Determinants of rib motion in flail chest

We have previously developed a canine model of isolated Rail chest to asses s the effects of this condition on the mechanics of breathing, and these st udies have led to the conclusion that the respiratory displacement of the f ractured ribs Is primarily determined by the fall in pleural pressure (Delt a PpI) and the action of the parasternal intercostal muscles, The present s tudies were designed to test the validity of this conclusion. A flail was i nduced in six supine anesthetized animals by fracturing both dorsally and v entrally the second to fifth ribs on the right side of the chest, after whi ch the phrenic nerve roots were bilaterally sectioned in the neck. Sectioni ng the phrenic nerves caused a 34% decrease in Delta PpI, associated with a 39% increase In parasternal intercostal inspiratory EMG activity (p < 0.05 ), and resulted in a marked reduction in the inspiratory Inward displacemen t of the ribs. In three animals, the inward rib displacement was even rever sed into a small outward displacement, When the airway was then occluded at : end-expiration to increase Delta PpI during the subsequent inspiration, a ll animals again showed a clear-cut inward rib displacement. These observat ions therefore confirm that in dogs with frail chest, the inspiratory displ acement of the fractured ribs is set by the balance between the Force relat ed 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 para dox in such patients.