Chest wall mechanics in dogs with acute diaphragm paralysis

To investigate the action of the inspiratory intercostals, we studied the patterns of rib cage and abdominal motion during tidal breathing in eight dogs before and after bilateral phrenicotomy. Hooks screwed into the sternum and the ribs were used to monitor the anteroposterior and transverse diameters of the rib cage and the axial displacements of the sternum and the ribs. In all animals, we found that during tidal inspirations performed with the inspiratory intercostals alone, 1) the rib cage moved outward while the abdomen moved inward; 2) the rib cage was displaced near its relaxation curve (defined by a plot of its transverse vs. anteroposterior diameter) but expanded more in its transverse than in its anteroposterior dimension; and 3) the ribs in the midaxillary line moved cephalad, whereas the sternum was displaced caudally. Additional experiments performed on four dogs demonstrated that contraction of the parasternal intercostals was responsible for the inspiratory caudal displacement of the sternum. These findings indicate that in the dog, 1) the inspiratory intercostals act essentially on the lateral walls of the rib cage, and 2) those of the parasternal area produce a caudal displacement of the sternum. In addition, they suggest strongly that the axial displacement of the sternum constitutes an additional degree of freedom of motion of the rib cage and that contraction of the neck accessory muscles is mandatory for the sternum to be displaced in a cephalad direction during inspiration.