Mechanism:

The physical processes behind pulmonary contusion are poorly understood. However, it is known that lung tissue can be crushed when the chest wall bends inward on impact. Three other possible mechanisms have been suggested: the inertial effect, the spalling effect, and the implosion effect.

In the inertial effect, the lighter alveolar tissue is sheared from the heavier hilar structures, an effect similar to diffuse axonal injury in head injury. It results from the fact that different tissues have different densities, and therefore different rates of acceleration or deceleration.In the spalling effect, lung tissue bursts or is sheared where a shock wave meets the lung tissue, at interfaces between gas and liquid. The alveolar walls form such a gas-liquid interface with the air in the alveoli. The spalling effect occurs in areas with large differences in density; particles of the denser tissue are spalled (thrown) into the less dense particles.

The implosion effect occurs when a pressure wave passes through a tissue containing bubbles of gas: the bubbles first implode, then rebound and expand beyond their original volume. The air bubbles cause many tiny explosions, resulting in tissue damage; the overexpansion of gas bubbles stretches and tears alveoli. This effect is thought to occur microscopically when the pressure in the airways increases sharply.

Contusion usually occurs on the lung directly under the site of impact, but, as with traumatic brain injury, a contrecoup contusion may occur at the site opposite the impact as well. A blow to the front of the chest may cause contusion on the back of the lungs because a shock wave travels through the chest and hits the curved back of the chest wall; this reflects the energy onto the back of the lungs, concentrating it. (A similar mechanism may occur at the front of the lungs when the back is struck.)

The amount of energy transferred to the lung is determined in a large part by the compliance (flexibility) of the chest wall.Children's chests are more flexible because their ribs are more elastic and there is less ossification of their intercostal cartilage. Therefore, their chest walls bend, absorbing less of the force and transmitting more of it to the underlying organs.

An adult's more bony chest wall absorbs more of the force itself rather than transmitting it.Thus children commonly get pulmonary contusions without fractures overlying them, while elderly people are more likely to suffer fractures than contusions. One study found that pulmonary contusions were accompanied by fractures 62% of the time in children and 80% of the time in adults.

Pathophysiology:

Normally, oxygen and carbon dioxide diffuse across the capillary and alveolar membranes and the interstitial space (top). Fluid impairs this diffusion, resulting in less oxygenated blood (bottom).Pulmonary contusion results in bleeding and fluid leakage into lung tissue, which can become stiffened and lose its normal elasticity. The water content of the lung increases over the first 72 hours after injury, potentially leading to frank pulmonary edema in more serious cases. As a result of these and other pathological processes, pulmonary contusion progresses over time and can cause hypoxia (insufficient oxygen).