DISPLACEMENTS AND STRAINS IN THE COSTAL DIAPHRAGM OF THE DOG

Radiopaque markers were attached at 1- to 2-cm intervals along three n earby muscle bundles to cover rectangular regions of the mid-costal di aphragms of seven dogs. The markers were tracked by biplane video fluo roscopy during spontaneous breathing (SB), mechanical ventilation with the same tidal volume (MV), and at inflation to total lung capacity ( TLC) in the prone and supine positions. The three-dimensional position s of the markers at functional residual capacity (FRC), at end inspira tion during SB and MV, and at TLC were determined, and the strains in the plane of the diaphragm relative to FRC were calculated. The princi pal strains were found to lie nearly along the muscle bundle direction and perpendicular to it. The principal strains along the muscle bundl es, which describe muscle shortening, were uniform among the three bun dles and uniform along the bundle for MV. For SB, in the prone and sup ine positions, shortening was similar to 30% greater in the middle of the bundle than near the central tendon and chest wall. Although the t idal volumes were the same for SB and MV, the shortening was larger fo r SB. The strains perpendicular to the bundle direction were not signi ficantly different from zero. It appears that, for the loads that occu r during tidal breathing, the diaphragm is inextensible in the directi on perpendicular to the muscle direction. There is a very small displa cement of the costal diaphragm at its insertion on the chest wall. The displacement at the central tendon is primarily a result of muscle sh ortening and rotation of the are of the muscle around its insertion on the chest wall.