Shape and tension distribution of the passive rat diaphragm

We developed an in vitro preparation to investigate shape and stress distribution in the intact rat diaphragm. Our hypothesis was that the diaphragm is anisotropic with smaller compliance in transverse fiber direction than along fibers, and therefore shape change may be small. After the animals were killed (8 rats), the entire diaphragm was excised and fixed into a mold at the insertions. Oxygenated Krebs-Ringer solution was circulated under the diaphragm and perfused over its surface. A total of 20-23 small markers were sutured on the diaphragm surface. At transdiaphragmatic pressure (P(di)) of 3-15 cmH(2)O, curvature was smaller in transverse direction than along fibers. Using finite element analysis we computed membrane tension. At P(di) of 15 cmH(2)O, tension in central tendon was larger than muscle. In costal region maximum principal tension (sigma(1)) is essentially along the fibers and ranged from 6-10 g/cm. Minimum principal tension (sigma(2)) was 0. 3-4 g/cm. In central tendon, sigma(1) was 10-15 g/cm, compared with 4-10 g/cm for sigma(2). The diaphragm was considerably stiffer in transverse fiber direction than along the fibers.