Actions of the inspiratory intercostal muscles in flail chest

Pattern and presentation of blunt chest trauma among different age groups

We assessed the presentation and mechanisms of blunt chest injuries sustained in different age groups, and the impact on management options. We retrospectively reviewed the records of 486 patients with blunt chest trauma who were treated between 2004 and 2007. They were stratified into 3 age groups: pediatric, adult, and elderly. Clinical data, mechanism of injury, associated injuries, treatment strategies, and mortality were analyzed. The majority of patients were male (86%), and the largest age group was 12–60-years old (79%). Motor vehicle accidents accounted for 93% of cases. Isolated chest injuries were more common than combined injuries (69% vs. 31%). Tube thoracostomy was required in 39% of patients, predominantly in the elderly age group. Combined injuries involved bone fractures (21%), head (5%), and abdominal trauma (5%). Most blunt chest injuries were treated conservatively (95.5%). Open thoracotomy was required in 4.5% for clotted hemothorax (3.7%), empyema (0.6%), and massive air leak (0.2%). Mortality was 1%, predominantly in the adult group (0.8%). Blunt thoracic injuries are a major cause of morbidity and mortality. As the majority of such injuries are related to motor vehicle accidents, preventative strategies should be enforced.

Disorders of the respiratory muscles

The act of breathing depends on coordinated activity of the respiratory muscles to generate subatmospheric pressure. This action is compromised by disease states affecting anatomical sites ranging from the cerebral cortex to the alveolar sac. Weakness of the respiratory muscles can dominate the clinical manifestations in the later stages of several primary neurologic and neuromuscular disorders in a manner unique to each disease state. Structural abnormalities of the thoracic cage, such as scoliosis or flail chest, interfere with the action of the respiratory muscles-again in a manner unique to each disease state. The hyperinflation that accompanies diseases of the airways interferes with the ability of the respiratory muscles to generate subatmospheric pressure and it increases the load on the respiratory muscles. Impaired respiratory muscle function is the most severe consequence of several newly described syndromes affecting critically ill patients. Research on the respiratory muscles embraces techniques of molecular biology, integrative physiology, and controlled clinical trials. A detailed understanding of disease states affecting the respiratory muscles is necessary for every physician who practices pulmonary medicine or critical care medicine.

Determinants of rib motion in flail chest

We have previously developed a canine model of isolated flail chest to assess the effects of this condition on the mechanics of breathing, and these studies have led to the conclusion that the respiratory displacement of the fractured ribs is primarily determined by the fall in pleural pressure (Delta Ppl) and the action of the parasternal intercostal muscles. The present studies were designed to test the validity of this conclusion. A flail was induced in six supine anesthetized animals by fracturing both dorsally and ventrally the second to fifth ribs on the right side of the chest, after which the phrenic nerve roots were bilaterally sectioned in the neck. Sectioning the phrenic nerves caused a 34% decrease in Delta Ppl, associated with a 39% increase in parasternal intercostal inspiratory EMG activity (p < 0.05), and resulted in a marked reduction in the inspiratory inward displacement of the ribs. In three animals, the inward rib displacement was even reversed into a small outward displacement. When the airway was then occluded at end-expiration to increase Delta Ppl during the subsequent inspiration, all animals again showed a clear-cut inward rib displacement. These observations therefore confirm that in dogs with flail chest, the inspiratory displacement of the fractured ribs is set by the balance between the force related to pleural pressure and that generated by the parasternal intercostals. These observations also point to the critical importance of the pattern of inspiratory muscle activation in determining the magnitude of rib cage paradox in such patients.