail chest. On inspiration, the flail part of the chest wall becomes indrawn by the negative intrathoracic pressure, as it is no longer in structural continuity with the bony thoracic cage. Similarly, in expiration the flail part of the chest is pushed out while the rest of the bony cage becomes contracted. This is termed paradoxical movement. The patient becomes grossly hypoxic due to failure of adequate expansion of the affected side and also because of shunting of deoxygenated air from the lung on the side of the fracture into the opposite side. The pendulum movements of the mediastinum also produce cardiovascular embarrassment so that the patient becomes rapidly and progressively shocked.