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. Childrens 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.
It takes an average of six hours for the characteristic white regions to show up on a chest X-ray, and the contusion may not become apparent for 48 hours. When a pulmonary contusion is apparent in an X-ray, it suggests that the trauma to the chest was severe and that a CT scan might reveal other injuries that were missed with X-ray. Computed tomography
Computed tomography (CT scanning) is a more sensitive test for pulmonary contusion, and it can identify abdominal, chest, or other injuries that accompany the contusion. In one study, chest X-ray detected pulmonary contusions in 16.3% of people with serious blunt trauma, while CT detected them in 31.2% of the same people. Unlike X-ray, CT scanning can detect the contusion almost immediately after the injury. However, in both X-ray and CT a contusion may become more visible over the first 24–48 hours after trauma as bleeding and edema into lung tissues progress. CT scanning also helps determine the size of a contusion, which is useful in determining whether a patient needs mechanical ventilation; a larger volume of contused lung on CT scan is associated with an increased likelihood that ventilation will be needed. CT scans also help differentiate between contusion and pulmonary hematoma, which may be difficult to tell apart otherwise. However, pulmonary contusions that are visible on CT but not chest X-ray are usually not severe enough to affect outcome or treatment. Ultrasound
Pulmonary ultrasound, performed at the bedside or on the accident scene, is being explored as a diagnosis for pulmonary contusion.