Role of the mediastinum in the mechanics of the canine diaphragm.

The objective of this study was to evaluate the role of the mediastinum in the mechanics of the canine diaphragm. Two sets of experiments were performed. In the first experiment on five animals, the mediastinum was severed from the sternum to the vena cava, and radiopaque markers were attached to muscle bundles in the midcostal region of the diaphragm. The three-dimensional location of the markers during relaxation at different lung volumes and during phrenic nerve stimulation at the same lung volumes was then measured using computed tomography. From these data, accurate measurements of muscle displacement and muscle length were obtained, and these measurements, together with the changes in airway opening pressure, were compared with those previously obtained in animals with an intact mediastinum. Severing the mediastinum per se appeared to have no influence on the pressure-generating capacity of the diaphragm or on the lung-volume dependence of this capacity. The great vessels and the esophagus in these animals, however, were left intact, so the possibility remained that these structures continued to impact on the diaphragm through their close attachments to the muscle. In the second experiment, therefore, loads were applied caudally to the central tendon to assess the force-displacement relationship of the entire mediastinum, and this relationship, combined with the known displacement of the diaphragm dome during phrenic nerve stimulation, was used to infer the force exerted by the mediastinum on the muscle during contraction. The results showed that this force is small compared with that developed by the diaphragm, except at very high lung volumes. It is concluded, therefore, that the mediastinum has only little influence on the mechanics of the canine diaphragm.