Adverse effect of hyperinflation on parasternal intercostals.

The force-generating ability of the parasternal intercostals is maintained during acute hyperinflation in the dog (Jiang et al. Am. Rev. Respir. Dis. 139: 522-528, 1989). In the present studies, we assessed whether the ability of these muscles to expand the rib cage and inflate the lungs is really maintained. We thus measured the electromyogram and the changes in length of these muscles, the axial motion of the sternum and the ribs, and the changes in pleural pressure and tidal volume in anesthetized vagotomized phrenicotomized dogs during selective parasternal stimulation and during spontaneous breathing at different lung volumes corresponding to 0, 5, 10, and 15 cmH2O positive airway pressure. Compared with functional residual capacity, parasternal stimulation at 15 cmH2O positive airway pressure caused a mild decrease in muscle shortening, a large reduction in cranial rib motion, and a large reduction in pleural pressure fall. The caudal displacement of the sternum, however, was increased. Similar alterations in rib and sternal motions and in muscle shortening were seen during spontaneous breathing; tidal volume was markedly reduced as well. These observations thus indicate that hyperinflation affects the action of the parasternal intercostals on the rib cage; their rib-elevating action is decreased, whereas their action on the sternum is increased. As a result, their ability to inflate the lung is reduced. Thus, the inflationary actions of both the diaphragm and parasternal intercostals are reduced by hyperinflation.