Cardiac output (the volume of blood pumped by the heart) may be reduced, and hypotension (low blood pressure) is frequently present. The area of the chest wall near the contusion may be  tender or painful due to associated chest wall injury. Signs and symptoms take time to develop, and as many as half of cases are asymptomatic at the initial presentation. The more severe the injury, the more quickly symptoms become apparent. In severe cases, symptoms may occur as quickly as three or four hours after the trauma. Hypoxemia (low oxygen concentration in the arterial blood) typically becomes progressively worse over 24–48 hours after injury. In general, pulmonary contusion tends to worsen slowly over a few days, but it may also cause rapid deterioration or death if untreated. Causes
Pulmonary contusion is the most common injury found in blunt chest trauma, occurring in 25–35% of cases. It is usually caused by the rapid deceleration that results when the moving chest strikes a fixed object. About 70% of cases result from motor vehicle collisions, most often when the chest strikes the inside of the car. Falls, assaults, and sports injuries are other causes. Pulmonary contusion can also be caused by explosions; the organs most vulnerable to blast injuries are those that contain gas, such as the lungs. Blast lung is severe pulmonary contusion, bleeding, or edema with damage to alveoli and blood vessels, or a combination of these. This is the primary cause of death among people who initially survive an explosion.
Fibrosis of the lungs can occur, resulting in dyspnea (shortness of breath), low blood oxygenation, and reduced functional residual capacity for as long as six years after the injury. As late as four years post-injury, decreased functional residual capacity has been found in most pulmonary contusion patients studied. During the six months after pulmonary contusion, up to 90% of people have difficulty breathing. In some cases, dyspnea persists for an indefinite period. Contusion can also permanently reduce the compliance of the lungs. Complications
Pulmonary contusion can result in respiratory failure—about half of such cases occur within a few hours of the initial trauma. Other severe complications, including infections and acute respiratory distress syndrome (ARDS) occur in up to half of cases. Elderly people and those who have heart, lung, or kidney disease prior to the injury are more likely to stay longer in hospital and have complications from the injury. Complications occur in 55% of people with heart or lung disease and 13% of those without. Of people with pulmonary contusion alone, 17% develop ARDS, while 78% of people with at least two additional injuries develop the condition. A larger contusion is associated with an increased risk. In one study, 82% of people with 20% or more of the lung volume affected developed ARDS, while only 22% of people with less than 20% did so.Pneumonia, another potential complication, develops in as many as 20% of people with pulmonary contusion. Contused lungs are less able to remove bacteria than uninjured lungs, predisposing them to infection.