Mechanical arrangement of costal and crural diaphragms in dogs.

To assess the mechanical arrangement of the costal and crural parts of the diaphragm, we studied changes in diaphragmatic length with piezoelectric crystals in 17 supine anesthetized dogs. During control resting inspiration, the crural part usually shortened more and earlier than the costal part. After phrenicotomy, the crural part always lengthened during inspiration, whereas the costal part shortened or lengthened. These interanimal differences disappeared after opening of the abdomen; the costal part then always lengthened during inspiration. During stimulation of one part, the relaxed nonstimulated part always lengthened. However, when compared with the relationship between length and transdiaphragmatic pressure (Pdi) obtained during passive deflation, the lengthening of the relaxed part during stimulation of either part was small. This difference between predicted and measured Pdi-length relationship decreased in magnitude as lung volume increased above functional residual capacity (FRC) and increased as residual volume was approached. These results indicate that 1) even during quiet breathing the diaphragm in the dog is not a single functional entity; 2) at FRC the costal and crural portions of the diaphragm behave as if they were mechanically arranged partly in parallel and partly in series; and 3) they gradually move into a pure mechanical series arrangement as lung volume increases.