Just about everyone at some time or another has bumped an arm or leg which is then followed by swelling, discoloration and pain over the injured site. This is a bruise, which in med speak is called a contusion.
What if a similar injury (ie bruise or contusion) were to occur in the lungs? Well, this type of injury has some potential very serious complication. The bruise can happen after hitting chest on something such as a car door in an accident or falling off a bike or numerous other possibilities.
The ribs do not have to be broken for this type of injury to occur. In fact some of the most serious lung contusions can occur without any rib fractures. At first the patient will have pain, at times severe. If the contusion (bruise) is large then the ability of the lungs to exchange oxygen will be impaired. This of course leads to immediate shortness of breath.
Sometimes the bruise may take several hours to develop. The problem here is that the patient will feel ok, especially after a little pain medications. And the chest xray might even be normal at first. A few hours later and the patient can have severe shortness of breath. A repeat chest xray will then show the bruising.
What this means is that if you have a pretty good story for a significant chest injury, then you probably should stay in the ER for a few hours to see how you are doing.