What are the mechanisms of morbidity and mortality from flail chest?
Flail chest is defined as fracture of several ribs at two or more sites, or disarticulation of ribs from their cartilaginous attachment to the sternum in addition to a fracture. The resulting respiratory impairment may lead to arterial hypox-emia and/or hypercarbia. Two mechanisms are involved: paradoxical ventilation and pulmonary contusion. Para-doxical chest wall motion—manifested by “caving” of the flail segment on inspiration and “bulging” on exhalation—is dyssynchronous with movement of the uninjured part of the chest wall and diaphragm. By itself, a flail segment may increase the work of breathing, but it usually is not the primary cause of acute respiratory failure, unless there is a pneumothorax from injury to the underlying pleura. The “Pendelluft” effect, a pendulum-like motion of gas from one lung to the other during respiration as a result of inequality of pressures between two hemithoraces, does not seem to be a significant cause of respiratory impairment either. It is currently believed that the primary cause of morbidity and mortality after blunt chest trauma is severe pulmonary contusion. An increase in elastic recoil from this cause makes it difficult to expand the lung during inspiration, which not only increases the work of breathing but also causes a decrease in functional residual capacity (FRC) and lung compliance that may not return to normal for several weeks. All these events result in exaggeration of paradoxical chest wall movement.
The pathology of lung contusion involves atelectasis, interstitial and intra-alveolar hemorrhage, and alveolar disruption. Although these changes begin within a few minutes after injury, they may take up to 3–4 hours to complete. Thus, chest radiographs and arterial blood gases will deteriorate gradually within the first few hours of injury. Generally, age >45 years, pre-existing diseases, a higher injury severity score, and the need for large volumes of fluids predict increased morbidity and mortality.