DIAPHRAGM THICKNESS HETEROGENEITY AT FUNCTIONAL RESIDUAL CAPACITY ANDTOTAL LUNG CAPACITY

One of the determinants of muscular force is the number of myofibrils in parallel, which is approximated by thickness. To better understand the heterogeneity of diaphragm thickness, we quantified the interregio nal and radial patterns of thickness of nine canine diaphragms rapidly perfusion fixed in situ with glutaraldehyde at functional residual ca pacity (FRC) (n = 6) and total lung capacity (TLC) (n = 3). Thickness was determined gravimetrically from punch biopsies radiating from the central tendon to rib cage insertion in ventral, middle, and dorsal co stal and crural regions. For comparison, the contralateral unfixed hem idiaphragm was sampled in the same fashion. The findings of this inves tigation include the following. 1)The costal diaphragm exhibits the sa me pattern of interregional heterogeneity at FRC, TLC, and in the fres hly excised state. 2) The costal diaphragm is significantly thinner at FRC in situ (0.17 +/- 0.01 cm) than is the freshly excised contralate ral diaphragm (0.21 +/- 0.01 cm; P < 0.05), whereas there is no signif icant difference between thickness at TLC and the freshly excised stat e. 3) There is significant, previously undescribed, radial tapering fr om the rib cage attachment (0.24 +/- 0.02 cm) to the central tendon in sertion (0.15 +/- 0.01 cm; P < 0.05) that is exaggerated at TLC. 4) Wi th passive inflation from FRC to TLC, the greatest increase in thickne ss occurs close to the rib cage attachment for the ventral and medial costal regions but close to the central tendon in the dorsal and crura l regions. We conclude that the diaphragm at FRC and TLC exhibits radi al thickness heterogeneity that cannot, be predicted from dimensions o f the freshly excised diaphragm. The thicker areas are predicted to ei ther develop greater maximum force generation or lower wall stress for the same degree of activation.